Paul Langevin, my father: The man and his work 9782759827831

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André Langevin

Paul Langevin, my father The man and his work

English edition by Francis Duck

Translation and adaptation by Francis Duck of the book « Paul Langevin, mon père : l’homme et l’œuvre » by André Lanvevin, with permission, © Les Éditeurs français réunis, 1971.

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© 2022, EDP Sciences, 17 avenue du Hoggar, BP 112, 91944 Les Ulis Cedex A

This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, re-use of illustrations, recitation, broad-casting, reproduction on microfilms or in other ways, and storage in data bank. Duplication of this publication or parts thereof is only permitted under the provisions of the French Copyright law of March 11, 1957. Violations fall under the prosecution act of the French Copyright law. ISBN (print): 978-2-7598-2782-4 – ISBN (ebook): 978-2-7598-2783-1

Table of Contents Preface to the English edition . . . . . . . . . . . . . . . . . . . . . . . . . .

1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7

Paul Langevin’s origins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

From Cambridge to the Collège de France . . . . . . . . . . . . . . . . . .

37

War in 1914 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

67

Between the two world wars: scholar and activist . . . . . . . . . . . . .

75

Anti-fascist leader: educationalist: philosopher . . . . . . . . . . . . . . .

103

Paul Langevin, symbol of the resistance . . . . . . . . . . . . . . . . . . .

139

After the liberation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

191

Testimonials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

213

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

237

Preface to the English edition Paul Langevin was a French physicist, educationalist and political activist. He is sufficiently honoured in France to have been laid to rest in the crypt of the Pantheon in Paris. Here, his remains lie alongside his friend Jean Perrin and, more recently, his friends Pierre and Marie Curie. These few have been accorded this highest honour, together with only a handful of other French physicists and mathematicians: Joseph-Louis Lagrange, Paul Painlevé, Lazare Carnot, Nicolas de Concordet and Gaspard Monge. National pride in his memory was sustained in France after his death in 1946. There are numerous roads, schools and institutes that carry his name. By the centenary of his birth, celebrated in 1972, two full biographies had been published in French and two in Russian, together with a full collection of his work and several compilations of commentaries on his life. However, by then, in the English-speaking world, the memory of Langevin’s name was only retained by a few scientists associated with his particular scientific contributions: to the theory of magnetism, statistical mechanics, ultrasonic detection or ion transport. No biography in English has, to this date, ever been prepared. This year marks the 150th anniversary of Paul Langevin’s birth, on 23 January 1872, in the Montmartre district of Paris. The purpose of the present translation is to make easily available in English the personal account of Paul Langevin’s life that was prepared by his son André 50 years ago. This book was written in collaboration with his wife Luce Dubus, and was first published as Paul Langevin, mon père : l’homme et l’œuvre in 1971 by Les Éditeurs français réunis, 21, rue de Richelieu, Paris 1. The publisher closed in 1994. The author, André Langevin (1901–1977) was the second son of Paul Langevin and his wife Jeanne Desfosses. He was a student at the École supérieure de physique et chimie industrielle (ESPCI) in Paris1, where he became head of works in applied electricity in 1938. He published on piezoelectricity and petrography. He completed his doctorate in 1942, awarded by the Faculty of Sciences in Paris. During the post-war period, he was active in the National Union of 1 André Langevin did not use any abbreviations in his text, usually referring to the École as the ‘School of Physics and Chemistry’. In this translation, I have used the abbreviation used now, ESPCI.

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Scientific Researchers and the National Education Federation. His biography of his father was written after his retirement in 1965. During the intervening 50 years since this book was published, there have been infrequent additions to the literature about Paul Langevin’s life and scientific contributions. Most notably, the outstanding biography by Bernadette Bensaude-Vincent, published in 1987, remains the most complete and deserves its own translation2. The published proceedings of a meeting held at ESPCI on 12–13  June 19973, 50 years after Langevin’s death, comprised 15 contributions covering all aspects of Langevin’s life. These include his central role in the Solvay Physics Congresses4 and his continuing development of ultrasonics for civil underwater detection during the 1920s5. Langevin’s contributions to relativity have also been reviewed6. Nevertheless, since these sources are in French, access for English-speaking students remains a challenge. Elsewhere, there have been occasional reviews in English. Shaul Kazir’s account of the early development of ultrasonics7 finally clarified Walter Cady’s assertion that Langevin is ‘the originator of the science and art of ultrasonics’8. Other short reviews of Langevin’s life9 and his work in ultrasonics10 have also appeared in English. Frédéric JoliotCurie’s biographical obituary for the Royal Society, first published in French in 1950, has appeared in translation11. Undoubtedly, much still is to be learned from and understood about this extraordinary man. Some years ago, a substantial archive of Langevin’s

2 Bensaude-Vincent B.  Langevin science et vigilance  : un savant, une époque. Paris, Belin. 1987. 3 Epistémologiques, vol. 2 (2002) Paul Langevin, son œuvre, sa pensée, Science et engagement. 4 Bensaude-Vincent B. 5 Lelong B. Paul Langevin et la détection sous-marine, 1914–1929. Un physicien acteur de l’innovation industrielle et militaire. Épistémologiques. 2002;2:205–232. 6 Paty M.  Paul Langevin (1872–1946), la relativité et les quanta. Bull Soc Française Phys 1999;119:5–20. 7 Katzir S.  Who knew piezoelectricity? Rutherford and Langevin on submarine detection and the invention of sonar. Notes and Records of the Royal Society, 2012;66(2):141–157. 8 Cady WC. Piezoelectricity. New York. McGraw-Hill. 1946. p. 5. 9 Bok J, Kounelis C.  Paul Langevin (1972–1946) From Montmartre to the Pantheon. Eur Phys News 2007;38:19–21. dx.doi.org/10.1051/epn:2007001. 10 Lewiner J.  Langevin and the birth of ultrasonics. Jpn J Appl Phys 1991;30: Suppl 30-1. 5–11. 11 Duck FA, Thomas AMK. Paul Langevin (1872–1946): The father of ultrasonics. Med Phys Int J. 2022;10:84–91. http://www.mpijournal.org/pdf/2022-01/MPI-202201-p084.pdf

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papers was deposited with ESPCI12 where a selected list related to Langevin’s life and work can be found13. An extensive archive of Langevin’s papers is now available on-line14. These resources will undoubtedly be of use for serious researchers who wish to explore more of Paul Langevin’s life. It was a coup for the French Communist party when Langevin took out membership in 1945. After his death a year later, this association with the communist cause was a significant factor in the celebrations of his life and work, especially the focus on education that consumed the last months of his life. But, by the time that this book was published in 1971, the world was a very different place. A capitalist/communist dichotomy had replaced the fascist/communist dichotomy that had underpinned Langevin’s political activism. The USA was fighting in Vietnam. It was the height of the Cold War. The threat of nuclear annihilation was real. The UK had joined Europe. This is the backdrop against which the author’s narrative about Langevin’s politics may be judged. The story told in this book merges the three main aspects that were equally important in Langevin’s life: his family, his science and his politics. His son was part of all three. Much of the story is told through quotations, from letters, publications and accounts of speeches. Here we find the words of Marie Curie, of Peter Kapitza, of Albert Einstein, of J.D. Bernal, of J.J. Thomson and of Ernest Rutherford. Elsewhere we find Langevin’s words of greeting to his own family, with the little trivial but intimate matters that were important at the time. The text contains, in Langevin’s own words, his bold clear philosophy promoting science and justice. His many friends, whose names are beginning to fade from the record, are listed at length. Throughout his life, Langevin sought to move beyond conflicting theories and conflicting positions, both in his science and in his personal affairs. He was generous and disinterested in personal wealth yet received considerable income from his ultrasonic patents. He was promiscuously affectionate and was loved in return, balancing conflicting relationships and remaining permanently committed to each who came close to him. His political enemies opposed his socialist views yet respected him. He was a pacifist who opposed appeasement with Hitler’s fascism. He was known as a theoretical physicist but was also a consummate experimentalist. He led his colleagues through the tangle of twentieth-century physics, quanta, relativity and uncertainty, seeking understanding with them from a detailed discussion of seemingly incompatible versions of reality. His scientific colleagues revered

12 Bensaude-Vincent B, Blondel C, Monnerie M. Les archives de Paul Langevin à l’école supérieure de physique et de chimie industrielles. La gazette des archives 145 1989 150–153. 13 https://www.esp ci.psl.eu/fr/esp ci-paris-psl/bibliotheque/centre-deressources-historiques/bibliographie 14 https://bibnum.explore.psl.eu/s/psl/item-set/249135

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him, unsuccessfully nominating him for a Nobel Prize for Physics on fifteen occasions. He himself successfully made one nomination, for his friend Albert Einstein. The book holds tiny cameos of illumination: the image of Langevin calculating with burnt matches on toilet paper in his Gestapo cell: the note that Langevin helped with military radiography during WWI: the boy Langevin, running freely with the street urchins of Montmartre: the experimental scientist, climbing to the top of Eiffel Tower to explore ion transport in unpolluted air. In such details, we find his true complex personality. Above all, it is the humanity of Paul Langevin that flows out from this account of his life. Certainly, he was one of the most talented physicists of the early twentieth century. His unique contributions to the understanding of relativity, of magnetism, of ion transport, of statistical mechanics, of ultrasound and of neutron dynamics emerge as natural actions in his story, as though anyone could have achieved what he did. And he believed that to be true. For Paul Langevin, the study of science was an integral part of being human, drawing on the senses and the intellect, an activity that recognized no boundaries set by race or nation, to be shared freely for the improvement of the world. Attempts to monopolize the outcomes of scientific discovery, whether by nation states or by large capital enterprises, were anathema to him. He drew his inspiration from Pierre Curie and went on to inspire a generation of physicists in France and beyond its borders, whose deep love and respect for the man they called ‘Boss’ shines from these pages. He was a brilliant teacher, devoted to the principle of education for all, irrespective of wealth or gender, to allow all to contribute to the rational development of the human race. But he felt a deeper commitment to all men, and his engagement against injustice and for freedom placed him in the danger that is still experienced by some scientists today who publicly challenge governments. His natural antipathy to war as a means to resolve conflict was set aside in the face of the threat of fascism. Langevin’s life story can still teach us great deal about what it is to be human at the highest level, to retain humanity in the uses of science, to fight against inequality, and to believe in the fundamental truth that rational thought remains the best defense against falsehood and exploitation. I would like to express my sincere thanks to those members of the Langevin family who have kindly agreed to this republication of André Langevin’s book in English. In particular, I acknowledge gratefully the enthusiasm and guidance of Paul Langevin’s grandson, Paul-Éric Langevin, without whom this translation may not have succeeded and Yves Langevin, André Langevin’s grandson, for his support and comments. I would also like to record my personal thanks to Bernadette Bensaude-Vincent for her kind guidance as I started to learn more about Paul Langevin, Catherine Kounelis, Chef de Service of the Bibliothèque et Centre de ressources historiques at

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ESPCI, for her assistance in access to archives, Jacques Lewiner, Stèphane Holé and Brigitte Leridon from ESPCI for their kind help and hospitality and Martha Bustamante De La Ossa who guided me to French sources of which I had been unaware. In preparing the English text, I have retained, very largely, the style and structure used by the author. Where the text included terminology or references that would have been familiar to French readers 50 years ago, but might be unfamiliar to a contemporary English reader, I have inserted additional footnotes of explanation. These are marked with an asterisk  (*) to distinguish them from the footnotes that were added into the original text. F.D.

Introduction Why compile this collection of memories when much has already been written about my father? It is a fact that numerous publications have demonstrated Paul Langevin’s scientific, philosophical, pedagogical and political work. Apart from the volumes published abroad15, there is a book published by C.N.R.S.16, Œuvres scientifiques de Paul Langevin, which my brother Jean completed after much research17. There are also two fundamental works, one edited by our friend Paul Labérenne with the title Paul Langevin, la pensée et l’action18, with prefaces by Frédéric Joliot-Curie and Georges Cogniot. This book analyses Paul Langevin’s thinking on a number of fundamental questions in science, philosophy and education. It also succeeds in reviving the ardent struggle for justice and peace that my father waged throughout his life. The other book is by his collaborator P. Biquard19. In addition, there are all the articles and essays on educational reform from the Langevin-Wallon Commission, particularly those by our friends Madame Seclet-Riou and Georges Cogniot. One might think that almost everything important has already been said if there were not, in France at least, a book on the man that Paul Langevin was, on his behaviour in everyday life. Such a book, I thought, would not be without interest, for Paul Langevin was not only a man of science, a profound thinker, a courageous man of action, he was also a great honest man, a heart of gold, a father of admirable devotion, and at the same time an accomplished humanist.

15 O.  A.  Starosselskaya-Nikitina: Paul Langevin, State Publishing House for Physical-Mathematical Literature, Moscow 1962, Ion Ghimezan: Paul Langevin, Youth Publishing House. Bucharest 1964. 16 * Centre national de la recherche scientifique. 17 * The author omitted a yet earlier Russian biography: Yu G Geivish: Paul Langevin – Scientist, Fighter for peace and democracy. Academy of Sciences of the USSR. 1955. 18 La pensée et l’action de Paul Langevin. Texts collected and presented by Paul Labérenne. Prefaces by Frédéric Joliot-Curie and Georges Cogniot, Éditions sociales, Paris 1964. 19 Paul Langevin scientifique, éducateur, citoyen. Collection Savants du monde entier, Seghers, Paris 1969.

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Who else but one of his close friends or a member of his family could attempt to bring to life, with sufficient intensity and truth, this strong and endearing personality? This was my primary aim in writing these memoirs. The other justification for this book is the recent discovery, in the family archives, of numerous unpublished documents, some of which are extremely important, such as the letter from Dr. Paul, in which the Hitlerites tried to induce my father, who was then under house arrest in Troyes, to make his scientific knowledge available to them. I hope, by publishing a certain number of unknown documents, to contribute to a better knowledge not only of Paul Langevin and the scientists of his group, but also of the time when he lived and fought. A.L.

Paul Langevin’s origins The family As the name suggests, my family probably comes from Anjou. In this connection, I will tell you an anecdote: in 1961, during a trip to Budapest, while crossing Heroes’ Square to view the paintings in the National Gallery, we passed in front of the statues of the kings of Hungary that adorn this beautiful square. When I saw the statue of Charles I, one of the country’s great kings, I was surprised to see a striking resemblance between the face of this monarch and that of my father: the same oval shape, the same nose and... the same moustache! This resemblance can of course be explained by simple chance. But, having then explored more, I learned that this Charles I was the grandson of Charles II of Anjou, known as the Limping One. This Charles II of Anjou was himself the son of Charles of Anjou and Provence, who was King of Naples, and son of Louis VIII, brother of St Louis. The dukes of Anjou were known to have had particularly free morals, which may explain the many possibilities of “bastard” births. Whatever the case, one point is certain: our ancestors left Anjou before the 1789 Revolution. At that time, our family lived in Falaise. Despite the terrible destruction of the last war, you can still see the proud castle that, from the top of its rock, dominated the old town and where William lived in before he left to conquer England in 1066. My ancestors were registered under the name of Angevin. As usual the apostrophe disappeared from the spelling. This name, borrowed from a province, probably indicates workers who had done their Tour de France20 and then settled in Falaise. The most distant ancestor known to us is a Joseph Langevin, who settled in Falaise as a wool carder. He had a son, also named Joseph, who was born in the same town on 8 ventôse year V (26 February 1797) and who was a locksmith. 20 * The traditional journey around France doing apprenticeships with master craftsmen.

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Many Langevins lived in Falaise after the Revolution. One of them was Father G. Langevin, a priest and scholar, who lived under Napoleon I and wrote a book on the history of the town21. In due course, Joseph Langevin settled in Versailles as a craftsman. A son, my grandfather Victor-Charles Langevin, was born there on 11 April 1836.

Father My grandfather used to tell us that as a child that it was a tradition in the family for men to walk around France to learn their trade before becoming journeyman locksmiths. As Paul Langevin said in response to the speeches given at the Sorbonne during the tribute paid to him on 3 March 1945, “My father, who had to interrupt his studies in spite of own wishes at the age of eighteen, inspired in me the desire to learn”. It is true that my grandfather had a taste for intellectual work; he had succeeded in his brilliant studies at the Lycée de Versailles (his prizes, which my father kept, are evidence of this). Then at the age of eighteen, on a whim, he enlisted in the 17th regiment of the line. This was on 4 February 1854, as confirmed by his military booklet kept in the family archives. He arrived at his corps the following day and, thanks to this booklet, we can follow his military career in detail. He was appointed corporal on 16 November 1856 and took part in the Algerian campaign from 1 February 1863 to 31 January 1864. He was then transferred to the 2nd Zouaves regiment and continued the campaign until 1868, when he returned to France. He was then a sergeant-major. In 1869, shortly after his return, he suffered from the loss of his mother, born Marie-Louise-Julienne Maillet, who had lived in Sèvres. The Franco-German war broke out almost immediately afterwards. Victor Langevin joined the National Guard; he was assigned to the 3rd marching company of the 148th  Guard battalion, which was commanded by Captain Flayose. He was himself appointed lieutenant of the 2nd section. My grandfather took part in several sorties with his company. In those days, Parisians were reduced to eating anything they could get their hands on, first horses, then dogs, cats and finally rats and mice. A menu that my grandfather said he saw at the door of a restaurant during the siege illustrates this:

21 P.G.  Langevin, prêtre  : Recherches sur Falaise. Published in 1814 by Brée l’aîné, printer and bookseller, Place de la Trinité in Falaise.

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Potage Horse consommé with millet Relevés Dog liver skewers à la maître d’hôtel Sliced saddle of cat with mayonnaise sauce Entrées Braised dog shoulders and fillets in tomato sauce Cat stew with mushrooms Dog chop with peas Rat Salmis in sauce Robert Rôti Leg of dog flanked with baby mice in pepper sauce Legumes Begonias in juice Entremets Plum pudding with rum and horse marrow. On 19  January 1871, the National Guard took part in the attack on Buzenval and the Montretout redoubt. It rushed into attack, with the cry of “Long live the Republic” and occupied the redoubt after having behaved valiantly. My grandfather was bitter to see that all the acts of courage of the Guard were rendered useless by the treachery and incapacity of Trochu’s staff. The betrayal and negligence of the bourgeois government were, as we know, at the origin of the insurrection of 18 March 1871, which brought to power a true representation of the people: the Paris Commune. We have not found any precise information on my grandfather’s actions in the period following 18 March, but what we do know certainly is that he was on the side of the Communards. Paul Langevin testified to this at the ceremony already mentioned: “The fact that I believed, like so many of those who are here, that I should divide my strength between the service of science and that of justice, is certainly due to the atmosphere in which I grew up in the aftermath of the 1870 war, between a father who was a republican to the core of his soul and a mother who was devoted to the point of self-sacrifice, amidst the admirable people of Paris, of whom I have always felt so profoundly united.”

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In 1870, my grandfather met Marie Adèle Pinel at a friend’s house. Her great-uncle was the famous neurologist Philippe Pinel, born in 1745, who is credited with having substituted a reasoned and humane treatment for the violence that had been inflicted on the insane until then. This young girl was cultured and had spent a few years as a schoolteacher in an English family. He fell in love with her. They married in 1870  and came to live in Montmartre. My grandparents had three sons within a short time: − Victor Langevin, born in 1871, became a locksmith in the family tradition. Unfortunately, he died young in 1906; I did not know him well. He had two children, my cousins Paul and Jeanne Langevin. He has disappeared, she is still alive. − Paul Langevin, the second, was born on 23  January 1872, at 8 rue Ravignan in La Butte Montmartre, close to Place J.-B. Clément and next to the Place E.  Goudeau, made famous by the famous Bateau lavoir. − Finally, Jules Langevin, known as Julien, who, like his brother Paul, did not have a strong taste for studies; he became a wine and liquor merchant and succeeded brilliantly in his profession. He became a wholesale wine merchant and was elected courtier-gourmet. It is worth remembering that there are forty courtier-gourmets who are members of the Institute. To become an official courtier-gourmet, it is necessary to pass several examinations that are, of their type, very difficult. The candidate is served various wines in bottles of any shape without labels. After tasting, the candidate must state the vintage and the year. Other tests of the same nature concern liqueurs and regional dishes. Paul Langevin was indebted to his brother Julien and his dear friend Paul Montel for his perfect gastronomic training.

The Chinese uncle It was noted before that my grandmother was related to the neurologist Pinel. But she also had a brother who had a rather extraordinary life. His name was Louis Pinel. The romantic times that my great-uncle lived through go back to the beginning of colonial expansion. An expedition was sent to China in 1860. Uncle Pinel’s regiment was part of it and his adventures began. Details on this subject may be found in an article published in Le Journal Wednesday 22  July 1896, reporting a trip to Paris of Li-Hung-Tchang, Viceroy of Petchili. The headline is “Li-Hung-Tchang and the Pinel drum”: “In connection with Li-Hung-Tchang, one of our colleagues referred to a Frenchman named Pinel, a drummer in the 101st line regiment, who took part in the Chinese expedition of 1860. He remained in the country after

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the expiry of his military service, entered the Chinese army and was, after some time, appointed general by Li-Hung-Tchang himself. “The story of Pinel is reported in a curious and picturesque letter addressed in 1892 to one of our colleagues who died 3 years ago, from Mr. G. Dévéria, Consul General, who had been charged by the Minister of Foreign Affairs to accompany Li-Hung-Tchang during his trip to France.” It would be a disservice to the content of this letter to add any commentary. Here it is in extenso: “Dear Sir and friend, “Monsieur Patenôtre, Minister of France in Peking, asks me to reply on his behalf concerning the Pinel drum. “In 1860, Pinel was a drummer in the 101st line regiment, attached to the 102th line regiment, which took part in General Cousin-Montauban’s expedition to China. At the end of his military service, he was in Shanghai, where he had perfected his study of the Chinese language. Rather than demand his repatriation, to which he was entitled, Pinel asked for an audience with the famous Li-Hung-Tchang, now viceroy of Tché-Ly, but who at that time was simply commanding the Nanking volunteers he had recruited to fight the Taiping insurrection. “Pinel knelt down with great grace before Li-Hung-Tchang and asked for the honour of serving him without any conditions. A Westerner at the feet of a Chinese! This had never been seen before! Li-Hung-Tchang was flattered, gave the rank of sergeant to Pinel, who, with unfailing bravery, soon became a general. “After the end of the Taiping insurrection, Pinel took part in an expedition against the Setchouan Muslims. In 1870, he followed Li-Hung-Tchang to the north and was appointed imperial commissioner to draw up an enquiry after the massacre of the French at the port of Tien-Tsin (1870). “On his way to this place, Pinel was interviewed by a missionary friend of mine. It was believed at that time that France would retaliate against the Chinese. Asked what he would do in the event of a war against us, Pinel replied that he would stay on the Chinese side. He could hardly do otherwise. As a general commanding the artillery, he received a fairly large salary, but he was obliged to invest his savings in the purchase of buildings that remained in Chinese hands, which were proof of his loyalty. He had successively married three Chinese women and still spoke sufficient French. You don’t forget your language in 11 years. “One day during an official reception with Li-Hung-Tchang, he was among the red-bloodied officials who formed a barrier as I passed through the Yamen courtyards. At least that was what I was told. I could not distinguish this strange character, who had perhaps become more Chinese than the Chinese. It appears that this ease of assimilation is particular to the French.

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“The war of 1870 broke out; our difficulties with China had to be settled peacefully from then on. The Chinese government decided to send an ambassador to France to make us accept the apologies of the Son of Heaven. “The choice of this ambassador was not easy. “The Chinese wanted him to be “persona grata” with us, on the other hand, the high Chinese officials, more ignorant than today, still dreaded visits from Europe. One day, at the Council of Foreign Affairs in Peking, in an interview with the Count de Rochechouart and me, Prince Kong suddenly said to us: We have made a choice which seems to please you. We have a very distinguished man in our service who will make a very nice ambassador for you, especially as he is French: his name is General Pi-nei-eurl (Pinel). “He is a vulgar drummer! cried Rochechouart. Do you think that it is with “rra” and “fla” that you are going to satisfy us? If this is your admiration for a simple “hustler”, what do you think of the officers and generals who commanded him? “Prince Kong did not insist. This was in September 1870. “Since then, I have not heard from Pinel. Did he die in his Chinese bed like a simple bourgeois from the land of flowers, entrusting his soul to Buddha, whose cult he had consented to practice? “Or, still alive, was he among our adversaries in Langson, Bac-Ninh, TuyenQuang? I don’t know. During the siege of this last place, in the frightful hand-to-hand combat which took place at night on a breach which had just been opened by a mine, a Chinese man was heard shouting as he withdrew: “There will be tomorrow!” These words were spoken in the purest French. “Sorry for the disjointedness of this account, or rather of these historical facts. “Tout á vous. G. Devéria” “Mr. Devéria could have added that Pinel was introduced in 1879 to M. de Montmorand, our minister in Peking, the predecessor of M. Patenôtre. One last piece of information: Pinel, now between sixty-three and 65 years of age, lives very quietly in Peking: his infirmities would have prevented him from taking part in the war against Japan. Signed: Fabrice” In spite of the very unpleasant tone taken by these pen-pushers when they deign to speak of a simple “hustler”, it can be understood why Paul Langevin carefully preserved this newspaper cutting in his archives. As for the date of Pinel’s death, we were also able to find details, thanks to a letter addressed to my grandmother by M.L. Dunoyer de Segonzac on 1st April 1889. Here is the text:

Paul Langevin’s origins

“Chinese Mission No. 4343 G.

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100, avenue Victor Hugo Paris, 1 April 1889

“Madam, “I am very saddened to learn from your letter that M. Frandin has confirmed the news that I had given you as probable, of the death of General Pinel. Please accept my condolences. “Although I did not know your brother, I can say from everything that was repeated to me in China that he was very much appreciated by the Viceroy of Tien-Tsin, his Excellency Li-Tchoug-Tang, to whom he was attached. “I was unable, for lack of precise information, to provide you with any other information than that which I had already given you. That is why I referred you to M. Frandin, who, having recently returned from China, could enlighten you better than I could myself... “Please accept, Madam, a renewed expression of my condolences. Signed: L. Dunoyer de Segonzac” Finally, a word about the place where my great-uncle was buried, which my father was very keen to know, for he had always felt a great interest in this unique man who became a general in the Chinese army. He was only able to find this out much later, in 1931, when he was sent to China by the League of Nations to study the reorganisation of education. He put this question to the French Legation in Peking and received this answer: “French Legation in China Peking, 17 November 1931 “Dear Sir, the tomb of Pinel, who died in the service of China, is situated at Fong-houang-tai, in the south of the district of Hou-ngan, in the province of Ngan-Houei. A temple known as Tchongmiao guards his headstone at Hu-Tchéou, some 180 li away and about 20 kilometres from Lake Tchao.22 “As you know, the expeditionary corps of 1860 was, on its return to France, detained at Shanghai for some time on account of the disorders which then ravaged the Yangtze valley. Several officers and men, especially of the artillery, joined the detachment that helped the imperial government to defeat the rebels. Perhaps Pinel was one of them. “The legation does not possess in its archives any document concerning Pinel; but I would be surprised if, armed with the above information, our Consulate General in Shanghai, which at the time was directly involved in these affairs, had not kept track of the passage of your relative. “Please accept, dear Sir, my best wishes. Signed: Lagarde” 22

One li corresponds roughly to 576 metres.

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My grandmother was very fond of her brother and my father admired him, often speaking in the family about this extraordinary uncle. For a while after 1931, he had intended to follow in his footsteps by moving permanently to China with his whole family. Unfortunately, I did not know my grandmother, who died of cancer on 13 September 1902, when I was only one year old. Paul Langevin found it very difficult to recover from the great pain that he felt from this loss. A year later, on 9 September 1903, he wrote a touching letter from Germany to his father, in which he recalled what he owed to his mother: “I often think of you and my beloved mother, and I work with more courage when I remember the long good days of work that I spent with her. I was so happy to work near her and I am so aware that it is to her that I owe the pleasure of working.”

Paul Langevin’s birth Paul Langevin was born in Montmartre on 23 January 1872, in the aftermath of the Commune: “As eyewitnesses of the siege and the bloody repression of the Commune, my parents, through their writings, instilled in my heart a horror of violence and a passionate desire for social justice.” It is known what Zola, Camille Pelletan and Lissagaray had to say about the atrocity of the Versailles repression. My grandfather was a Communard sympathiser. In spite of this, he had the incredibly good fortune to escape repression and denunciation. Paul Langevin was born into this atmosphere of hatred against the Versailles executioners. 8 rue Ravignan was a beautiful but very simple old house, with its small rectangular single door, with a curious “bull’s eye” window above it. The large-paned windows of the apartments were each surmounted by an elegant and very sober coat of arms23. My grandparents did not stay long in this house after my father was born. Less than a year later, they moved to a small house at the top of the Montmartre hill, 3 rue Saint-Vincent. Unfortunately, this house has long since been demolished. An extraordinary panorama could be seen from its windows. The view extended not only over the northern slope of the hill but also over the whole of the northern suburbs to the plains of St-Ouen, St-Denis and Aubervilliers.

23 * The five-story building now houses a small bar on the ground floor. The bull’s eye window and decorative triangular window pediments still remain, each with its small coat of arms.

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As is known, my father was always a secular and anticlerical activist. Ironically, the site of this house was used to build the restaurant and catholic centre of Sacré-Coeur, which depend on the Action catholique française. By a curious coincidence, the main building of this organisation lies on the rue du Chevalier de la Barre at number 4024. But, during my father’s early childhood, the Sacré-Coeur was not built, and the hill remained very rural. There were only small houses with gardens, of the kind that still exist in some corners of old Montmartre. The Sacré-Coeur construction sites and almost the whole of la Butte Montmartre were a real paradise for children. Thanks to the absence of traffic, it was one of the few places in Paris where they could play safely in the street. And what a great time the little “poulbots” (street urchins) had, sliding down the iron railings of the stairs on the slopes of the hill! It is only natural that my father should have retained a very pleasant memory of his childhood in Montmartre, and it is easy to understand why it was always a pleasure for him to remember this happy period. When he was in a good mood, to put us in the atmosphere of those old days, he would sing one of his favourite songs, which has been recently revived by Germaine Montero25: A Montmerte Malgré qu j’soye un roturier, Le dernier des fils d’un Poirier D’la rue Berthe, Depuis les temps les plus anciens Nous habitons, moi’s et les miens A Montmerte! En mil huit cent soixante et dix Mon papa qu’adorait l’trois-six Et la verte, Est mort à quarante et sept ans, C’qui fait qu’il repose depuis longtemps A Montmerte! Deux ou trois ans après, je fis C’qui peut s’app’ler pour un bon fils 24 * Rue du Chevalier-de-La-Barre is named after Jean-François Lefèvre, Chevalier de La Barre (1746–1766), a young man from Amiens sentenced by civil justice to torture and death by beheading, in 1766, for various blasphemies. 25 * Germaine Montero  (1909–2000) was a French singer and a stage, television and film actress.

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Une rude perte. Un soir su’ l’ Boul’vard Rochechouart Ma pauv’ maman s’est laissé choir A Montmerte! Je n’fus pas très heureux depuis. J’ai ben souvent passé mes nuits Sans couverte, Et ben souvent, quand j’avais faim, J’ai pas toujours mangé du pain A Montmerte! Mais on était chouette en c’temps-là. On ne sacrécœurait pas sur la Butte déserte, Et j’faisais la cour à Nini, Nini qui voulait faire son nid A Montmerte, etc. But Paul Langevin’s life as a free spirit would soon come to an end.

Paul Langevin’s youth and studies Paul Langevin’s youth was essentially studious. From an early age, he showed a passion for learning and above all for understanding. He had a republican and anticlerical upbringing. I have material proof of this, since I found in my father’s books the “Republican Catechism”, an illustrated work which deals with the evolution of worlds, species and man, and whose author, the engineer Henri Arnould, summed up his entire doctrine of faith in science and solidarity in this appeal: “Forward for Humanity, France and the Social Republic!” My father’s parents had him start his studies at the age of four in a small boarding school very close to their home, probably in Rue des Saules. He stayed there until about 1879. My grandparents always had a hard time. Paul Langevin and his brothers usually had dry bread as a snack on their way home from school. But, despite their dry bread, my father and my uncles never used the trick my grandfather used in his youth and which he liked to tell us about. My grandfather’s parents lived in Versailles at that time and, on his way home from school, the child would pass a small grocer’s shop. There was a barrel of molasses in front of the shop. My grandfather invented the following trick. When he passed the shop and saw that the grocer was busy with a customer inside, he would deliberately

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throw his toast into the barrel and come in whining: “My toast slipped out of my hand and fell into your molasses barrel, I have nothing left for my snack, would you be so kind as to fish it out?” Of course, the trick didn’t work every time, but if the grocer was in a good mood, he would fish it out instead of kicking the boy out of the house. My father, always very proud, would have preferred not to eat than to feel like a beggar. In 1880, the family left Montmartre to settle in Pré-Saint-Gervais, at 57 Grande-Rue. Then in 1882, my grandparents moved again to live in the 15th arrondissement. Paul attended primary school in the rue Falguière until he was eleven. He passed his school certificate on 10 July 1883. Once the child had been noticed by his teachers, they advised my grandparents to have him continue his studies. It was then that his mother, an admirable woman, agreed to make the great sacrifices that were necessary, despite the financial difficulties in which the family found itself at the time. Paul Langevin passed the entrance exam to the École primaire supérieure Lavoisier with flying colours in 1883 and continued his studies there until the age of sixteen. He passed the Certificat d’études primaires supérieures on 28 October 1886 and, a year later, on 11 October 1887, he passed the Brevet élémentaire, thus already securing a possible career as a teacher. In the same year, 1887, my father was ranked among the best students of the 4th year. He was invited by the school to go on a tourist trip to Brittany from 1 to 12 September (this was the twelfth such trip that the school had organised). Paul Langevin was delighted with this wonderful excursion, as he had never seen the sea before. However, disappointment first awaited him: he found the eleven-hour journey between Paris and Saint-Brieuc rather long, as reported in the account of his trip which the headmaster of the Lycée Lavoisier kindly made available. But, shortly before arrival, the teenager’s enthusiasm was renewed: “With shouts of joy, about six in the evening, we welcomed our first sight of the sea; it was the cove of Yffiniac, close the route before arriving at Saint-Brieuc.” Paul Langevin’s teachers at Lavoisier advised him to prepare for the competitive examination for the École de physique et chimie industrielle de la Ville de Paris, (ESPCI) which had recently been formed, in 1882. After only one year of preparation, he was awarded first place in July 1888. In August of the same year, my father was chosen to take part in the thirteenth trip of the Lavoisier school, which took place this time in the Jura. This was his first contact with mountains, and he retained an unforgettable memory of this excursion. In La Chaux-de-Fonds, my father bought a watch, with the shape of a large onion, which he kept all his life and which he always liked to show us.

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In the School of Physics and Chemistry Paul Langevin was consistently ranked at the top of his class at ESPCI. The premises where he continued his studies were historic but dilapidated. The school was then housed in the Saint-Michel convent for girls at 42 rue Lhomond (formerly rue des Postes). The Saint-Michel girls were neighbours of the nuns of the Notre-Dame: their chapel was that of the Collège Rollin. The remains were converted in 1893 for the installation of a laboratory known as the 4th year, which was later demolished. The 1888 ESPCI was also close to the orphanage for young girls built on the ruelle des Vignes (now rue Rataud) and run by the Dames de Saint-Thomas de Villeneuve. This relic of the eighteenth century was still intact at the time when Paul Langevin was studying; the same religious order continued to live there, and its beautiful orchard was still in place, when I myself was a student at the same school in 1920, with its apple trees strung on ropes. The Dames de Saint-Thomas de Villeneuve withdrew in 1926 leaving the place vacant for the extension of the chemistry laboratories at the school. My father was the very active designer of this extension. Paul Schützenberger, the first director of ESPCI, “Father Schütz” as Paul Langevin used to call him, was still in office in 1888 when he entered the old walls of this secular house. It was in these shabby-looking buildings, in these more or less ruined sheds, that Paul Langevin pursued his engineering studies and received a solid scientific training. Pierre and Marie Curie, with the help of Gustave Bémont, were to make them famous a few years later with the discovery of radium. His time at Lavoisier, and subsequently at ESPCI, was one of Langevin’s happiest periods of his life. It was here that he met the excellent teachers who awakened his vocation as a physicist. At Lavoisier, it was Victor Urbain, professor of physics and chemistry, who steered him towards physics, deciding his entire career. Moreover, it was there that he met one of his best friends, Georges Urbain, the son of this professor, and his companion during the trip to the Jura in 1888. Paul Langevin said at the time of Georges Urbain’s anniversary: “Since that time, our two lives have been very close. We were 2 years apart when you joined me in Rue Lhomond and you left just as I was entering the École normale. “For five laborious years, from 1888 to 1893, we had a lot of influence on each other within a small group of students that your brother Édouard animated with his energy and his gaiety. We used to talk a lot during our evening walks in the Latin Quarter or along the endless paths of the Fontainebleau Forest. I already admired the richness and diversity of your natural gifts, which your father’s example and influence helped

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to develop in both science and art. You were writing verses at the time, painting, sculpting and composing music, as you have always continued to do with the extraordinary activity that makes you find rest only in a change of creative effort. Your first attempt at sculpture was a bust of your first teacher, your father, thus preparing you to express, by fixing their features, your gratitude to Schützenberger, to Friedel, to those who have exerted on your mind and on your career the most profound influence. And if the memory of your father is linked to your beginnings as a sculptor, that of your wife is linked to your passion for music, which I am grateful to you for having taught me to love...” Georges Urbain, at the beginning of the century, sculpted a bust of Paul Langevin with a more than faithful talent, a bust that has always remained on the mantelpiece of the dining room in my father’s flat in rue Vauquelin. Then, when Paul Langevin was elected to the Académie des sciences in 1934, he struck a commemorative medal for him. It is worth repeating that it was under Georges Urbain’s influence that my father developed a taste for classical music and the enjoyment of attending great concerts. Georges Urbain’s memoirs provide information on the birth of the two scholars’ friendship and gives us picturesque details of their life as students: “At the École de Physique, Langevin had a nickname; he was called the “Canaque” 26, because he had long hair, going to the right and to the left, rebellious to any discipline, which made him look like a savage. Later, at the École normale, he was called the “Cacique” 27. I always thought that calling him that must have suited him very well. We had met at the École; Langevin was more advanced than I; he was a studious pupil...” Then Urbain spoke about his friend’s career: “... Langevin has been particularly successful: he is the soul of modern physics. Physics is a science of abstraction, and it takes an intelligence like Langevin’s to have overcome all the difficulties of this very abstract science. Langevin’s most important scientific work is his theory of magnetism. He is to this field what Ampère was to electricity. Langevin’s theory has stood the test of time without the need to modify its principles... “Langevin is a Humanist, with a capital H. “Langevin’s politics is the politics of the heart. When he asks himself what he should do, he looks for the generous gesture, the chivalrous gesture to

26 27

* Kanak – Native of New Caledonia. * Tribal chief of the Taino people of the Bahamas and Antilles islands.

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make, he hardly hesitates, and he always makes the best gesture. What matters to him is that the gesture is generous...” When he entered the old buildings of the ESPCI, Paul Langevin was not bothered by the lack of comfort. He came from a working-class family and was not used to luxury. He might also have complained that the teaching staff was small: there were eight professors in all (two in mathematics, three in physics, three in chemistry) and four assistants or chefs de traveaux, including Pierre Curie, who was 29 years old. If Paul Langevin began to work with passion, it was, as he himself said, because he had the great good fortune to have Pierre Curie as his teacher, who made him enthusiastic about scientific work. Let’s listen to my father: “Pierre Curie was at that time barely older than the students, comrade and friend as much as teacher, and to whom he was able immediately to relate, as much by his science as by his kindness.” Paul Langevin always kept a moving memory of this physics laboratory, a half-ruined shed, where Pierre Curie took care of the students alone, helped, just for the practical part of the work, by his laboratory boy, a man named Petit, whom he had brought with him from the Sorbonne and from whom he was never to be separated. Paul Langevin wrote in an obituary on Pierre Curie28, published in 1906: “It is difficult to express how his pupils remunerated his feelings of affectionate admiration, his attachment to the school and his devotion. He was 29 years old when I myself became a pupil: the mastery acquired from 10 years spent entirely in the laboratory transferred to us, in spite of our ignorance, through the confidence of his gestures and his explanations, through the ease, tinged with timidity, of his approach. We returned with enthusiasm to the laboratory, where it was good to work close to him, because we felt him working close to us, in the large, bright room filled with apparatus that was still a little mysterious, where we were not afraid to consult him often, where he also sometimes let us see a particularly delicate manipulation. Perhaps the best memories of my school years are of the moments spent there, standing in front of the blackboard, where he took pleasure in chatting with us, awakening in us some fruitful ideas and talking about the work that created our taste for the things of science. His lively and communicative curiosity, the breadth and reliability of his information, made him an admirable awakener of minds... “Pierre Curie was young, he was perhaps closer to us than most of the other teachers at the school and he had a need to express, to externalise 28

Paul Langevin: Pierre Curie. «Revue du mois», No. 7, 10 July 1906, pp. 5-37.

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in a way, his love for science which made him call us to him to explain to us the new things that excited him. I remember him standing in the room where, at that very moment, he was continuing his work on piezoelectricity with his brother Jacques and explaining to us Van der Waals’ ideas on the compressibility of gases. This was in 1889. A little later he did me the great honour of a modest collaboration in his work. I was in my third year when he was pursuing his studies on damped motions and the possibility of representing their laws in a general way. He asked me and my classmate Planzol29 to help him with his numerical calculations. I was extremely proud of this. I used an old Thomas de Colmar calculating machine, which still exists at the school and which I had the affection to use again for my personal work (1931–32)30. This machine has lost a few teeth. Its workings are not as new as they used to be and the results it gives are not very reliable... “This first initiation into creative science, which this tiny collaboration with Pierre Curie gave me, was perhaps the event that triggered in me the desire to also participate in the scientific effort.” But there was also another extraordinary personality who exerted a certain influence on Paul Langevin: this was Gustave Bémont, whom the students had nicknamed “Bichro” because of his red beard, the colour of sodium bichromate. This very curious man, head of Étard’s laboratory (professor of general chemistry), was a passionate chemist. A great worker and bachelor, he also spent all his time in the laboratory. It was he who helped Pierre and Marie Curie separate radium from the other neighbouring mineral elements that accompanied it in pitchblende by fractional crystallisation. Paul Langevin also had fond memories of the lessons given by another teacher: Schützenberger, “who really”, he said, “illuminated chemistry for me, organic chemistry in particular. He knew how to create enthusiasm around him... A large part of my love for science, chemistry in particular, I owe to Schützenberger.” It is understandable why Paul Langevin remained so passionately attached to the School of Physics and Chemistry for the rest of his life. He graduated from the school in 1891 having been consistently top of his class.

29 Planzol, one of P. Langevin’s intimates, was a good specialist in accumulators as well as in electrical equipment for railways at the Compagnie du Nord. He died in 1935. 30 * Charles Xavier Thomas invented his arithmometer in 1820, the first commercially-available calculating machine.

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Students at the École normale Paul Langevin wanted to complete his scientific education. This is how he did it according to his own account: “I had to find a way to continue my studies by giving lessons31. One of the heads of the school, the one we called “Father Fink”, an excellent man of Alsatian origin who was introduced by Schützenberger, provided me with this opportunity by asking me to go every day to spend two hours at his place to help his boarders solve their problems. They were preparing for various competitive exams, one for the École Centrale, another for the Institut Agronomique, a third for the Saint-Cyr competitive exam, etc. “For 2 years I went every evening from five to seven to “Father” Fink’s house, where I was bombarded as soon as I arrived with problem statements. I got into the habit of solving them quickly, and this had an influence on my scientific career and on the ease with which I later passed the dreaded exams for which I was preparing... “I had already passed (first) the physics degree (in November 1892) and was preparing for the mathematics degree when the influence of the School of Physics and Chemistry returned to influence my career. “Passing along rue Lhomond one day in February 1893, I met Dommer (engineer of arts and manufacturing), a professor of industrial physics, one of the teachers that people of my generation had at school. Dommer asked me in a very friendly way what I was doing. I told him where I had got to and of my desire to teach. Why”, he said, “don’t you go to the École normale?” I replied that this was far beyond my ambition, that one needed Latin and mining32 to get in and that I had done neither. “Perhaps you could try”, he said.

31 Although Paul Langevin needed to teach lessons to live, he only wanted to teach students who could benefit from them, as his basic honesty would not allow him to accept money from the parents of a hopeless student. We have direct proof of this in the book called Noces d’or avec mon passé by Charles Oulmont, brought to our attention by Miss Marie Cérati, who we thank here. Here is the passage: “I was sure that mathematics was a world forever forbidden to me. Yet my father had entrusted me to a scientist who was to become famous: Paul Langevin. I can see him again with his Rostand-style collar, his bib tie, in dark suit, and I remember his deep black eyes. After a few lessons, he said to my father: “I feel, Sir, that I am stealing your money. I don’t know why, but your son doesn’t understand what I’m explaining to him. It seems to me to be clear.” It certainly was, but I wrapped his luminous explanations in a poetic haze. 32 * The original text has ‘et du taupin’.

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“This suggestion made up my mind. In February I started to do Latin for two hours a day. I gave two hours of lessons at Fink’s house and sometimes another two hours. “It was a good time in my life. I had a few months of continuous tension, of really mobilising my underlying strength. But in the end, I was able to succeed.” As we have just seen, from February 1893 onwards, Paul Langevin worked intensively to complete his literary and scientific knowledge and thus manage in four months, in all subjects, to bring himself up to the level of the entrance examination to the École normale supérieure. In this short time, he managed to learn enough Latin to obtain an honourable mark (12 out of 20) in Latin translation, a compulsory subject, and enough special mathematics and physics to be awarded first place in the entrance examination in July– August 1893. Jacques Nicolle recalled this stage in my father’s life: “A very young candidate in a black frock coat, holding a top hat in his hand, entered the examination room. These were the only “best” clothes he owned and which he wore every evening to his “luxury pupils”. The supervisor33 was astonished by the youthfulness of the newcomer and from time to time he would come and have a look at the script. After a while, he said to a member of the panel: “He’s not doing badly, that boy”. And indeed, “the boy” did so well that he passed all the tests with flying colours.” If one wants to get an idea of how Paul Langevin performed orally, one must refer to Aimé Cotton’s recollections34: “It was in 1893 and the examiner in physics was our excellent master, Marcel Brillouin. Montel, from the same class as Langevin, recently asked Brillouin in front of me if he remembered the examination of his former “cacique”. Brillouin replied that when Langevin had entered the competition, he had been told that he would probably fail, since he had never taken the special mathematics classes. But Brillouin remembered above all that he had tried in vain, at the oral examination, to see where this candidate’s knowledge would end. Jules Tannery had made the same observation when questioning our fellow student in mathematics...” During the time he was preparing for his competitive examination, Paul Langevin was not content only to give paid lessons at Father Fink’s for a

33 This supervisor was in fact Jules Tannery himself. 34 Aimé Cotton: L’œuvre scientifique de Paul Langevin, «  La Pensée  », n°  12, May–June 1947, p. 21.

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living, he also found time to give free lessons in the evenings; here are the details that he himself provided: “At that time there were evening classes at the School of Physics and Chemistry in the electricians’ section of l’Association philotechnique. When I graduated from that school, I was 19 years old. I was asked to give the electricity course in that section. The people who offered it to me were unwise because I had never taught before, and I accepted with some audacity. This was my first experience in teaching. I immediately enjoyed it very much. “I continued for several years and when, having become more confident, I carried out more experiments, I felt the need to have an assistant; it was again the School of Physics and Chemistry that provided me with one in the person of Gratzmüller I remained very grateful for his help, which was as free as my teaching. This was before I entered the École normale supérieure: I was admitted in 1893 but only entered in 1894 after my military service.” Indeed, before entering the École normale, Paul Langevin did a year’s military service in the infantry at Sens. He entered a platoon of student corporals and finished his service as a sergeant. Paul Langevin adapted easily to the life of a soldier. He always told me that he had relatively good memories of his military service. He had recently spent most of his nights working hard, so he felt a certain relief at being able to sleep peacefully. It was only, he told us, from a sprinkling of cold water, from the classic joke of a quarter of a litre of water suspended and tipped over the sleeper’s head, that succeeded in waking him up. My father took advantage of this period to take a complete intellectual rest, which would be beneficial for his further studies. In October 1894 Paul Langevin entered the École normale supérieure. One can imagine the feelings of this child of the people as he crossed the threshold of this place, which had been difficult to enter, haunted by the memory of the great Pasteur. Naturally he was soon noticed as he was a “cacique” of his class. The general supervisor of the School, Paul Dupuy, wrote about Paul Langevin for La Pensée immediately after my father’s death: “In 1894, I saw him arrive at the École normale, at the head of a promising class since, with Paul Langevin, there were Henri Béghin, Henri Bénard, Noël Bernard, Henri Lebesgue and Paul Montel. “He was both their “cacique” and their elder, for he was already 22 years old. He was one of those recruits that the Normal School, especially the Science School, has increased in number, from the primary and higher schools. Many of them came to us with something of a rough edge, and it was at the school that they took on the appearance of men of science. Langevin, for his part, already had it. Gravity mingled with youth in his

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face, where his beautiful deep eyes shone. His whole person exuded a distinction for which he had only himself to thank. “As the “Cacique” of his class, and later the “Cacique” of the General Science Department, he naturally had a closer and more frequent relationship with me than any of his fellow students. There is no “cacique” general who understood better than he did the spirit in which I perceived my own role as general supervisor... “... With Langevin representing these young people the game was played without any difficulty. “I lived then, not above, but alongside the students, during the years in which my comradeship as a great elder gave me the richest pleasures of my life. That was in 1894–1895, the year of the centenary which was prepared in close collaboration with them and when Langevin was a third-year student. He was an agrégé préparateur, during the years of the Dreyfus affair, when, thanks to Herr, the École normale was one of the most vigorous centres in the struggle against lies and iniquity. I do not need to say what part Langevin played in this struggle; he is one of those who, throughout his life, have remained Dreyfus supporters, but I have no particular memory of this period which relates to him. He, on the other hand, had kept the memories of my action at the time and which, in 1944, at their first meeting, made him embrace the author of two books, one in defence of Picquart, the other in defence of Dreyfus35...” Paul Langevin settled as a boarder in his student room and got to know his classmates, among whom was Paul Montel, a mathematician of great importance who has since become a member of the Institute, and who distinguished himself by his work on the theory of functions of complex variables and in the field of finite geometry; Noël Bernard, a botanist of the first order, who discovered the secret of orchid germination and showed the necessary association of a fungus with orchid seeds; Henri Bénard, who was later professor of experimental fluid mechanics at the Faculty of Poitiers and who became famous for the development of the theory of vortices in liquids; Beghin, who was also a professor of fluid mechanics, but at the Sorbonne; Henri Lebesgue, one of our greatest mathematicians. He died in 1941 and revised the old notions of integration. He replaced the integral defined by Cauchy and the more general one proposed by Riemann with the integral that bears his name. This work contributed greatly to the establishment of the theory of functions. Lebesgue was later appointed professor at the Sorbonne, then elected to the Collège de France and made a member of the Institute. Alongside these were others less well known, such as Angelloz-Pessey, who was a professor of mathematics at the Lycée Buffon, Cambefort, Foulon, Pierre 35

Paul Dupuy: Souvenirs. « La Pensée », No. 12 (May–June 1947), p. 63.

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Massoulier, who ended his career as Inspector General of Secondary Education in physics and chemistry, Meynier, Patte, who was a professor of physics at the Lycée Charlemagne in Paris, and Jules Renaud. In 1945, when we celebrated my father’s seventy-third birthday, we read the picturesque and amusing message of the “archicubes” 36 of the École normale, signed by Louis Blaringhem: “Around 1900, Jean Perrin, Paul Langevin and Noël Bernard were the heroes of a curious company, the “Jus des Archicubes”, of which I was, by the grace of the gods, the conscript in charge of preparing the coffee... From the lair of the Méga 37, where Lucien Herr and Matruchot, encyclopaedists, were on the lookout for sensational discoveries, the company emigrated, established its headquarters at la Nature38, where Noël Bernard, with his pious assistant, received illustrious guests. Illustrious because the discoveries announced at the Institut de France were rewarded with a bottle of three-star cognac from the authors. In winter, in the Skeleton Room, in summer on the edge of the pool, discussions and collaborations were initiated with cordial and sincere enthusiasm. “Matter was thus in vibration, the tangible cohesion of an infinitely small world whose vortices, like the solar worlds, solved the enigmas of the universe, the small ones moving at the speed of light, the large ions in slow motion. What astonishment for the painters to learn that the blue of the sky was the proof of this discontinuity! And Noël Bernard did not hesitate to affirm that the mysterious life of the splendid orchids, that the tuberisation of the potato was the consequence of a tragic struggle between thousands of fragile embryos and a mycelium of parasitic fungus. “These mutations in space and time [led] to heredity, to equilibrium, to beauty. Paul Langevin, the calm hero of the trinity, translated into precise terms the objects, causes and effects...39 ” On the literary side, my father’s classmates were Charles Péguy and Étienne Burnet, who had a talent for writing that was highly appreciated by Paul Langevin; he came to visit my father on each of his trips to Paris. But among the literati, there were also Albert Mathiez, Julien Luchaire, who was one of the directors of l’Institut de coopération intellectuelle. Marie Roques, who was a professor at the Collège de France, Felicien Challaye, a man of the left who turned to total pacifism and thus played into Hitler’s hands, Paul Léon, a very amiable man, who was director of the Beaux-Arts

36 * “Cube” was the term for a third-year student. 37 The megaphone and acoustic equipment room. 38 The natural sciences office. 39 Extract from the tribute to Paul Langevin on his 73rd birthday in Les cahiers de l’Université libre, 3 U.F.U. p. 71.

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for many years, and Léon Bloch, better known as a physicist than as a literary scholar, because after his literary studies, he wrote a doctoral thesis in physics on the ionisation of gases, a fact which is quite rare in the annals of the École normale. Paul Langevin became friends with many of his classmates, but he became more deeply attached to Paul Montel, Henri Lebesgue, Patte, Étienne Burnet, Massoulier, Paul Léon and Noël Bernard, whose life ended prematurely by tuberculosis.

FIGURE 1. Student at the École nationale supérieure with the physicists of his class (1895–96). © Bibliothèque de l’ESPCI.

The affectionate concern of my father and his friend Jean Perrin for the botanist Noël Bernard, who was in very poor health and died at the age of 38 in 1911, is evident in this undated letter from Perrin to Paul Langevin: “My dear Paul, “Forgive my delay in explaining my dispatch to you. Bernard had just written to me that he would be very glad to see you and me but that, at the moment, he has severe haemoptysis and might pay with half a pint of blood for the pleasure of exchanging a few words with us, and that it is better to wait for a slight improvement. So perhaps we should just shake his hand. If we bothered to do so, it might hit him hard. There would have to be a “pretext” for “passing” through Poitiers. Let’s say something like the following: organisation of meteorological apparatus, ion collector, for example at the Observatoire du Puy-de-Dôme (if Poitiers is on the way) or something similar. Under these conditions, he would understand the

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passage and would only retain the pleasure of seeing you for a moment. You yourself would have the satisfaction of having seen him once again and perhaps, despite everything, you would come across a better passing situation that would allow us to talk a little. All precautions taken, it seems to us that it would be better for you to pass by like this (of course, you would have to warn him by giving the pretext, not forgetting that Bernard is extremely intelligent, and that you have to take the trouble to make him not be suspicious and to give him a bit of gentleness without running an extra risk)...” My father was an intimate friend of Paul Montel and Henri Lebesgue. Not only did he see them often, but he chose them as witnesses at my wedding in 1925. This young “freshman”, who had matriculated nineteenth at the refectory (the “Normaliens” called him “le Pot”), had already, unlike his fellow students, a scientific and technical training which he had gained at the ESPCI. However, his stay at the École normale was beneficial to him above all because it gave him the opportunity to make contact with the elite of scientists and literary figures around him. There he met many of the older Normaliens. These are those who were his closest friends and who had the greatest influence on him: first of all, Jean Perrin, who was his assistant on the rue d’Ulm and with whom he collaborated when he was still a third-year student (in 1896) on a study of the ionisation of gases due to X-rays. They became inseparable friends. Aimé Cotton, a physicist of great renown, discovered the magnetic birefringence of colloids and then that of pure liquids. He is also well known as the creator of the great Bellevue electromagnet, which was unique in the world at the time (1928). Aimé Cotton, who I knew well, was a very simple man, almost painfully shy, very friendly, and Paul Langevin always remembered the charming welcome he had given him at the École normale. I should also mention Charles Maurain, who had a very successful career as a globe-trotting physicist, a man as kind as he was learned, who died recently; I met him not long ago in Antony, at the home of my children Michel and Hélène. These are the great friends of Paul Langevin who belonged to the class of 1890. From the previous class, Paul Langevin was very fond of Émile Borel. In 1894 Borel introduced the new concept of set metrics, the basis of set theory, and who made a very important contribution to the calculation of probabilities. I knew Émile Borel, who was the son of a Cévenol pastor. He was a man of strong build; he wore a full beard, which was very black. A powerful brain, he seemed to me to be a little taciturn and when my father and I went to visit him at his beautiful property in St-Paul-des-Fonds, he did not seem to be overly interested in us children. His behaviour contrasted with that of Madame Borel, a writer known under the pseudonym of Camille Marbo; she was a very pretty woman with a cheerful character and great amiability.

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Finally, as a member of the class of 1888, Paul Langevin became close friends with Élie Cartan, one of the greatest French mathematicians. He made a very important contribution to the theory of groups, in the study of discontinuous and finite groups or Galois groups as well as continuous and finite groups or Sophus Lie groups. In geometry, he studied particularly the symmetric Rieman spaces which bear his name. To complete the list of scientists to whom Paul Langevin was attached, we must add the name of Pierre Weiss, from the same class of 1888, who was one of the world’s leading specialists in magnetism. Moreover, my father knew the physicist Henri Abraham at the École normale, class of 1886, with whom he was to collaborate in 1905 in writing a two-volume book Ions, Électrons, Corpuscules, published by Gauthier-Villars. Paul Langevin was also in touch with the ideas of the literary scholars from the École normale supérieure who were destined to great fame, especially Romain Rolland, with whom he would collaborate much later in the fight against the war in the Amsterdam-Pleyel Committee. He made friends with Foulet, an Anglican from the class of 1895. Foulet was one of my father’s close friends and often came to lunch at our house when he was in Paris.

Paul Langevin’s teachers at the École normale Paul Langevin had eminent teachers at the École normale. Among these teachers, those for whom he felt the most admiration were undoubtedly Eleuthère Mascart, Marcel Brillouin, Henri Poincaré and Jules Tannery. Mascart was not only an admired teacher for my father, but it was he who appointed Paul Langevin as his worthy successor to the chair of experimental physics at the Collège de France in 1902, when he had just completed his thesis on the ionisation of gases. My father published his obituary, shortly after the death of his teacher in 1908, in the yearbook of the Collège de France: “... With a more varied activity than his two great predecessors (Ampère and Regnault), Mascart knew how to be a scholar, a teacher, a man of action, always served by the same luminous spirit and the same energetic will. He passed, with equal ease, from the most elevated theories to the most delicate experiments, leaving results of very great theoretical scope as well as numerical determinations of high precision... “Mascart’s most important work was in optics. Here he made the most effective use of his ability as a theorist, the clarity of his mind as well as his skill as an experimenter and his great familiarity with delicate optical measurements. He carried out a series of studies on the influence of the movement of the Earth on optical phenomena, undertaken

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in response to a question posed by the Academy of Sciences in 1870 for the grand prize of mathematical sciences: “On the modifications which light undergoes as a result of the movement of the source of light and the movement of the observer”. “The problem had been raised by Arago at a time when people were looking on all sides for ways to test the two opposing theories of emission and waves. Arago had announced that if one observes two stars so placed that the Earth approaches one while receding from the other because of its translational motion, the apparent refraction undergone in a prism by the light coming from these two stars is exactly the same... Mascart remarks, by virtue of Doppler’s principle, that if the two stars emit the same light, the period of the waves received by the Earth is not the same on both sides: one receives more vibrations in the same time from the star towards which one is moving than from the one from which one is moving away, and Mascart concluded that if Arago’s experiment is correct, a difference in refraction must be observed if one replaces the two stars with two sources linked to the Earth, or, which amounts to the same thing, that a change in deviation must be observed by operating with a terrestrial source whose rays will propagate alternately in the direction of and in the opposite direction to the Earth’s movement. An experiment using only devices linked to the Earth would therefore make it possible to demonstrate a translational movement of the Earth. “Mascart set up the experiment with extreme care, following from noon to midnight the refraction of a light ray in a prism to see if the turning of the Earth, reversing the direction of the light rays in relation to the movement of translation, would bring about the slightest change in the deviation produced by the prism, and after multiple trials, he concluded in the negative: the phenomenon observed is absolutely independent of the overall movement of translation and Arago’s primitive result could not have been completely accurate. “This led to the question whether the same negative result applied to all the optical experiments using only earthbound apparatus. Mascart repeated, one after the other, all these experiments: diffraction by gratings, double refraction in Spath, rotation of the plane of polarisation by quartz, various phenomena of interference; he repeated them under conditions of extreme precision, the possible variations being extremely small, and in all cases the result was absolutely negative; he concluded the last of his Memoirs on this subject in the following manner: “The general conclusion of this Memoir would therefore be that the translational motion of the Earth has no appreciable influence on the optical phenomena produced with a terrestrial source, that these phenomena do not give us the means of appreciating the absolute motion of a body, and that relative motions are the only ones we can observe.

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“This was the first statement of what is now called the principle of relativity, in its definitive form for optical phenomena, whose perfect accuracy has been established by subsequent experiments (in particular those of the American physicist Michelson) in all fields, and whose importance appears to be increasingly great, at least equal to that of the principle of the conservation of energy. The very recent introduction of the principle of relativity into physics and mechanics already has consequences of the utmost importance, and it is essential to remember that Mascart, after a considerable experimental effort, was the first to assert its accuracy.” It was certainly this very fine work that was the main cause of Paul Langevin’s admiration for Mascart. But in order to complete his judgement of Mascart, I think it is essential to give some extracts from his impressions of Mascart’s merits as a teacher: “From 1872 onwards, Mascart’s teaching at the College led him to deal with electricity in a more complete way than he had before, more complete especially than anyone else in France at that time. In this field, French science was considerably behind. Perhaps because optics had absorbed the attention of Fresnel’s followers, the considerable progress made in electricity by Faraday, Gauss, Weber, W.  Thomson and Maxwell had remained almost unknown in France. Ampère had not made as much of a name for himself as Fresnel. The fundamental notions of electric field and potential were understood here only by a few mathematicians, Faraday’s ideas on lines of force and the role of the dielectric, taken up and developed by Maxwell, the still barely formulated conceptions of the same Maxwell on the electromagnetic theory of light, the work of Gauss, Weber, Thomson on electrical units and methods of measurement were more or less ignored. “Mascart took it upon himself, in accordance with the teaching role that the Collège de France should have, to introduce these essential ideas. It was not without opposition; he reminded me with a laugh how a famous populariser of the time had nicknamed him “Knight of Potential” and it is not necessary to go back very far to find traces of the fear that the mysterious word “potential” inspired in the students, and even in the teachers. All this has become more familiar to us now, but it is worth remembering that it was originally the work of Mascart…” Reading this, one can see that it was through Mascart’s remarkable lessons that Paul Langevin reinforced his solid training. The second of Paul Langevin’s revered teachers was undoubtedly Marcel Brillouin, who was not only a teacher but also a very dear friend. Marcel Brillouin had a great deal of respect for my father and never ceased to support him in his career, up to and including his appointment as President of the Solvay Councils.

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Paul Langevin spoke highly of Marcel Brillouin in an address he gave at the ceremony for his teacher’s scientific anniversary on 17 December 1935 at the Collège de France: “... The extreme richness of your work becomes clearer when we place it in relation to the powerful scientific movement of which you were a contemporary and which you have followed for almost 60 years with such passionate attention. “Your start in research coincided with the development of electromagnetic theory and electrotechnics: Maxwell had just published his immortal Treatise and the 1881 Congress of Electricity was being prepared, with the central concern of measurements and units. Our common teacher, Mascart, had introduced you to this movement through his teaching at the Collège de France, where he chose you as an assistant as soon as you left the École normale. So, your first works were essentially concerned with the circulation of currents and methods of measurement. This was the subject of your two theses for the doctorate in mathematics and physics... “As early as 1893, probably under the influence of Lord Kelvin’s admirable lectures in Baltimore, you solved the difficult problems concerning the relations of a supposedly spherical and rigid atom surrounded by a fluid or solid medium, either from the point of view of the vibrations of this medium in order to account for atomic spectra, or from the point of view of its deformations around a moving atom. “It is with extreme clarity that you took a position in favour of this revival and that you participated in the discussion to which it gave rise exactly 40 years ago. I remember, as a student at the École normale, the pleasure I took in your reply to the ill-advised article that Wilhelm Ostwald had just published in the Revue générale des sciences, under the title “The rout of contemporary atomism”, in which the most metaphysical and anti-material positivism was asserted. Your answer claimed for the physicist the right to admit an external reality progressively accessible to our mind, which we can understand, on which we can act, and which reacts on us by determining our physical and intellectual evolution. Your conclusion is in keeping with the whole spirit of your work and your whole attitude as a physicist: “For freedom and for matter...” A third teacher whom Paul Langevin greatly admired was Henri Poincaré. The reader will therefore not be surprised to learn that it was a great joy in 1904 for my father to meet Henri Poincaré again at the International Congress of Sciences and Arts in Saint-Louis (USA). Paul Langevin had been appointed rapporteur for electron physics at this congress, and he travelled with Henri Poincaré on his return: “During the week in 1904 he gave me the pleasure of spending time alone with him in the vast plains of North America. In the course of

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our conversations, on his return from the Congress of St. Louis, I had the opportunity to see with what passionate interest Henri Poincaré followed all the phases of the revolution that were taking place in our most fundamental concepts. He saw with some anxiety the shaking, thanks to instruments forged by himself, of the old edifice of Newtonian dynamics which he had recently crowned with his admirable work on the three-body problem and the equilibrium form of the celestial bodies. But if his enthusiasm was more thoughtful than mine, he was, like all of us, dominated by the excitement of entering an entirely new world...”

From Cambridge to the Collège de France At the Cavendish Laboratory After graduating from the École normale supérieure in 1897, Paul Langevin was awarded a grant by the city of Paris to spend a year in a foreign laboratory. He chose the Cavendish Laboratory at the famous Trinity College in Cambridge, directed by the great physicist Sir J.J.  Thomson. Indeed, having collaborated in 1896 with Jean Perrin at the physics laboratory of the rue d’Ulm, in experiments which aimed to determine the nature of cathode rays, he was aware of the fine work of J.J. Thomson on the same question. Paul Langevin arrived at Cambridge, as he himself said in his 1908 report of his scientific work: “After the discovery of the Roentgen rays, at a time when the theoretical ideas previously put forward on the discontinuous structure of electric charges were confirmed and supported by the experimental study of the conduction produced in gases by the passage of the new rays. This resulted in a considerable scientific advance which extended far beyond the limits of the field of electricity where it had originated, to magnetism, optics, heat, chemistry and even to mechanics, to shed unforeseen light on the most fundamental ideas.”

FIGURE 2: Paul Langevin in Cambridge (1897). © Bibliothèque de l’ESPCI.

So, Paul Langevin went to work with J.J.  Thomson at Cambridge to take part in this scientific movement. His companions in the laboratory there were Rutherford, C.T.R. Wilson, Aston and Townsend.

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After studying secondary radiation, Paul Langevin began his work on the ionisation of gases in the same laboratory. He later continued at the Sorbonne, and this was the subject of his thesis. He defended it in 1902 and it was published in 190340. The importance of Paul Langevin’s research at the Cavendish Laboratory can be recognised, where his portrait is still displayed next to Thomson’s. This research was carried out quickly, thanks to the very warm welcome given the young foreign scientist by Sir J. J. Thomson and his team. Moreover, he had the good fortune to work in the great calm of the English countryside, on a green and fresh site, on the banks of a quiet river, in the most favourable conditions for thinking. He always had warm memories of his time at Cambridge. Staying in the homes of local people and being a frequent guest in the homes of Sir J. J. Thomson and the other researchers, he improved his skill in the English language. This would be very useful in his career. In 1947, in an article entitled “Langevin and England”41, J.  D.  Bernal would say: “Paul Langevin’s first contacts with foreign countries go back a long way, and their importance was great, both for him and for science. He was lucky enough to be born early enough to enter scientific life at the very moment (1896) when that great movement known as modern physics was being born. He was also fortunate to be in the two cities, Paris and Cambridge, where the most important scientific discoveries took shape 50 years ago; one year after his death, we are celebrating the fiftieth anniversary of the discovery of the electron by J.  J.  Thomson, and we are approaching the fiftieth anniversary of the discovery of radium by the Curies, those two pillars of modern science and technology for peace as well as for war. Paul Langevin was himself influenced by this movement and reacted simultaneously to both aspects: the intuitive and powerful empiricism of Rutherford and the profound mathematical generalisations of Einstein. In his scientific work he stands between these two trends and thus one of his main merits is to have been, perhaps more than anything else, a clear interpreter in the discussion of questions which could so easily turn into either a catalogue of experimental facts or to a form of mystical recipes.”

40 Paul Langevin: Recherches sur les gaz ionisés, doctoral thesis 1902, published in Annales de chimie et de physique, t. 28, p. 500, 1903. 41 La pensée, No. 12 (May-June 1947), p. 17.

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On my father’s 73rd birthday, he spoke fondly of J. J. Thomson, who had welcomed him to Cambridge 48 years before and who he was pleased to have met again in January 1940, a few months before his death. My father’s relations with English scholars were always close and cordial. In 1925 he was made an honorary doctor of the University of Bristol. New buildings were inaugurated at the university in the presence of the King. Félix de Grand’Combe, who was then at the Faculty of Languages in the same city, tells what happened to my father: “As is customary in similar circumstances, on the evening of the inauguration a large banquet was held... “After this meal, which brought together several hundred guests, the deans each made an excellent speech and proposed the health of the new honorary doctors. To my great disappointment, Paul Langevin was asked to answer on their behalf, because his English was not brilliant... “At first, I was rather embarrassed as I listened to him, but then, little by little, this impression vanished and was replaced by that felt by all the guests; the style was no longer important, only the brilliance of the content shone through, for he spoke from the heart and it was only our hearts that were listening to him. All of us were electrified and the English, so proverbially phlegmatic, were powerless to control their emotions and gave him an indescribable, unforgettable ovation, such as I have never heard. If I may use an adjective that was not yet used, it was truly “atomic”. The guests unanimously declared his speech to be “the speech of the evening.” After his stay in Cambridge, my father maintained a permanent correspondence with English scientists year after year; he always remained in contact with Townsend, Thomson, Rutherford and Richardson. It is possible to quote many letters testifying to deep friendships. In 1901, Paul Langevin translated and read Thomson’s report to the Paris Physics Congress. As an example, here is the letter sent by Thomson to my father after receiving his doctoral thesis: Holenleigh West Road Cambridge 31 December 1902 “Dear Mr Langevin, “It is with the greatest pleasure that I received today the magnificent copy of your research on ionisation; it is a wonderful specimen of the union of science and art. I have followed your work with the greatest interest, and I congratulate you on having obtained such beautiful results. I often think of the year you spent at Cambridge and wish I could see you more often. My wife and I were very sorry that you could not come in the summer, perhaps we can hope to have the pleasure of seeing you and Madame

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Langevin next summer. We are expecting Rutherford and his wife, and so the old days will live again if you can come. Please remember me to Madame Perrin and the Professor; we were very pleased to see from the photograph they sent us a day or two ago: what a lovely little girl they have. My wife particularly begs me to convey to you and Madame Langevin her most sincere New Year’s wishes. I hope we can meet again before the year is out. I cannot conclude without thanking you once again for your magnificent work. “Believe me most sincerely yours, Signed: J. J. Thomson”

Paul Langevin’s marriage My father was already engaged during his stay in Cambridge. On 24 December 1896, while still a student at the École normale, he was staying with mutual friends and had met a pretty and elegant young woman of 22 years old, Emma Jeanne Desfosses, and he immediately fell in love. Jeanne’s father (for Jeanne was her usual first name), Theodore, was a ceramic artist-craftsman who made copies of famous statues on commission, but who also produced original works, remarkable in particular for the quality of the enamels and the beauty of the colours. The Desfosses family approved, and they got married on 22 September 1898 at the Choisy-le-Roi town hall, as Jeanne Desfosses was living at that time at 27 avenue de Paris, in Thiais. Paul Langevin’s witnesses were Jean Perrin, then aged 28 and living in Paris, 37 rue Rousselet, and Louis Pillet, aged 30, a trader living in Paris at 88 rue de Rivoli. Those of Jeanne Desfosses were her cousin Victor Marion and Georges Corvol, a sand merchant. After his marriage, Paul Langevin moved into a small flat at 50 boulevard de Port-Royal, immediately next to the house of his revered teacher, Marcel Brillouin. His choice to live near the Latin Quarter was apparently for two reasons. First, his parents lived in the 5th arrondissement (4 rue Guy de la Brosse) and secondly, he intended to prepare a thesis at the Sorbonne and he was a mathematics examiner at ESPCI, just a stone’s throw from the boulevard de Port-Royal. At the beginning of the school year, he was awarded a scholarship to prepare his thesis at the École normale and the Laboratoire d’enseignement de physique at the Sorbonne. This scholarship enabled him to continue the work he had so happily begun at Cambridge. But this grant was hardly enough for the two of them to live on. He had to increase his income; he was obliged to give lessons to make ends meet. At

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this time, I believe, he gave a few tutorials to Jean-Richard Bloch, thanks to an introduction by Richard Bloch who he had known during his military service in Sens. In my father’s own words, throughout this period he was “pulling the devil by the tail”. And, from 18 December 1899, Paul Langevin’s problems increased, due to the birth of my brother, Jean. Fortunately, a few months later, my father was appointed as assistant in the physics laboratory of the Sorbonne (Professor Bouty’s department) and his salary increased a little (100 to 150 francs per month). His colleagues were Dongier and Victor Crémieu, with whom he became friends. Victor Crémieu was a very original personality: after marrying the daughter of the publisher Alcan, he divorced her, then married his wife’s maid. He was an inquisitive person and interested in many things. After working for a few years in Professor Bouty’s laboratory, he set up a vineyard in the Hérault at Poujol-sur-Orb. But, still thinking about physics, he studied the radioactivity of the thermal springs at Lamalou-les-Bains and Colombières-sur-Orb, and created an “emanatorium” where patients were treated with the radium 42 emanating from the waters. Curious about everything new, he was one of the first speleologists and, at his own expense, explored the abyss of Mas Raynal in the Causse du Larzac. When we were young, he rented out a very pleasant house to my father, next to the Rodié mill in Colombières-sur-Orb, where we often spent our holidays. He also had the idea of participating in technical developments by building a small hydraulic power station with a turbo-alternator, exactly on the site of the old mill of Rodié with its dam across the Orb. This power station allowed him not only to have electric light and power in the house of Rodié and in his residence of Ombriès, about one kilometre from Rodié in the middle of the vineyards, but also to sell electric energy to the commune of Colombières-sur-Orb! Paul Langevin’s financial situation was no better; his family expenses increased as a result of my birth on 21 September 1901. It did not help that I needed special care, as my birth was said to be premature following my mother’s fall down the stairs of a holiday home, which my father had rented from the local doctor, near the fort of Palaiseau. To make up the deficit, in October 1901 Paul Langevin agreed to replace, temporarily, Georges Urbain’s father, who was professor of physics and chemistry at Lavoisier.

42

* Clearly this should read “radon”.

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Paul Langevin’s thesis and the substitution at the Collège de France In 1902, at the Sorbonne, Paul Langevin defended the very brilliant thesis on the ionisation of gases that has already been mentioned. His thesis attracted the Mascart’s attention, who was then professor at the Collège de France where he held the chair of experimental physics. Mascart chose him as a substitute in 1902, then again as a substitute in 1903. This is best described in the words of Professor Edmond Bauer. He was an eminent physicist, who collaborated with my father at that time and was later chosen by him as director of his laboratory at the Collège de France, before being appointed professor at the Sorbonne43: “I met Paul Langevin in December 1902, during his first lecture at the Collège de France. He was 30 years old. He had just defended a thesis at the Sorbonne that had become a classic, in which he presented his “Recherches sur les gaz ionisés”. Mascart, who then held the chair of physics at the Collège de France, was struck by the novelty and beauty of this work, in which ingenious and delicate experiments and elegant and profound theoretical calculations complemented and supported each other. He immediately proposed to the young physicist, a simple assistant at the Faculty of Science, to replace him in his chair. Langevin accepted. A few years later, he became the holder of this post, which he was to retain for over 40 years. “I can still see him standing behind the experimental table; his straight and youthful figure, which he was to keep all his life, his black frock coat and his brushed hair gave him almost a military appearance44; his luminous eyes communicated their flare to us, his clear and calm speech developed into lessons of admirable clarity. The mere arrangement of his calculations on the blackboard was a work of art. “I still have in my library the five books of the notes that I took in that first course. Langevin introduced us to the experiments and theories which, for some years, had begun to revolutionise and clarify our ideas 43 Edmond Bauer: Talk broadcast on the national channel on 25 November 1956, under the aegis of the “Union Rationaliste”, entitled: Souvenirs sur Paul Langevin and published in the “Courrier rationaliste” of 25 November 1956, No. 11, third year, pp. 167-170. 44 Bauer’s impression is quite correct. It is true that my father had, with his brushed hair, long moustache and imperial goatee, quite the air of an officer. Madame de Noailles, who he used to meet, even claimed (as Jean Perrin said) that he reminded you of a cavalry officer. What I can say myself is that during our walks through France, my father, in all the villages we passed through, caught the attention of the priests, who certainly took him for an officer in civilian clothes. They came over to talk to him, the army attracting the clergy (le sabre attirant le goupillon).

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about matter and electricity. This is a revolution that is still going on today and in which he had already taken an active part, either as early as 1897 with his friend Jean Perrin, or alone, or also as a member of Professor J. J. Thomson’s team at the Cavendish Laboratory. This astonishing team included four future Nobel Prize winners, to whom it would have been fair to add Paul Langevin. “At that time, physics courses at the Sorbonne were generally content to teach a science that was well established, – it was believed –, that is to say, a little dead, having been young around 1860. Langevin’s lessons, on the other hand, were an admirable initiation into research. Delicate quantitative experiments were carried out in front of the audience, and new theories were explained and discussed clearly, whatever their difficulties. And this lasted for 40 years; the old physics amphitheatre of the Collège de France was the source where several generations of French physicists came to learn about the most recent advances in their science.” At the end of 1902, tragedy struck my father; his mother died of cancer. It was a huge loss for him, from which he would recover with difficulty. As the family grew, he had to abandon the small flat on Boulevard de Port-Royal and move to a slightly larger one in the same neighbourhood, 21 Boulevard Saint-Marcel. In 1903, Paul Langevin was appointed substitute professor at the Collège de France in Mascart’s chair. Despite the additional burden imposed on him by the birth of my sister Madeleine (on 27 January 1903), his financial situation began to improve. After his mother’s death, he wanted to look after his father; he had to look for something bigger and not too expensive. Before the end of 1903 he found a detached house very close to the Parc Montsouris, – which was ideal for children – at 29 rue Gazan. We only had to cross the street to go and play in the park. We could even play in the small garden of the house while still enjoying the fresh air from the park. To tell the truth, the choice of the new residence was also influenced by the fact that this street is very close to Boulevard Kellermann, where Paul Langevin’s best friends, Pierre and Marie Curie and Jean Perrin, lived. Pierre and Marie Curie and the family of Jean Perrin occupied twin houses at the corner of Boulevard Kellermann and rue de l’Amiral Mouchez45. Long afterwards, Aimé Cotton remembered “the two neighbouring houses on the boulevard Kellermann, open to faithful friends such as Debierne, Urbain, Sagnac...”, these houses the scene of “famous jousts in which Curie’s gentle seriousness was enlivened by the spectacle of Jean Perrin’s brightness

45

Pierre and Marie Curie’s house was at 108 bd Kellermann.

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of mind and bold intuition, grappling with Paul Langevin’s broad, precise and profound views.” I remember the games we used to play with my brother, who was eighteen months older than me, around a powerful focus of interest, the tap for watering the small garden, attached to one of the walls of the Gazan Street house. My mother used it to wash clothes in a tub. However, my brother was already old enough to use it for less rational purposes, creating mud puddles in which we delighted to paddle. One day these games almost turned tragic. I cannot say how I managed to climb onto the edge of the tub, but what is certain is that I fell headfirst into the water and almost drowned. I remember very well that, from then onwards, my father took us very often to the Curies’ house, where they had guests every Sunday. While my parents talked with Pierre and Marie Curie and their friends inside the villa, in the summer we romped in the garden with Irene and later with Eve. On other occasions, we went to the Perrin’s house next door and played in the adjoining garden.

The International Congress in St. Louis and its aftermath His thesis on the ionisation of gases publicised Paul Langevin as a theoretician and experimenter who was well aware of the problems of modern physics and the corpuscular theory of electricity. This led to his appointment as rapporteur on the “Physics of Electrons” at the International Congress of Sciences and Arts in St. Louis46. He was very pleased to participate actively in the development of the new physics and to meet his revered teacher, Henri Poincaré, at this congress. Paul Langevin had done a lot of work to write his presentation on issues that were, at the time, quite new. These meetings were important. It was the first major congress since the crisis in classical physics. It was there that the construction of a new physics was started and, above all, its foundations were clarified. My father’s report was eagerly awaited. On 11  July 1904, Rutherford wrote to him from Montreal: “I am delighted to hear that you have decided to come to St. Louis to take part in the Congress. I hardly need to tell you that there is no one with whom I would more readily associate than with yourself.”

46 The full text of this report can be found in the book La physique depuis 20 ans, by Langevin, Librairie Octave Doin, 1923, 1 to 69.

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The excitement with which my father embarked did not prevent him from thinking with tenderness of all his family back in Paris. A letter found in his papers bears witness to his feelings for his father: “Royal Mail Line, 2 September 1904 “My dear Dad, “I did not have time before leaving to reply to your kind letter and I am taking advantage of a stop in Ireland on our steamer which left Liverpool last night at five o’clock, to send you a note and say goodbye again before starting on the Atlantic crossing. “I had a very bad Channel crossing the night before last, on a small ship which the sea shook a little roughly, with sudden dives from which the passengers seemed to suffer quite a bit; I managed not to be sick, but it would not have taken much to get me there. In contrast, the present liner, though not one of the great transatlantic liners, hardly stirs at all, so that we can enjoy the abundantly served table, albeit of English cuisine. We have a lot of French Canadians on board, so that more French is spoken than English; the accent with which these Canadians speak is amusing, and I thought at first, I heard our language being barked by foreigners. “We shall arrive in Quebec on Saturday at eight or on Sunday, and I do not yet know where I shall go. I shall try, as I told you, to join the French Mission. “Jeanne will probably leave for Fontainebleau on Saturday or Sunday with the children, who will get some fresh air from the forest, so you might not find anyone when you get home; they will return at the end of the month. “Stay well, my dear Papa, I will come and kiss you when I return. Signed: Paul Langevin” Paul Langevin did not rest after the congress from the enormous effort he had put into his presentation and the numerous discussions that followed it. He worked on the boat, with his characteristic enthusiasm and energy, on a new theory of magnetism, a theory that was certainly one of the most beautiful he had constructed, because it remained valid for more than 50 years. Here is what Edmond Bauer says about it in his “Souvenirs sur Paul Langevin”, already quoted: “In 1904, on his return from a trip to America, while crossing the Atlantic, Langevin began to reflect on the problems posed, from the atomic and electronic point of view, by the well-known but poorly understood phenomena of magnetism. He managed to keep to himself on the ship. These few days of calm concentration were enough for him to lay the foundations of a theory that he published a few months later and which is perhaps his greatest scientific achievement.

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“Relying on his profound knowledge of the laws of electro-magnetism, boldly generalising certain formulae of statistical mechanics, and clarifying from the electronic point of view the image of “molecular currents” which Ampère had once used, he arrived at a satisfactory explanation of paramagnetism or weak magnetism and diamagnetism or “inverse” magnetism. Although in the last 40 years the quantum revolution has forced a complete revision of the fundamental principles of physics, Langevin’s theory of magnetism has remained intact. On the contrary, some of his postulates have been justified, and some of the consequences have been verified and used by experimenters, for example cooling by demagnetisation which today allows us to approach absolute zero in temperature…” Bauer’s opinion on the value of this work was confirmed by Louis de Broglie and Marcel Brillouin. “Paul Langevin’s beautiful theory of diamagnetism”, writes Louis de Broglie, “has retained all its value today despite the modifications undergone by atomic physics since the time it was created. Quantum theory has been able to find exactly the formulas given by Langevin and it has brought the fundamental theorem he proved into the general framework of the theory of adiabatic invariance due to Ehrenfest.” In 1905, Paul Langevin, continuing his work on the ionisation of gases, studied the “conductivity of the atmosphere”. Here is exactly how he discovered large ions, again according to his own work record: “In order to work in Paris in the purest possible air, I proposed, first of all, to go to the top of the Eiffel Tower, by day and by night, and undertake a systematic study of the variations in the free electric charges in the atmosphere... “...The rather unexpected result showed me that the ions of the atmosphere are very clearly divided into two categories, the ordinary ions produced directly by the action of radiation and, without any intermediary, ions several thousand times slower, to which I gave the name of large ions.” It is interesting to give here the impressions of Edmond Bauer, who witnessed these atmospheric ion experiments on the third floor of the Eiffel Tower: “As far as the ions or mobile electrified particles in gases are concerned, I will simply tell you about the aerial walks he made me take happily, up an endless staircase to the top of the Eiffel Tower. It was the middle of winter, and the lift didn’t work. He wanted to show me the apparatus he had installed and the experiments that demonstrated the existence, in our atmosphere, of two distinct species of ions: the “small” ones, whose dimensions are of the order of those of molecules, and the “large” ones, or “Langevin ions”, ultra-microscopic electrified dusts. At low altitudes, the latter are the most efficient at condensing water vapour: the drops

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that make up the low rain clouds or nimbus form on them. Above 5000 metres, only the small ones exist. Cirrus clouds condense on them, the white clouds that float high in the sky. Higher still, at very low pressure, the negative ions become highly mobile free electrons, the positive ions remaining as electrically charged molecules or atoms: this is the “Heaviside layer”, whose essential role in the long-distance propagation of radio waves is now known.”

Paul Langevin, successor to Pierre Curie In 1905, as well, Paul Langevin was appointed professor at ESPCI to replace Pierre Curie. In fact, Pierre Curie’s replacement from 1903 to 1905 had already been entrusted to my father (I found the official notebooks of the ESPCI in his papers, where the chapter headings of the nineteen lessons given to the first-year students during the second semester of 1904 are recorded, as well as the notebook relating to the lessons my father gave to the students of the second and third years in 1904–1905). These substitutions and especially this appointment brought a very serious improvement to my father’s financial situation, because the salary of a professor at ESPCI was 6000 gold francs. Most classes started in the morning at half past eight. So, when we left the little villa in rue Gazan, at the beginning of 1906, to go and live in Fontenay-aux-Roses, at 52 rue Boucicaut, my father was obliged to get up very early. He had decided to move because he thought, like Marie Curie, that the air of the suburbs was preferable to that of Paris for his children. At that time, Fontenay-aux-Roses was not served by the metro, as it is today, but by a small, rather slow steam train that took almost an hour to get to Luxembourg. In addition, the house was about 1 km from the station, so my father often had to hurry to avoid missing his train, which ran infrequently. A childhood memory comes to mind: when my father was in a hurry to leave, on days when he had classes, he would dispatch his breakfast extremely quickly. At a young age I was totally amazed by the extraordinary performance of swallowing two fried eggs in a single bite, before heading off for the station. The beautiful flat that my father discovered in Fontenay-aux-Roses was on the ground floor of an old 17th century house that had belonged to the famous doctor Antoine Petit, physician to Louis XVI. Immensely rich, Petit had given this house to the commune of Fontenay-aux-Roses to house his health officer. There was a beautiful, paved courtyard with a wrought iron gate at the front of the house, on the edge of rue Boucicaut. To the right and left of

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the courtyard, there were the caretaker’s lodge and the old stables. Behind the house lay a magnificent park with centuries-old trees. This park was a paradise for children like us. From the terrace and the windows of the apartment, we had a beautiful view of the valley that stretches from Fontenayaux-Roses to Sceaux-Robinson. This was where I spent a large part of my childhood. How many games of hide and seek, blind man’s buff or croquet we played in this park, my brother, my sisters and I, with Irène Curie, Aline and Francis Perrin, Isabelle, Marguerite and Fernand Chavannes! The parents of the latter lived in a house very close to us, with a common wall with ours. The apartment on the first floor of our house was occupied by Ferdinand Lot, a professor of history at the Sorbonne and a very kind man, whose wife, née Borodin, was the granddaughter of the famous Russian musician. We thus had neighbours who were both very cultured and very pleasant. My father was very concerned about our intellectual development. He himself trained in primary and then higher primary education and enrolled us first in the public primary school in Fontenay-aux-Roses. It was there that my brother and I acquired our basic knowledge under the sympathetic guidance of M.  Fréchoux. I stayed there until my parents came to live in Paris, when I was ten, and my brother until the year before. Paul Langevin was concerned about our scientific education and was convinced by an idea of Madame Curie’s. She thought that it was in the interest of children to introduce them to science as early as possible (around eight or ten  years old) to communicate to them a love of science and the need to understand, and the passion of research. On 30 January 1907, Henriette Perrin wrote to my father, who was then in the Midi: “...One would be afraid of being too presumptuous, of asking too much of Destiny in wishing these children to be better and more intelligent than their fathers, but one can hope that they will have, thanks to the efforts made by the latter, more happiness, a sunnier life, easier victories, and a quicker and more joyful flowering of the riches they carry within them. “We have formed a project with Madame Curie – we will discuss it with you as soon as you return – of combining our efforts to teach our group of children together, with classes to be held at your home, our home and Madame Curie’s home. We will try to organise this as well as we can, for the greatest intellectual benefit and for the greatest happiness of the nice little gang.” My father collaborated in this project of a kind of “teaching cooperative”, where world-renowned scientists were to try to apply what we now call “new educational methods” to their children in a group. The innovation, at that time, was that all lessons were illustrated by elementary but well-chosen

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practical work. The group included all the children of the participants (Curie, Perrin, Chavannes, Langevin and Mouton). Chemistry was taught by Jean Perrin at the Sorbonne’s physical chemistry laboratory; mathematics by Paul Langevin at Fontenay-aux-Roses; physics was taught by Madame Curie; Madame Henriette Perrin taught French and literature, the sculptor Magrou taught modelling, and Professor Mouton taught natural sciences. Unfortunately, this exciting experiment lasted only 2 years: parents like these were too busy to devote enough time to this demanding form of teaching!

Lecturer at the École normale de Sèvres In 1905, Paul Langevin was put in charge of teaching electricity at the École normale supérieure des jeunes filles de Sèvres. He was always very interested in this subject. “My joy in teaching, he said, was perhaps never greater than at the École de Sèvres where, for more than 25 years, I felt deeply how fervently these classes of young girls welcomed new ideas and acknowledged, through sustained attention, the efforts made to open the doors of the temple of science to them a little more widely than the official curricula allowed. There, too, I made valuable friendships, such as that of Madame Cotton, who became director after I left, and also forged very dear family ties47.” The opinion that the Sévriennes had of my father’s lectures was reported by Madame Cotton in the article she wrote for La Pensée (12, May-June 1947)48, on the occasion of the publication of a special issue dedicated to the memory of Paul Langevin. She was a tutor at Sèvres at the time, before becoming its director: FIGURE 3: Reception at the École Nationale Supérieure de Sèvres (1904). All rights reserved.

“The students at Sèvres have a glowing and lasting memory of their teacher Paul Langevin. 47 My father alludes in this text to my marriage to Luce Dubus, his former student. He also met Éliane Montel, my wife’s classmate at Sèvres, who later became Paul Langevin’s most devoted secretary and to whom he became deeply attached. Paul-Gilbert Langevin (* musicologist 1933-86) is my half-brother. 48 This issue is long out of print. The article is entitled: Paul Langevin, maître de conférences à l’École normale de Sèvres. p. 41.

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“Paul Langevin began teaching electricity in 1905, and he did not abandon this until 28 years later. He lived in Fontenay-aux-Roses at the time and frequently came to Sèvres on foot in the summer, crossing the woods of Verrières and Meudon. Like his teacher Pierre Curie, he liked long solitary walks, conducive to reflection. He entered through the small door of the École, at the very top of the park49. As he descended the steep slope, he could see his students watching for him on the sunny pavement of the courtyard of honour, the famous “tarmac” dear to the Sévriennes. The supervisor had no difficulty in gathering them for Langevin’s course, for none of them wanted to lose a word of the magnificent teaching they were being given. “Langevin expounded all subjects from the most general and highest point of view. He linked the ideas with the confidence of a philosophical mind from which no aspect escaped the question under discussion. Images prepared the mind with elegant and clear language, given with admirable diction. Mathematical demonstration followed. “It’s very simple”, said Langevin, glancing at the starting formulae placed carefully at the top of the large blackboard! And, without notes, with his left hand in his pocket, Langevin gradually covered the whole board with impeccable calculations, arranged with an elegance and neatness which made us admire him. While following this beautiful sequence of equations, we listened to his well-modulated voice and his deep gaze, the sensitive means by which Langevin communicated his thought, the inner impact revealing its perfection. “The women of Sèvres were grateful to their teacher for the confidence he placed in the female intellect. This was not common around 1900... “...With such a teacher, the hours of class flew by without anyone noticing – for him as for us, in fact. The dinner bell may have been ringing and ringing, the supervisor may have appeared behind the glass door to discreetly remind us of the time. Paul Langevin could neither hear nor see anything: his teaching absorbed him comFIGURE 4: At the École pletely. Suddenly, he became aware of nationale supérieure the time; the pupils left, fatigue suddenly de Sèvres (1910). © Bibliothèque de l’ESPCI. 49 I think that Mrs Cotton makes a slight mistake here. Paul Langevin always told us that he never had the key to the little gate in the park at his disposal. He boasted about this, because he was proud of the fact that he was still considered... dangerous to young girls at an age when he could have been considered venerable, whereas his colleague and friend Jean Perrin, who was older than he was, had been given the key.

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overtook him, he raised his hand to his forehead, closing his eyes for a moment, then, after drinking a little tea, he went to wait at the end of the Avenue des Marronniers for the old Louvres-Sèvres-Versailles tramway, which would take him home very slowly. We then considered, with much gratitude mixed with surprise, how much time and strength it had taken of this of great scholar to teach at Sèvres.”

Work on magnetism and relativity In the same year, 1905, Paul Langevin began to publish his first work on the theory of relativity. From his deep knowledge of classical electromagnetic theory and Lorentz’s studies on this question he was led to reflect on the ideas of time and mass, reaching the same conclusions as Einstein. The summary that he himself compiled in 1934 on his scientific work, the testimonies of Louis de Broglie, Edmond Bauer and Einstein himself, and the content of the lectures given at the Collège de France, allow us to confirm my father’s original contribution to the relativistic theory of time and space, and to the notion of the inertia of energy, with Langevin following paths that were different from Einstein’s. Edmond Bauer notes in his memoirs on Paul Langevin: “During the years 1905-1906, his course at the Collège de France led him to examine in depth the properties of electrons and of light and their relationship to each other. His calculations soon showed him that light has mass, that it is inertia. Then he found that for a given quantity of light, as for the electron, the mass is proportional to the energy, more precisely that it is equal to the energy divided by the square of the speed of light.” “He kept me informed of his research and said: I am sure that this is a fundamental law of nature. I hope to be able to establish its complete generality soon.” L. de Broglie, for his part, writes in his 1947 obituary50 about Paul Langevin: “Although he did not publish anything on this subject at the time, it seems that Paul Langevin saw one of the most important consequences of the new dynamics almost at the same time as Einstein: the principle of the inertia of energy. This principle asserts, as we know, that to every form of energy corresponds a mass equal to the value of this energy divided by the square of the speed of light in vacuum. As a result, the principle of conservation of mass as it has been stated since Lavoisier is not rigorous; 50 L. De Broglie: Notice sur la vie et l’œuvre de P. Langevin, lecture given at the Académie des sciences during the annual prize-giving session on 15 December 1947, Paris, Gauthier-Villars.

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in order to rigorously apply the idea of conservation of mass in physical or chemical reactions, it is necessary to take into account the masses corresponding to the kinetic energies, the quantities of heat and the radiation involved in the reaction. Because of the enormous value of the speed of light, the corrections to be introduced in this way into the usual calculations of chemistry are insignificant, which explains the success of Lavoisier’s ideas; but this is no longer the case if we consider nuclear transformations, where the kinetic energies of the constituents of the reaction and the emitted radiation can intervene in a significant way in the energy balances. “Langevin was the first to realise this fact, when he showed that the large differences (of the order of 1 %) between the masses of atoms other than hydrogen and integer multiples of the mass of the hydrogen atom could be interpreted by the inertia of energy. This undeniable fact seemed until then to formally oppose the old hypothesis of the English physician Prout, according to which the atoms of all the elements would be constructed from the hydrogen atom. By removing this obstacle, Langevin made it possible to return to this beautiful hypothesis of the unity of matter, which today, as a result of the definite presence of neutrons in the nuclei of atoms, has taken a slightly more complicated form than in the past, but which has nonetheless become the keystone of all our knowledge of the nucleus of the atom. Together with Albert Einstein, our colleague is thus at the origin of the whole movement of ideas that led physicists to understand the enormous reserve of energy contained in the nuclei of atoms and to see how it would be possible for mankind to tap into this inexhaustible treasure that is hidden at the very heart of matter. And so, we see how his work is linked to the most admirable, and in some respects the most formidable, discovery in contemporary physics.” For his part, Marcel Tournier, who was a pupil and then a collaborator of Paul Langevin, confirmed what we have said. He wrote in the 1958 review “Physique et Chimie”, page 15: “Developing the consequences of the theory in dynamics, he came to discover the fundamental relation of the inertia of energy. He showed the calculations to one of his friends51, who could not believe the extraordinary novelty of the result. This made Langevin hesitate; he delayed publication, which was made just several months later by Albert Einstein.” Bauer says: “At that time, I was in charge of reviewing the articles for the journal Le Radium that had appeared in Annalen der Physik, a German periodical. Delivery had been delayed. Leafing through a few issues from 51

This is Jean Perrin, as mentioned above.

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1905-1906, I came across a formula linking mass and energy; it was the same as the one my teacher had been talking about for several months. I read the article. Without yet understanding its full significance, I was struck by the simplicity of the proof. I ran to Langevin and said to him – I remember it clearly – In this issue of the Annalen, a German called Einstein publishes your formula and proves it.” He said, “Give me that. I doubt that his results are as general as mine. “In fact, these were Einstein’s first papers on special relativity. The demonstration of the inertia of energy, although a little sketchy, was general. Langevin understood this. He gave up publishing his own research, became enthusiastic about the theory of relativity and became a friend of Einstein. “Later, he was to give a completely rigorous and general demonstration of the Theorem of the Inertia of Energy. The importance of this theorem, which is the basic principle of the use of atomic energy, is well known.”

Paul Langevin after the death of Pierre Curie The year 1906 was a very dark one for Paul Langevin. On Thursday 19 April of that year, a great catastrophe befell him. His most revered teacher and close friend, the physicist Pierre Curie, was killed in a stupid accident. That day, Pierre Curie had attended a lunch of the Association des professeurs de la Faculté des sciences at the Hôtel des Sociétés savantes, rue Danton, and on his way out went to Gauthier-Villars to correct some proofs, but he found the door closed, as the workshops were on strike. On his way back from there, he was hit by a heavy truck and died with his head crushed52. The premature death of Pierre Curie, at the age of 47, was a terrible blow to Paul Langevin. This can be seen from the obituary he wrote a few months later in the Revue du mois53: “Nearby, at the bottom of the valley, now full of flowers and perfumes, where a great part of his youth passed, lies the great physicist and excellent man that was Pierre Curie. “It’s been almost two months since we took him to sleep the eternal sleep in the quiet cemetery, overlooking these slopes of Sceaux and Fontenay where, for a long time, he walked and thought. It is now nearly two months since, as if by brutal and stupid revenge, blind matter, the

52 For more details on the circumstances of Pierre Curie’s death, the reader is referred to Ève Curie’s book on Madame Curie, published by Gallimard in 1938. 53 Revue du mois du 10 juillet 1906, n° 7, pages 5 to 37.

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mother of life and pain, destroyed the brain that understood it, dominated it and loved it best. In a moment of the struggle we are waging against them, the physical forces were once more asserting their power over the most beautiful of all blooms, over human intelligence and goodness. “It is almost two months since his death, and yet those who lived near him, who used to submit their ideas and doubts to him, find it difficult to believe in this loss, to “realise.” it, as the English say. The hour when we knew we could meet him and when he liked to talk about his science, the path we usually took with him, recall his memory every day, evoke his benevolent and thoughtful appearance, his luminous eyes, his beautifully expressive head, shaped by 25 years spent in the laboratory, by a life of hard work, by his continual concern for moral beauty, by an elegance of mind which had created in him the habit of believing nothing, of doing nothing and saying nothing, of admitting nothing into his thoughts or actions that was not perfectly clear to him and that he did not fully understand. “This constant need for truth and clarity extended to all activities that made him, in a natural and simple way, always keeping his thoughts directed towards the most important issues. This gave so much nobility and strength of example to his life of courageous and free experiment, and to the invigorating and healthy atmosphere he created around him. “The same discipline, applied to all his mental activities, will give us the key to his personality, which always kept him well away from the hustle and bustle, in the serenity of an existence entirely devoted to science and to the people he loved, in the calm of the laboratory where glory came to find him, so as to disturb for a moment the admirable unity of his life. “My memories, still so new, come more easily to remind me of him in his laboratory, which has actually hardly changed as we have aged during the 18 years that have passed since, as a timid and often awkward beginner, I began my experimental education with him. I see him again, tall and slim, a little bent over from stubborn labour, his thick hair prematurely grey on his open forehead, his fine features a little drawn for some time by physical sufferings suffered without any complaint, his face lit up by a smile which reflected his exquisite kindness, made alive by his child-like laughter, or attentive in his constant wish to observe and understand...” It would be very easy to show, with the help of Paul Langevin’s texts, to what extent the keen admiration he had for his teacher extended to his companion and collaborator Marie Curie, who, at the time, already enjoyed a worldwide reputation due to the discovery of radium and polonium.

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The friendship between Paul Langevin and Pierre and Marie Curie is evoked in the following lines, which I borrow from my father’s text, quoted above: “Their friends were chosen to be few in number, because of the same concern for sincerity that dominated their whole moral life, not wanting to disperse their affections any more than their minds. These friends had begun to form a small colony around the house (108 boulevard Kellermann). Debierne came, the most intimate friend and collaborator during the long processing required for the preparation of radium salts; also Perrin, immediate neighbour for a year, Sagnac and Urbain. As a neighbour myself, I spent some very good times there, liking to go to find them in the evening, always ready for long talks, in the room on the first floor, where they worked together above the garden that the grandfather cultivated. We continued their ongoing work of examining ideas and facts, guided by Curie’s keen mind and supported by his wife’s persevering need for clarity. We touched on everything, but we preferred to return to the marvellous movement that is currently sweeping physics and in which their joint work holds such a large place. “How bitter it is to think that this is all over, that those moments have passed and that we will no longer feel, after too few years of a wonderful business, the driving force that Curie gave us! “These talks also continued on the road or in the laboratory, either at the School of Physics, in the large wooden building, half-destroyed by the weather, where they somehow had to shelter their processing of radioactive materials, where she put on a chemist’s coat, while he covered himself warmly, sensitive to the rheumatism caused by the dampness of the place, or in their new laboratory at the P.C.N., where they had settled themselves for the last 2 years and where they barely had time to begin a new period of quiet work.” How could it be considered abnormal, when Paul Langevin considered it his duty to encourage and to help Marie Curie in her misfortune, and when he continued to discuss with her the new physics that radioactivity had brought about and which fascinated them both? Is it not also quite natural that this friendship, together with mutual admiration, gradually turned into a love and an affair, several years after Pierre Curie’s death? The xenophobes, anti-feminists and ultra-nationalists of the time, as is well known, organised a hate campaign against Marie Curie and my father. I would like to emphasise the baseness of this. The home where we had been raised was destroyed for a while. My father and mother lived apart until the 1914 war. Most of the time we children stayed with our mother and with my aunt, Madame Bourgeois. Her children, Pierre and Thérèse, became like our brother and sister.

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A caring and loving father But my father’s affection for us did not wane for a moment. He continued the Sunday walks and holiday trips with us. He was always very concerned for our health, and he believed that outdoor exercise and especially walking were excellent for it. So, he organised, from our earliest years (4 or 5 years old), walks with us in the forests around Paris. As we lived at the beginning of the century in the beautiful house in Fontenay-aux-Roses that I mentioned, we could go for walks with him either in the surrounding countryside or, very often, in the Verrières woods, which were not very far from Fontenay, passing through Robinson. In those happy days, there was still a real farm in Malabry and our father used to make us drink good fresh milk with jam for our afternoon snack. In those days, there were no housing estates in Malabry and the area remained quite rural. A few years later, our father took us out on summer Sundays to more distant suburbs. So, from the age of ten, we walked through all the forests around Paris. Paul Langevin was an excellent walker, and it was not unusual for us to cover 20 to 25 kilometres on foot in one day. Much later, in 1921, the three of us, he, my brother and I, made a great and beautiful journey on foot through the Massif Central. We left from Le Lioran and ended up in Colombières-sur-Orb, where we were to spend the rest of the summer holidays. From the Plomb du Cantal, we passed to the Truyère valley. Then, via Saint-Chely d’Aubrac and the curious town of Espalion, we came to the Aveyron valley to reach Rodez. Then we visited the Gorges du Tarn as far as Millau, and crossed, still on foot and in the rain, the monotonous landscape of the Causse du Larzac, to finally descend into the valley of the Orb and reach the Rodié mill in Colombières-sur-Orb, where we met the rest of the family. At the beginning of 1947, a friend of my father’s, Pierre Vaillant, honorary professor at the Faculty of Science in Grenoble, wrote an article for the newspaper Le Travailleur alpin, about Paul Langevin, who he had known well since 1915, when they had worked together for la défense nationale. Among other things, he recalls their hikes together: “... During my stay in Paris, I had often spoken to Langevin about Grenoble, which he had only passed through, and its magnificent surroundings. At my invitation, he came there several times after the war, almost bought a house on the outskirts of Grenoble, and our two families spent a whole summer together in a small village in the Vercors, which has since been destroyed54.

54 Pierre Vaillant is referring here to the Baraques des Grands Goulets, a small village at the entrance to the Vercors plateau from Pont-en-Royans.

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“It was an unforgettable holiday for me. A tireless walker, he livened up our walks with his dazzling conversation, tackling all subjects with equal mastery, interested in everything and everyone. “In our little village, everyone knew and loved him and I was able, some years later, to find that he had not been forgotten, nor had his children who accompanied him everywhere and for whom he was, as much as an admired and loved father, a sort of affectionate and tender elder brother who was full of intelligence.” In addition to our physical development, my father also took care of our musical culture from an early age. He often took us to hear classical music, either at the Concerts Colonne, or later at the Concerts Poulet (at the Sarah-Bernhardt theatre), or to listen to a string quartet in a tiny hall on the Boulevard de Strasbourg. If I am not mistaken, it was the Quatuor Calvet. If piano lessons were of little use to me, it was probably because I had very little aptitude, but it was also because my parents had chosen a surly old lady as our teacher, who did not succeed in making me take a liking to playing the keys. I dare say that if this lady had been young, pretty and agreeable, I might have taken to playing the piano. On the other hand, going to good concerts allowed all the children in the family to develop a musical culture. It was a complete success, we all became more or less passionate about classical music55. It was the same for painting and sculpture. My father took us to museums, especially the Louvre; moving from room to room, he never tired of making us admire the beauty of the paintings of the great masters or the sculptures of Greek and Egyptian antiquity. This delightful father also took great care in the formation of our literary tastes. From our earliest childhood, he regularly read to us in the evening before bedtime. He introduced us to all the great novels of Victor Hugo. His friend Giovanni Malfitano, when he came to the house at that time, told us from memory the stories that had enchanted his own childhood, such as “The extraordinary journey of Nils Holgersson”. During our adolescence my father continued to advise us in our reading. He passed on to me his passion for Anatole France and Balzac, for which I am very grateful. Paul Langevin was a great lover of the theatre. As soon as we were old enough to go to secondary school, he took us to classical performances, especially at the Odéon. In times of reduced financial means, we queued for the cheaper seats and went with our father to the Paradis under the roof of the Odéon. Later, when his means increased, he paid for our subscriptions 55 My father was also a great fan of vocal music, especially the lieds of Schubert and Schumann and the songs of Henri Duparc and Gabriel Fauré, which he enjoyed listening to when my sister Madeleine performed them for him.

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to see the classics. This does not mean that he was not interested in modern theatre: we often went with him to the Vieux-Colombier to see Louis Jouvet and Valentine Tessier. We went with him to hear Chaliapine in Prince Igor at the Théâtre des Champs-Élysées, and also accompanied him once to a performance of Amphytrion 38 and many other modern plays.

Paul Langevin’s successes in science from 1907 to 1912 Following his stay at Cambridge, Paul Langevin’s research, lectures and articles had established his scientific reputation in France and abroad. The value of his work was publicly recognised. Hugues Prize. – The Academy of Sciences awarded my father the Hugues Prize in its session of 2 December 1907. The commission that awarded him this distinction included Mascart and Henri Poincaré; the rapporteur was Becquerel. It emphasised especially the results obtained in the study of gas ionisation and magnetism. Election to the Collège de France. – At its meeting on 15 February 1909, the professorial assembly selected Paul Langevin as a new professor to the Collège de France. This election took place with an almost unanimous vote. My father’s lectures at the Collège de France were to have the most profound influence on the training of a whole generation of scientists. It took much hard work to prepare them. On the eve of these lectures, we, those close to him, noticed a great nervousness in him, which appeared to us, wrongly, as bad temper. We were too young to realise the effort of original thought and concentration that was required to reveal entirely new concepts. Besides simple students and a few friends, the audience included the greatest scholars of the time. “In this classroom, which resembled a small high school class”, writes Paul Le Rolland, “he arrived with astonishing simplicity, without the slightest pomp, and after a quick reminder of the ideas already accepted, he continued his lessons, which were undoubtedly difficult, but which, thanks to his fervent, confident and persuasive speech, seemed to us to be quite clear and lucid.” And the same witness adds: “I can still see on these benches of the Collège de France a young student with curly hair and the clear and gentle face of a child. This student was called Louis de Broglie.” Many years later, Louis de Broglie would say: “If Langevin, through his personal work, contributed to maintaining the reputation of French physics, particularly in the theoretical field, he also

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ensured, through his teaching, the link between the great school of French physicists-geometers of the nineteenth century and the younger scientists who are striving today to prolong a glorious tradition in our country. He also had a considerable influence over the training of researchers in the new school of experimental physics, because, as an eminent theoretician, he always knew how to keep in touch with the facts and concrete realities of the laboratory...” For his part, Frédéric Joliot-Curie, who was my father’s pupil and then colleague, liked to recall the decisive influence that Paul Langevin’s words had on him and on many of his fellow students56. Frédéric Joliot added: “We have carefully kept the notebooks filled with your masterly lessons and they are still the best documents that we consult. It is through your teaching that the great theoretical and experimental discoveries of contemporary physics have been assimilated and clarified in France. If it were necessary to give evidence of this, I would recall the action, one might even say the fight that you have led here to make relativity understood. All those who heard you were won over by the clarity and elevation of your thinking. Your words have left a deep impression on us all…” In the report to the secret committee of 18 June 1934, which decided to promote Paul Langevin at the Academy of Sciences, Marcel Brillouin was to say: “Everyone knows how quickly and unexpectedly experimental discoveries in the atomic and nuclear world have followed one another during the last 15 or 20 years, as well as the theories intended to explain them. However, from the outside, these theories appear almost as incoherent and disordered as the molecular movements themselves. Each is inspired by the discovery of a new phenomenon and adapts as best it can to classical phenomena. However, when adventurous theorists live close to experimenters, their hypotheses are not pure fantasy. “To connect them when possible, to make them fit into the general framework either by restrictions imposed on the new hypotheses or by a judicious extension of the previous frameworks is an extremely delicate task, which Langevin undertook every year in his teaching at the Collège de France. “The work of adaptation, requiring a universal knowledge of the most up-to-date physics and a transformation without betrayal of the findings of each author, was work which Langevin knew how to do with consummate skill. Thus his lessons, despite the absence of any final certificate, are attended by an extremely large and educated audience, as demonstrated by the weekly discussions that he has organised in addition to 56 Joliot’s speech, which we quote, is reproduced in La pensée et l’action, texts by Paul Langevin collected and presented by P. Labérenne, Éditions sociales, Paris.

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his lessons. The remarkable quality of the contribution made to the new theories by so many young Parisian students is almost entirely due to him and his first pupils, who have since become teachers themselves.” The International Congress of Philosophy in Bologna. – Following his studies on relativity and his lectures at the Collège de France, my father was invited to speak about “the evolution of space and time” at a congress organised by the Groupe de synthèse scientifique, whose publication was the journal Scientia. This congress was held in Bologna at the beginning of April 1911, with the participation of numerous scientists and philosophers from several countries such as Italy, France, England, Germany, etc... In his paper, which fills 24 pages of the journal Scientia57, he explained the various reasons that led to the abandonment of the notion of absolute space and time in the new conception of the theory of relativity. We have some information about how the congress was conducted thanks to a detailed letter that my father wrote to his children as was his custom; as usual, he related all the details of his trip: “... From the station, where Mr. Xavier Léon is waiting for me, I go quickly to the hotel, where many French people are gathered, almost all known to me. Then I dine and go to sleep for the previous night and for this one, in a small cold paved room, the only one remaining available in the hotel. I sleep until eight o’clock the next day. “I leave, once I am up, to look for the President of the Congress, Mr. Enriquez, who I had warned of my arrival to calm his anxiety, for I was told on my arrival that he was very worried about me58. He is very busy with the organisation of all these conferences, with everything that concerns the five hundred people who have come to the Congress, and I am running after him through the old city, which is all sunny. I go to the New University, where the sessions are held, then to the Old University, to the Archigymnase, where the offices are, and then to his house, where I arrive at the same time as a student who he has sent to run after me, because he has just arrived at the Archigymnase. I meet him there and he never lets me go, taking me to lunch at his place with his three young children, in the flat he occupies at the top of an old convent which is quite picturesque. After lunch, I return to the Archigymnase, where the room where I am to give my lecture later on overlooks a walled courtyard, surrounded by arcades and richly decorated. Another by a Frenchman, M. Bergson, is to precede mine. I first listen, then read my papers, and immediately afterwards I am taken to visit a children’s hospital, which is situated very

57 P. Langevin: The evolution of space and time, “Scientia”, Vol. 10, XIX, 3, p. 31 to 54, 1911, Bologna. Publisher: Nicola Zanichelli. 58 Just before the date of the Congress, Paul Langevin had fallen ill and his cheek was still swollen when he left; this explains the concern of the Congress President.

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close to Bologna on a hill overlooking the city and from which one has a magnificent view of it and of the whole valley under the setting sun. What one sees in this hospital is curious but leaves a painful impression; imagine a large factory, where strange machines are mechanically driven, to each of which is attached a stunted or ill-formed child, to whom the machine makes movements intended to cure him. “All this in a huge hall, from which I quickly escape to go outside to enjoy the glowing and peaceful panorama. There I meet some Italian professors, with whom I talk at length about the things I had just said in my lecture, and with whom I go back down to Bologna, just in time to attend the great dinner given by the mayor. The Italian food served there is very good; the speeches given afterwards are very beautiful, and all this takes us to eleven o’clock at night. I am at table with a deputy mayor, an engineer, who tells me some very nice things about the immense works of draining, of removing the water, which are being undertaken in the valley to transform into cultivation the marshes which extend below the level of the rivers, over tens of thousands of hectares. You really get the impression that these people are working hard. “I then meet a German professor, Max Abraham, who teaches in Milan and who suggests that the next morning I go with him to Venice, stopping at Padua, Padova as the Italians say, to see Levi-Civita, a scientist who I very much wish to meet. He also tells me that Venice in the sun is a delightful thing, and I am tempted, although I am in a hurry to get back to Paris and my cheek is bothering me...” Paul Langevin’s fame was already so great in 1911 that he was invited to two more assemblies during the same year. The first of these was the Karlsruhe Congress, which opened on 22 September and at which the new physics, in particular the theory of magnetism, was discussed. In a letter dated 24 September, my father gave us his impressions: “My beloved children, “I have been here since the evening before last and I have not yet been able to find a moment to write to you, so many invitations and kindnesses are surrounding me here. The great personage with whom I am staying, who is Grand Master of the Court of the Grand Duchy of Baden and Minister at the same time, had me taken from the station by his carriage: a large gallant lackey came to ask me, as I was getting off the train and looking after my luggage, if I was not myself, – and on my answering in the affirmative, led me to the carriage, to the coachman who was also gallant, to bring me here, to a beautiful house, where I have two rooms: a large room and a small sitting-room. It is from here that I am writing to you today, having a moment of rest before going to tea with my hosts. It’s Sunday, and the congress gives us a little freedom...

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“There are four French physicists here, Messrs Cotton, Perrin, Weiss and me, in the midst of innumerable (that means at least two hundred) German physicists, who are, however, very kind to us: we drink a lot of beer, which is very good, and we talk a lot, about very interesting things. It seems that the friendliness is exaggerated because of the recent difficulties with Germany59, which are more or less over now and which were never very popular among the German people. The people here were very well aware of the brutality of their intervention, which was intended to annoy us without any provocation on our part, and they seem eager to erase the painful impression these things may have made on us...” Shortly afterwards, on 19  October, a discussion meeting was held at the French Philosophical Society, where Paul Langevin was asked to give the main presentation on the following theme Time, space and causality in modern physics. In his very important contribution, which occupies 25 pages of the Bulletin de la Société française de philosophie of January 1912, Paul Langevin set out to indicate to philosophers the new facts which have obliged physicists to modify the usual conceptions of space and time, as imposed by the laws of classical mechanics and the conviction that these laws made it possible to explain these phenomena. Several speakers contributed to the discussion that followed, among them the philosopher Abel Rey, Jean Perrin, and, through the mouth of M.  Milhaud, the supporters of classical physics opposed to the theory of relativity. Paul Langevin stood out as a world-renowned physicist.

The first Solvay Congress (October-November 1911) A year before the foundation of the Solvay International Institute of Physics60, on the initiative of the wealthy industrialist Ernest Solvay, a first Council of Physics was convened in Brussels, where the main scientists from all over the world met. Among them were Lorentz, Madame Curie, Brillouin, de Broglie, Einstein, Nernst, Jean Perrin, Henri Poincaré, Planck, Rutherford, Sommerfeld and my father.

59 * This refers to the incidents in Agadir. 60 For more details on the Solvay Institute of Physics and the successive Solvay Congresses, see: Paul Langevin et les Congrès de Physique Solvay, “La pensée” no. 129, pp. 3 to 32 and no. 130, pp. 89 to 104, October and December 1966. - Ernest Solvay occupies a special place among the great industrialists of Belgian history; inventor, businessman, politician, he was also a wise and generous patron of science.

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FIGURE 5. The first Solvay Physics Council, Brussels 1911. Left to right. Back row: Goldschmidt, Planck, Rubens, Sommerfeld, Lindemann, de Broglie, Knudsen, Hasenöhrl, Hostelet, Hertzen, Jeans, Rutherford, Kamerlingh Onnes, Einstein, Langevin. Front row: Nernst, Brillouin, Solvay, Lorentz, Warburg, Perrin, Wein, Mme Curie, Poincaré.

This first Solvay Council tackled the development of the theory of radiation and quanta and worked from 29 October to 3 November 1911. Paul Langevin presented a report “On the kinetic theory of magnetism and magnetons”, in which he reviewed recent theories of magnetism and completed his theory of 1905. In his notice on the life and work of Paul Langevin (1947), Louis de Broglie indicated: “Paul Langevin was also one of the first, at the Solvay Congress of 1911, to notice that the introduction of quanta into the theory of the atom led to the consideration of an elementary magnetic moment. This unit of magnetic moment was later named, when Bohr had developed his quantum theory of the atom, the Bohr magneton... “... The theories of diamagnetism and paramagnetism are major achievements, which are of great importance in themselves. But they have also had a great influence on the progress of physics, because they have suggested to other authors to use their methods for the study of problems related to those examined by Langevin.”

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On Marcel Brillouin’s proposal, supported by Maurice de Broglie the honorary secretary, Paul Langevin accepted to be appointed assistant secretary of the Council. This meant a lot of work for him to finalise the publication containing the twelve papers that had been presented to this Council, in addition to the long discussions. The volume, published under the title Radiation Theory and Quanta, greatly helped physicists of the time to keep abreast of the rapid progress of their science.

Elections to the Royal Society of Sciences in Göttingen (1912) This election has a rather amusing history; it was the result of criticism by Paul Langevin of a paper by the German physicist W. Voigt, professor at the University of Göttingen who was very well known for his theoretical work, notably on piezoelectricity. On receiving the letter of criticism, Professor Voigt, instead of getting angry, published it in the very journal where he had written his article on molecular orientation61. He did even more, kindly inviting my father to spend an internship at the University of Göttingen to develop his theories on magnetism and molecular orientation and he had him elected a member of the Royal Society of Göttingen in the same year 1912. Paul Langevin became friends with Voigt.

Scientific publications of 1912 The lecture presented to the Société française de physique on grains of electricity and electro-magnetic dynamics is a masterly presentation of the modern conception of electricity, the development of the electromagnetic theory of light and electromagnetic waves and the phenomena of electromagnetic radiation in general (Roentgen rays, Zeeman phenomenon). My father ended with a presentation of the theory of electromagnetic dynamics. The Journal de chimie physique also published the paper presented to the Société de chimie physique on the kinetic interpretation of osmotic pressure62, in which Paul Langevin tackled a tricky theoretical interpretation. These revolutionary ideas on osmotic pressure, as well as the new notions on dia- and paramagnetism, provoked vigorous discussions before being

61 Paul Langevin: On molecular orientation (Letter to M. W. Voigt). Göttingen Nachr, Heft 5, S. 589, 1912. 62 Paul Langevin: L’interprétation cinétique de la pression osmotique, Journal de chimie physique, 10, p. 524 and p. 527, 1912.

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accepted. It took several discussion sessions, from June to November, to reach an agreement.

Work from 1912 to the 1914 war Paul Langevin gave a lecture on the inertia of energy and its consequences to the Société française de physique on 26 March 1913. In this important text, the differences between the atomic weights of the elements and the integer multiples of that of hydrogen are explained by the energy of condensation. This phenomenon is the basis for the operation of the H-bomb. Paul Langevin had not even considered the possibility of this use, since he says in his 1934 work note, p. 68: “It does not seem likely that we could ever cause or control such reactions, but it is probable that the solar furnace is fed by them. Jean Perrin has pointed out that the transformation of a mass of hydrogen equal to that of the sun into helium or other elements would be sufficient to sustain the sun’s present radiation for more than a hundred billion years.” The considerations that Paul Langevin then developed allow us to glimpse the possibility of greater energy than that provided by the condensation of hydrogen to create other atoms, and he even envisaged the possibility of a total destruction of atoms, which would release energies about a hundred times greater. What extraordinary prospects for the use of nuclear energy such calculations opened up for the future! My father also gave a very important presentation on the physics of the discontinuous at the French Physical Society63. Louis de Broglie appreciated this work, saying that “it summed up in a beautiful overall picture all the physics of the discontinuous to which Langevin had given so much thought. He had taught this physics of the discontinuous at the Collège de France for the greatest benefit of his listeners...” The same period saw publications on unusual collisions of gaseous molecules64 and on the measurement of the valence of ions in gases65. In the latter work, Paul Langevin used a theory by his colleague Townsend of Trinity College, Cambridge, to establish a new result simplifying the measurement of valence. 63 Paul Langevin: La Physique du discontinu (27 November 1913). 64 Paul Langevin and J. J. Rey: Sur les chocs exceptionnels des molécules gazeuses, Le Radium, 10, 142, 1913. 65 Paul Langevin: Mesure de la valence des ions dans les gaz. Le Radium 10, 113, 1913.

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With an authority reinforced by all this scientific activity, translated into numerous publications, Paul Langevin contributed very actively to the discussions of the second Solvay Congress held at the Solvay Institute of Physiology in Brussels from 27 to 31 October 1913. At this congress, Paul Langevin had the pleasure of discussing the new theories with the most eminent physicists of the time, many of whom, such as Sir E.  Rutherford, Madame Pierre Curie, Jean Perrin, Pierre Weiss, J. J. Thomson and Marcel Brillouin, President H. A. Lorentz, were excellent friends of his. We can imagine the importance that the meeting in Brussels had in my father’s life by adding that he met their other eminent scientists such as W.  H.  Bragg, of Leeds, R.  W.  Wood, of Baltimore, Lord Rayleigh, for whom he had a very high esteem, Van der Waals, of Amsterdam, and finally Professor G. Gouy, of Lyons. Almost at the same time as the Council was sitting in July 1913, a young Danish scientist named Niels Bohr, a pupil of Rutherford, was creating a model of the atom with discreet orbits, so the Council was in the thick of things.

War in 1914 Anxiety about the threat No one suffered more than Paul Langevin from the anguish of the threat of war, for he knew that if war broke out it would be global and would result in terrible bloodshed. Not only was Paul Langevin an anti-war activist by then, he also shared his convictions with us, and he took my brother and me to a socialist meeting against the war in the summer of 1913 at Le Pré Saint-Gervais and where Jaurès spoke. He was already involved in the anti-war struggle before 2 August 1914. If evidence is needed, it is given by the letter-card he received from his companion in that struggle, Hubert Bourgin, on 7 April 1914: “Pension Richemont Saint-Legien-sur-Vevey (Switzerland) 6-4-1914 “Dear friend, “We have come here to rest for a few days and we hope that you yourself will be able to rest a little. “I completed all the possible actions before we left. However many people had already gone. I was unable to see General Bourgeois. “Until the campaign resumes66 in about ten days, whatever you think you can do will be welcome and I thank you again. “We send you, my dear friend, our best wishes and compliments. Signed: Hubert Bourgin”

66 This was obviously the campaign organised by the socialists, led by Jaurès, against the threat of war.

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Paul Langevin had kept an extract from Maupassant’s story Sur l’eau in his papers, which he had cut from a newspaper: “When I think only of this word, war, I am frightened as if I were speaking of witchcraft, of the Inquisition, of something distant, finished, abominable, monstrous, unnatural…” The rest of this text is sufficiently well known that it does not require reproduction here.

Langevin’s attitude to the conflict On 31 July 1914, when Villain’s revolver shots extinguished the great voice of Jean Jaurès, Paul Langevin realised that from then on there would be no further decisive obstructions to putting war plans into action67. In good faith, the pacifists, except for Romain Rolland and a small minority, thought then that it was only the Germans who were warmongers, that the impending war would rid the world of militarism and that it would be the last war. These illusions, it must be said, were shared by Paul Langevin. My father, then aged forty-two, belonged to the Territorial Reserve and was not part of the fighting troops. He was mobilised as a sergeant in a battalion of army workers in Versailles. This regrettable use of his abilities did not last long. It soon became clear in high places that a physicist of his quality could be better used than in handling packages or guarding bridges. At the instigation of his friend Painlevé (I believe), my father was transferred to the department of research and inventions for national defence; there he carried out a long series of studies on a wide variety of subjects. To illustrate the state of mind of French scientists in this period, their lack of understanding of the imperialist nature of the conflict, their eagerness to place themselves at the service of so-called national defence, we can quote the following letter from Jean Perrin and Marie Curie68 to my father: 22 January 1915 “My dear Paul, “Here we are in Dunkirk in a semi-luxurious hotel, drinking tea that is too black, at a wobbly, pedestal table. We have had various minor incidents on the road, which have only delayed us, and we expect to be in Poperinghe 67 * Jean Jaurès was a French socialist leader and scholar who was assassinated on 31 July 1914 by a fanatic who believed that his pacifism was playing into the hands of Germany. 68 Jean Perrin and Marie Curie were behind the front line with a car equipped with X-ray equipment for treating the wounded.

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tomorrow, which will be the centre of operations for car number 1. We are being well received everywhere, especially because of the presence of Madame Curie. In Chantilly we received news of the Weiss experiments69. Satisfactory results, it seems. We think that you should definitely hurry to put your idea of the acoustic method into practice. In the meantime, the Weiss method will be of service. And I hope and believe that in a few months – let’s say two – your idea, which is more profound, more precise and requires fewer adjustments, will be of much greater service. “We are going through such hard times that a man like you must be keen to render the services that are only possible for him. You can do a great deal and must do so. Fortunately, you have not been summoned to this “statistical” mobilisation that assumes that we are all identical. The “fine” mobilisation is therefore possible for you. By using your intelligence as a PHYSICIST (and your energy can be used to make some first investigations), you can do more service than a thousand sergeants, despite all the esteem I have for this honourable rank. “Seriously, it seems to me that this is your great duty today: to find ways to help us win – ALL other duties go to the second or to the 25th place! You have the chance to have your real, simple and clear duty placed before you. Go for it and neglect everything else. Sorry to “advise” you, but it is precisely because I admire your intelligence, and all the more as I admire it the more. “And on that note, goodbye for today. Of course, I would have much more to say to you. But time is too short. “Yours Signed: Jean” “Dear friend, I have read Perrin’s very convincing letter and I think that it is indeed a very nice piece of work that you have before you. I for one would be very happy if this project came to fruition; it seems that the war will still be long and painful, so we can hope to help with a wellthought-out invention. “As for my service here, I don’t yet know what else there will be to do besides testing the Perrin car: may it perform well! “I send you my best regards. Signed: M. Curie”

69 * Probably a reference to Pierre Weiss’ acoustic method for determining enemy artillery positions.

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The problem of submarine detection – Use of ultrasonic sound At the end of 1914  an engineer of Russian origin, Constantin Chilowski, submitted several invention projects to the French government, one of which concerned the production of directed ultrasonic waves and the use of these waves, by means of ultrasonic echoes, to search for submerged obstacles (submarines, underwater mines, etc.) and for other applications such as the sounding of the sea bed. Painlevé, thinking that this idea was interesting, endorsed it. Mr. Chilowski was then called to Paris and, at the beginning of 1915, he was put in touch with my father and Jean Perrin. The invention was not applicable in the form originally conceived by Chilowski. It was Paul Langevin who proposed a practical means of emitting ultrasound, and this was put to the test in March 1915 in his laboratory at ESPCI. By January 1916, very clear signals were being obtained and the Naval Ministry requested that the method should be tested at sea. With Painlevé’s agreement, then Minister of Public Instruction and Inventions, it was decided to send a Langevin mission to Toulon to ensure the continuation of the research. In February 1917 Paul Langevin came up with the fruitful idea that would finally solve the problem of first receiving and then transmitting ultrasound waves. This idea was the use of the piezoelectricity of a quartz crystal, a phenomenon that had been discovered and studied by Pierre and Jacques Curie from 1880 to 188970. “I thought,” wrote my father, “of using the piezoelectric properties of quartz, first of all for reception. The great pressure amplitude in water which corresponds to a given power in the form of elastic waves allowed me to hope that, in spite of the smallness of the piezoelectric phenomena, it would be possible, under perfect conditions, to take advantage of the remarkable properties of quartz from the point of view of its robustness and fidelity.” The results were immediately very successful, allowing signals to be received at distances of up to six kilometres. The successful use of quartz as a receiver led to an investigation in April 1917 to see whether it could not also be used for the emission of ultrasonic waves. What was needed was a quartz crystal that was exceptional in size and purity, and which could give single slices of about 100  cm2 in area and 70 Pierre Curie: Œuvres complètes published by the Société française de physique in 1908, Gauthier-Villars, Paris, pp. 10, 22, 30, 35.

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15 mm thick. This historic crystal was cut by a skilled old craftsman named Werlein. Werlein had been recommended to Paul Langevin by Pierre Curie, and had cut the first Curie quartz. One of these first quartz slices is still preserved in Paul Langevin’s laboratory at ESPCI: it is 9 cm by 11 cm and 15 mm thick. In May 1917 my father travelled to London, where he gave a lecture at the British Admiralty called the Guthrie Lecture. He let my brother Jean know of his impressions in the following letter: “Board of Invention and Research London, 24 May 1917 Victory House, Cockspur Street, S. W. “My dear Jeannot, “The crossing went smoothly, our boat was accompanied by two torpedo boats and all the passengers had to put on a life-saving device consisting of four huge pieces of cork. This made the boat look very comical. The crossing took only an hour and a half from Boulogne to Folkestone and we arrived in London after being examined, interrogated and stamped. “I gave my talk last night and it went very well. I was a bit worried about having to speak English in public for the first time, but I heard that I was very well understood. I  now have a clearer conscience and I am busy seeing what people do here as we are doing in France... “I plan to leave on Friday evening and come to Bois-le-Roi on Sunday morning... Paul Langevin” After much thought and calculation, Paul Langevin adopted the quartzsteel triplet solution, which is still one of the best solutions in use today for underwater sounding. This triplet is a real “sandwich” with the quartz in the middle sandwiched between two thick steel frames. The thicknesses are calculated so that the triplet can vibrate mechanically with a half-wave with respect to the wavelength of the ultrasound in water71. The first trials of the devices were carried out in February 1918 and gave a signalling distance of eight kilometres while, for the first time, very clear echoes were obtained from a submarine. The development of the system continued until May  1918, when the results obtained were judged to offer a practical system and the apparatus was presented for official testing before final commissioning. The tests were carried out in June and July  1918  and the results are recorded in de Broglie’s report dated 10 July 1918. The maximum distance at which an echo was obtained from a submarine was 1300 metres.

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* This should say “with respect to the frequency of the ultrasound in water”.

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Sir Ernest Rutherford, chairman of the British delegation to the Allied Conference which dealt with the problem in October 1918, said: “I should like to express on behalf of the British Mission and the British Admiralty our appreciation of the great value and importance of the research of Professor Langevin and his collaborators, which opened a new approach. “Although the idea of using ultrasonic waves in water to detect the presence of submerged objects is not new to science and had already been proposed, notably at the time of the Titanic disaster, we owe to Professor Langevin and his collaborators the first experimental proof of the production of such high-frequency waves in water and the study of their properties. “We fully recognise the great value of his research not only from the point of view of submarine warfare, but also probably to be of great assistance to future navigation. My American friends can, I believe, testify that on my trip to the United States last year with the French scientific mission, I expressed the opinion that Professor Langevin’s research was the finest of all that was carried out during the war; I still hold this opinion. “In additional to expressing my admiration on behalf of the British Mission and Admiralty for this excellent research, I should like to add a more personal note; it is a further pleasure for me to be able to praise someone who I consider as an old friend and who was my fellow-student at Cambridge in 1896.” The official report presented by the English Professor Boyle72, collaborator of the British Admiralty, shows very clearly the considerable part played by Paul Langevin in the research carried out at Harwich on ultrasound. Not only had he communicated all the details of his research to London as early as March 1917, but he had done more; he had collaborated directly with Boyle, and the project for the creation of an ultrasonic device developed by my father had been communicated to the Board of Invention and Research at the beginning of 1916. Dr.  Boyle came to Toulon for six weeks to see the precise state of Paul Langevin’s research from the end of May 1917. The apparatus built by the British Admiralty on Boyle’s instructions conformed exactly to the first type of apparatus made by Paul Langevin, as he stated in a letter to Sir Richard Paget of 3 December 1918. It is therefore well established that it was Paul Langevin himself who supervised and directed, in accordance with his own achievements, the research done at Harwich in Professor Boyle’s department.

72 * Robert Boyle was a Canadian physicist who was recruited by Rutherford to investigate ultrasonic submarine detection, starting in September 1916.

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We give all these details because of the difficulties that arose afterwards concerning the ownership of the patent on the Anglo-Saxon side, an ownership that was long contested by Paul Langevin. Paul Langevin’s priority was not really recognised until a few days before his death. It would take too long to explain the painful steps that these difficulties forced on my father. The reader should not be left with the impression that Paul Langevin was a man of money, someone who had a “vested interest”, as the saying goes. The truth is quite different. No man was more disinterested than he was and he had no love of money. As soon as he had a little, he spent it with pleasure; he loved to give gifts to his family or friends, he was incapable of resisting a sponger and as the members of this honourable guild of hangers-on communicated to each other, my father assembled a very large number of lawyers. Precise facts demonstrate his total disinterestedness. I found in my father’s papers a draft from M.  Landrieu, the colleague at the Collège de France who looked after his interests, addressed to the British Admiralty, which includes a request for a lump-sum indemnity: “To fix the lump sum73, the inventors were guided by the precedent of the contract they signed with the French Navy. By this contract, the inventors received the sum of 300,000 francs (Mr.  Langevin had been mobilized and, being scrupulous, did not want to receive his share)74.” While discussions went on about the recognition of his patents and in spite of the war ending, Paul Langevin continued to perfect his ultrasonic apparatus in Toulon harbour in collaboration with the Navy. He tackled the problem of shallow water sounding, so important for navigation. My father continued his work on ultrasound for a long time afterwards. As a novice physicist, I was his technical assistant in the laboratory in the rue Vauquelin. On 18 March 1924, my father wrote to me on this subject from Grenoble, where he was the guest of Professor P. Vaillant: “My dear André, “I did not foresee that my wish to take a little more rest would result in you working for the Dutch and the Americans in such poor conditions and I thank you for all the trouble you have taken. “It seems to me that the self-interested enthusiasm of these gentlemen of the S.C.A.M.75 is making them go a little too fast. I am writing to them on this subject.

73 The lump sum requested was £50,000. 74 The other share went to Mr. Chilowski. 75 * The Société de condensation et d’applications méchaniques (S.C.A.M.) was the company that won the contract to manufacture an ultrasonic echo-sounder based on Langevin’s design.

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“Your letter was waiting for me at Vaillant’s, where I arrived a short while ago after a journey that was not too tiring. “I embrace you all very tenderly. Paul Langevin” In February 1926, a new study was to be undertaken which resulted in the ultrasonic lighthouse of Calais, intended to facilitate the entry to the port in foggy weather. The detection of icebergs was one of the other applications of ultrasonics. During the war, Paul Langevin was also involved in experimental ballistic studies, the development of a recoilless cannon and problems of military telegraphy, etc.

Between the two world wars: scholar and activist After the war my father came home to us, in the interests of the children. We lived at 10 bis boulevard de Port-Royal.

Courses at the Collège de France Paul Langevin returned to teaching at the Collège de France in April 1918. From the school year 1919-20 onwards he began to explain the various aspects of the theory of relativity. The title of his 1919 lecture was: “The successive aspects and experimental verifications of the principle of relativity”. It was an act of courage, for my father was well aware that, with such a programme, he was about to launch one of the bitterest battles that science had ever known. Indeed, there was still a strong hostility against such new ideas in scientific circles. My father’s work made him increasingly well known. His help was sought from all sides. Here, for example, is the letter from Henri Lebesgue on 20 November 1919: “My dear friend, “It is disgusting that you are not a member of the French Mathematical Society. Apart from general physics, you are interested in Mathematical Physics, which is mathematics. The proof of this lies in the problems that you posed to Montel and me at the beginning of the war, which you quickly solved because it amused you. “As such, you belong to us, and you are even a special and very rare species in the genre of mathematicians. And if, as the allies have requested, a mathematical congress is to be held on September 20 in Strasbourg, it will have to be you who represents the aforementioned Mathematical Physics, otherwise our collection of mathematicians would be woefully incomplete. “That you are from the Société math. is so obvious that I almost intended to introduce you automatically. But in order to be correct to the point of ridicule, I authorise you to write to me that you thank me for doing so...” In the following year (1920-21), my father’s lecture on “Experimental verifications of the principle of relativity and the theory of gravitation” was of vital relevance.

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On 29  May 1919, Einstein’s theory of gravitation was verified during a total eclipse of the sun, by measuring the deflection of the light from stars as it passed near the sun. Measurements by Grommelin in Brazil and Eddington on an island in the Gulf of Guinea confirmed the theory. The following year’s lecture (1921-22) was again devoted to the same questions, its title being: “The applications of the principle of relativity to the theories of gravitation and electromagnetism”.

Einstein’s lectures in Paris But Paul Langevin did not limit himself that year to giving his lecture and leading the usual scientific discussions that accompanied his talks. He had the courage to organise, not without difficulty, a series of lectures on relativity by Einstein at the Collège de France. After much lobbying, he succeeded in having Einstein invited as a foreign guest in 1922 by the G. Michonis Foundation. It was not the first time that Paul Langevin insisted that Einstein could come and explain relativity at the Collège de France. As early as 1911, he had suggested to the professorial assembly to “add M. Lorentz, from Leiden, and M.  Einstein, from Prague” to the list of foreign speakers. Lorentz, a Nobel Prize winner who had been a precursor of Einstein in the field of relativity, was invited first and gave his lectures in November 1912. On 16 November 1913, my father submitted a proposal for Einstein. It was accepted. The assembly decided to invite the German scientist to give the Michonis lectures in 1914. The declaration of war naturally prevented the completion of this project, which had received Einstein’s support. When, in 1922, Paul Langevin got Einstein to come to Paris, a campaign of hatred was unleashed against my father, fuelled by hard-line nationalists and the people of Action française. It was claimed that his efforts for international détente caused crime. He had written to Einstein on 18 February: “It is in the interest of science that relations be re-established between Germanspeaking scientists and us. You can help with this better than anyone else, you will be doing a great service to your colleagues in Germany and France and, above all, to our common ideal by accepting. Entry tickets had to be distributed to prevent the lectures of the Berlin University professor from being disrupted. Contrary to what Le Temps said on 2 April 1922, during Einstein’s first lecture, the room was not occupied by “privileged” people, but by those who usually listened to the College course, physicists such as Madame Curie, mathematicians such as Painlevé, astronomers and philosophers such as Bergson76.

76 Probably the only fashionable ladies were the Countess de Noailles, Madame Singer-Polignac, who had made generous donations to the experimental physics laboratory of the College, and the Countess Greffülhe, who gave similar patronage.

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The first meeting took place without incident on Friday 31  March at 5  pm. By 4  pm the 350-seat amphitheatre was full and the crowd even spilled out into the corridors. For a description of this session, it is best to refer to the article written at the time by Charles Nordmann77, astronomer at the Paris Observatory, who was additionally a scientific journalist and one of the first advocates of relativity: “It is undoubtedly unprecedented that Einstein’s recent presentation of his work at the Collège de France was followed by discussions. The famous physicist made himself available to this with inexhaustible patience. One could sense in him the desire to leave no misunderstanding in the shade, to ignore none of the objections, and on the contrary to provoke them in order to better embrace them face to face and agitate them. “... In Paris, Einstein was not content to speak didactically ‘ex cathedra’. He resolutely engaged in controversy, responding publicly, in the now famous discussion sessions, to everything that was objected to or asked of him by some of the most eminent representatives of science. “… The merit, which is not small, of having arranged these sessions, which are now famous, belongs above all to M.  Langevin, professor of experimental physics at the Collège de France, from whose suggestion Einstein was invited to Paris. It was M.  Langevin who set the agenda for the few meetings where so many subjects were to be discussed78. With a hand that was both firm and discreet, he steered the discussions, preventing the debate from going astray, when necessary setting boundaries to the exact position of the adversaries by a word that was always timely, taking part in the battle himself on rare but decisive occasions, rescuing those who were not very seriously wounded... or giving the coup de grâce to those whose condition was so desperate that it was important to cut short unnecessary suffering. Finally, it was he who played the indispensable and difficult role with Einstein, the role of the intellectual Pylades, the knowledgeable “prompter”, whose vocabulary and acute knowledge of the subject are never at a loss. “From time to time, Einstein leans towards Mr. Langevin, who is seated to his left and a little behind, to get the necessary word, the French word that he has difficulty, as he puts it, “extracting from his throat”. 77 Ch. Nordmann explained and discussed Einstein’s theory, Revue des deuxmondes, 42nd  year – 7th period, t.  9, 1  May 1922, pp.  129 to 166. I have found a copy of this journal where, on page  129, there is the following dedication: To Professor Paul Langevin with my affectionate gratitude and admiration. Signed: Ch. Nordmann. 78 There were five sessions in all, some of which were entirely devoted to discussion. The discussion sessions took place on 3 April, 5 April and 7 April 1922 in the physics amphitheatre of the Collège de France, which was even more cramped than the large amphitheatre.

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Sometimes it is an English word that comes to his lips, and I hear him murmur “assumption”, while Mr. Langevin softly whispers hypothèse... And then, as if to soften the sternness of the speech, whenever a word does not come easily, Einstein smiles, waiting for Mr. Langevin to deliver the desired term.” In the course of the discussions, Einstein paid tribute to the beauty of the work that had led Paul Langevin to establish the formulae of the new dynamics by simply starting from general relativity and the principle of conservation of energy. There followed an extremely brilliant and lively discussion in which Hadamard, Painlevé and Brillouin took part. After a while, Einstein raised his hand “like a schoolboy asking for a favour from teacher” and said softly: “May I say one little thing?” Einstein spoke into the restored silence, and within a few minutes everything was clear. In addition to these sessions of the College, another discussion was organised at the Sorbonne, on Thursday 6 April, by the Société française de philosophie. Again, this is what Nordmann says about it79: “This session was equal in interest to the physico-mathematical controversies of the Collège de France. “The philosophers had already had occasion to discuss the theory of relativity, notably with M. Langevin, “apostle of the new gospel”, but they were particularly numerous and eager at this meeting where the discussion was to take place “in the presence of the monster himself. “After an initial address by the President of the Society, Mr.  Xavier Léon, the debate was opened by a profound and remarkable presentation by Mr. Langevin, which could have been entitled: “Why philosophers should be interested in the theory of relativity”. The learned physicist described with masterly clarity all that, from the methodological and epistemological point of view, constitutes the strength and the seduction of the Einsteinian work... “The discussion that followed, in which several mathematicians took part, made it clear that, logically, the whole relativist doctrine is coherent and completely free of internal contradiction. This was already implicit from the discussion at the Collège de France.” In his memoirs of my father, which he kindly sent to me, Léon Brillouin recalls the feverish atmosphere during Einstein’s stay in Paris: “All the journalists, the onlookers and the snobs wanted to see Einstein and get a signature from him. Langevin, knowing his horror of publicity, 79 In the same article of the Revue des Deux-Mondes from which we have already given extracts above.

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had found a way to lodge him with friends; nobody knew this address, where he could find rest. I remember, one evening, a reception at the Borels’ house. At one point  I saw Einstein, Langevin and Paul Valéry in animated conversation. I approached; Valéry explained that he always carried a small notebook with him: “You can have an idea, suddenly, anywhere, in the street, in the theatre or on a walk. You have to write it down immediately, otherwise you risk forgetting it”. And Valéry gave us, of course, those fortuitous and often outstanding ideas that he later published. “You, Einstein, should do the same!” “Oh, me, replied Einstein, after a moment’s hesitation... It’s so rare that I have an idea! ” Einstein relaxed and became light-hearted among friends. It was at Langevin’s, I think, that I heard him complain about an “admirer” who had irritated him so much, during a dinner with German friends: I have read all your books, they’re wonderful! I have understood everything! Just tell me: what is the difference between concrete and concave? – It’s very simple, it’s exactly the same as between Gustav and Gasthof80!” After the College sessions, the friendship between the two physicists grew considerably stronger. From that date onwards, they never stopped writing to each other, except during the war of 1939 and the occupation of France by Hitler’s troops.

The work of spreading the theory of relativity Paul Langevin was enthusiastic about the beauty of the new theory and the progress it would make in physics, and worked to spread it throughout the world. Not only did he give a brilliant lecture to students in the presence of Einstein on 30 March 1922, entitled “The General Aspect of the Theory of Relativity”, a very complete presentation which was made accessible to the general public without distorting anything, but he also published a book entitled “The Principle of Relativity” 81. Paul Langevin not only devoted much of his efforts to developing and publicising relativity in France, but also undertook lecture tours, which took him to Zurich, where Einstein had studied at the Polytechnic (18961900). Following these lectures, an article by E.  Bovet appeared in the Swiss bi-weekly magazine Wissen und Leben (Knowledge and Life) on 1 June 1922, paying tribute to Einstein’s qualities. Bovet paid tribute to his ability as a scientist and above all as a teacher, to his clarity of explanation and to the elegance of his calculations. Mr. Bovet expressed all the

80 81

Gasthof hotel. P. Langevin: Le principe de relativité, Étienne Chiron, Paris 1922.

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“satisfaction one feels when hearing from a great scientist a truth which one knows to be new”. A little later, in 1928, Paul Langevin presented a paper at the International Meeting of Physical Chemistry, held in Paris from 8 to 12 October, entitled “The new mechanics and chemistry” 82. In 1929, the results of his reflections were set out in a lecture he gave at the University of Tiflis on “The structure of atoms and the origin of solar heat” 83. On 10 June 1931, at the General Assembly of the French Astronomical Society, my father spoke on “Einstein’s work and astronomy” 84 to a large audience of non-specialists, who greeted his talk with extended applause. He praised the great German scholar as follows: “It is impossible to know this man without loving him as he deserves. You know that Einstein will rank high in the history of science, in the history of physics in our time. He is and will remain one of those great stars in the sky of humanity. It is difficult to say whether he will be as great or greater than Newton, because his contribution to science has penetrated more deeply into the structure of the fundamental notions of the human mind. “It is very dear to me to first recall his face, reminding you that Einstein belongs to the Jewish people who have given so many great men to humanity... When you look at this man of about 50 years of age, when you see his face expressing not only intelligence, as his work demonstrates, but also finesse, kindness, courage, serenity, when you look at that crown of curly hair, now white, which forms a kind of halo, you feel immediately drawn to him, and you cannot know him without becoming deeply attached to him: independently of his qualities as a great man and a genius and if this word can be legitimately used it is in his case, he has those elements of goodness, courage and, simplicity which characterize a great heart and a great mind. “No human misery is foreign to him; he is never called upon in vain when a cause needs to be defended; I know of many examples, and in particular one that is very recent; I received a letter from him the day before yesterday concerning a case that he asked me to defend with him. “Alongside his goodness I say he has courage. You know what his attitude was during the war85, an attitude that earned him, in the Germany where he 82 Les nouvelles mécaniques et la chimie, published in L’activation et la structure des molécules, p. 550, Presses Universitaires, Paris 1929. 83 P. Langevin: Two lectures at the University of Tiflis. Bulletin of the University of Tiflis, No. 10, 22 November 1929, Tiflis. 84 P.  Langevin: Einstein’s work and astronomy, L’astronomie, 45e  year, p.  277, July 1931. 85 Einstein had refused to sign the famous manifesto of German intellectuals, the so-called “Manifesto of the 93”, which he rightly called “a capitulation of German intellectual independence”.

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lived and had taught for a short time, much opposition and many difficulties. The very fact that in 1922 he came to France earned him new animosities... “So he came to Paris: you know the difficulties to which he was then exposed for having done what was then an act of courage; he had to accept temporarily a situation at the University of Leiden, not only because of the misery into which he had fallen at a time when the marks he received for his salary as a professor were no longer of any value, but also because it was very uncertain whether there was any danger for him remaining in Germany. Since then, his pacifist convictions have remained very strong and he is not afraid to declare them whenever he feels it is useful to do so. “Einstein has goodness, he has courage and he has simplicity, though I can’t tell you how and why. You have to know him, you have to live with him to see that. “This human appearance that I have just described is completed by the fact that in the very shape of his mind Einstein combines courage and intelligence; he has these two qualities, but he also has intellectual courage, which is precisely the characteristic of his genius. He looks difficulties in the face, he does not let himself be impressed by habits of mind, fears or prejudices. “It is this disposition that has enabled him to accomplish such a great work and to introduce such a profound upheaval into the whole of our description of the world.” Moreover, my father’s interest in Einstein’s work was such that he nominated this brilliant physicist for the Nobel Prize in January  1922. His proposal had a definite effect, as he received the following answer from the Secretary of the Nobel Committees of the Royal Academy of Sciences: “Nobel Committee for Physics of Stockholm, 23 January 1922 the Royal Academy of Sciences in Stockholm “Mr Langevin, Professor at the Collège de France Paris “Your honoured letter in which you propose Professor A. Einstein, Berlin, for the Nobel Prize in Physics in 1922 has been received by the Nobel Committee for Physics of the Royal Academy of Sciences, which will consider it in accordance with the statutes and regulations of the Nobel Foundation. “Yours sincerely Wilh. Palmœr” In 1922, Einstein was awarded the 1921 Nobel Prize in Physics, which had not been awarded due to a lack of candidates.

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Paul Langevin and the Journal de physique Another of my father’s important contributions to the development of physics in France was to support physicists by renewing one of their main means of expression, the Journal de physique. During the 1914 war, scientific output was almost completely interrupted. What was left of it had been put to the service of national defence. It was therefore impossible to publish the results and this resulted in a considerable reduction in the activity of our scientific journals. Moreover, the disappearance and dispersion of some of their subscribers and staff, and the increase in prices had put two of the most important French physics journals, the Journal de physique and Radium, in a difficult situation. A reorganisation was necessary. A group of physicists led by Paul Langevin undertook it in 1919-1920, combining the two journals into one and setting up a new publishing company, whose shares – which, of course, never yielded any interest – were subscribed to by those physicists of the time who had some money. Paul Langevin was entrusted with the scientific direction of the new Journal de physique et le radium, whose editorial staff he housed at the ESPCI. Assisted by the Editorial Secretary, G.  Hache, his former student and assistant, and by L. Brillouin, then by L. Bloch for the bibliographical analyses, he started the publication. The disclaimer he wrote at the beginning of the first issue (JanuaryJuly 1920) said: “... In the mind of those who will have to write for the new mouthpiece, its essential aim will be to represent, in these various events, the activity of French-speaking physicists and of foreigners who will want to honour us with their collaboration, and to provide them, from the bibliographical point of view, with the abundant and rapid information that they need...” Paul Langevin devoted a great deal of his time and effort to the journal. He had the thankless role of deciding whether to accept or reject the submitted manuscripts. They were all sent to him, and he himself studied many of those whose inclusion posed a problem. He took the same scrupulous care in this examination as he did in all his work, and his insight saved some authors – some of whom have since gone on to successful scientific careers – from erroneous publications which they would later have deeply regretted. Under his leadership, the journal grew rapidly and became one of the two or three leading journals of its kind in the world. The 1938 volume was six times larger than that of 1920, and the standard of the published papers was first class. The theses of the brilliant young French physicists of the interwar period appeared in another important French periodical,

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the Annales de physique: those of L. de Broglie, L. Brillouin, I. Curie in 1925, that of P. Auger in 1926; R. Lucas in 1927, F. Perrin in 1929, L. Néel in 1932, A.  Kastler in 1936, but all the rest of their work appeared in the Journal de physique. For example, the fundamental paper by I. Curie and F.  Joliot on artificial radioactivity was published in 1934, as well as a number of works by physicists of the previous generation such as Marie Curie, P. Weiss, G. Friedel and others. The war in 1939  and the occupation in 1940 interrupted the journal’s development. On 30 October 1940, after Paul Langevin’s arrest by the Nazi authorities, the Vichy government demanded that he be replaced as editor of the journal. At the beginning of the first issue of 1941, the new director saw fit to include an editorial in which, after having denigrated the activity of his predecessor, he flattered himself that he would “return to the Journal the activity it should have, and make it fulfil its role”. In fact, the difficulties of the occupation combined with the disapproval raised by the attitude of the new management led to a collapse. Fortunately, a team of physicists, led by Ch. Fabry, saved its honour by creating a new review in Marseille, the Cahiers de physique, which took over from the journal until the end of the war. After the liberation, Langevin resumed his place at the journal. Only his death interrupted the eminent services he had rendered for more than 20 years.

Paul Langevin’s sympathy for Soviet Russia Paul Langevin had been among the first in France to be enthusiastic about the great October Revolution. Even if there were no other reasons, the mere fact that the first message from Soviet Russia was the decree on peace, written by Lenin and adopted on 8 November 1917, would have attracted my father’s sympathy. What also struck Paul Langevin was the behaviour of Lenin and the Bolsheviks towards science in general and physics in particular. Through an eyewitness, his friend the physical chemist Victor Henry, Paul Langevin learnt that in the winter of 1919, in the middle of the civil war, at the most difficult time when the inhabitants of Petrograd received only one eighth of a pound of bread per day and the houses were not even heated, the Bolsheviks created the first large research institutes of the Republic of Soviets (Institutes of Optics, Radiology, X-rays and Platinum). In frozen and barely lit Petrograd, the Bolsheviks succeed in running the first congress of Soviet physicists, thanks to the inexhaustible energy of O. Chwolson, a physicist known for his large treatise on general physics. This had the official support of Lunacharsky, then People’s Commissar for Public Education. At this congress, reports were presented by physicists with world-famous names, such as Ioffé, P. Lazarev, A. Krylov.

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How could Paul Langevin not have been enthusiastic about such an effort? He was the only one, along with Hadamard, to sign a “message of sympathy to the Russian intellectuals”, which Victor Henry had presented to French academics. Moreover, Giovanni Malfitano, a good friend of my father’s, a researcher at the Pasteur Institute and an enthusiastic Garibaldian, was in Russia at the beginning of the October Revolution. He described to him the strong feelings that had taken hold of the Russian people and the magnificent achievements that had been started there. A few weeks later, on 26 October 1919, a manifesto of intellectuals was published in L’Humanité under the title: “Against the blockade”: “... A great crime is being committed against men, a crime that can do no good for anyone. We refuse to be associated with it, even if only by our silence. We protest with all the strength of our hearts and minds against an act that is unworthy of the human conscience in general and the traditions of our country in particular.” This petition was immediately signed by 72 intellectuals including, of course, the great Anatole France, Henri Barbusse, Séverine, Charles Vildrac, Romain Rolland, Victor Basch, Émile Kahn, Ferdinand Buisson, Maurice Ravel and others. Paul Langevin joined enthusiastically.

Standing with the working class In May 1920, during the great strikes, Paul Langevin’s civic courage was again on display. The militant bosses reacted; they created organisations of strike-breakers, the Civic League and others. The government pretended to close the universities and the large écoles to allow the League to throw the student youth into the battle alongside trusted men. The majority of the students of the ESPCI were persuaded by the propaganda of the bourgeoisie, so the director at the time, the chemist Albin Haller, considered closing the school to leave the young people to their activity as strike-breakers. A minority of students, including Guy Emschwiller, a future professor at the school, sent a letter of protest to Paul Langevin, the Director of Studies at the time, asking him to ensure that the students’ right to intellectual work was respected without any reactionary constraints. His letter of 17 May to L’Humanité86 was considered by all as a courageous protest against those who wanted to turn the youth of the Latin Quarter into shock troops against the people.

86

See Labérenne’s book, La Pensée et l’Action, p. 256.

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Paul Langevin also participated in the campaign for the Black Sea sailors, who had mutinied in 1919 against the illegal military intervention of the Anglo-French forces and against the aggression of Soviet Russia. By relying on the working class, the Committee for the Defence of Seamen enlarged the demonstrations. Thus, in a big meeting at the Salle Wagram in 1920, Paul Langevin praised the civic courage of the mutineers. He was welcomed at the door of the hall by the president of the meeting, Daniel Renoult. It took my father a high sense of responsibility to speak on this occasion, together with Ferdinand Buisson, then president of the League of Human Rights, and Professor Auguste Prenant. That meeting at the Salle Wagram left me with an indelible memory, because it was the first meeting where I heard my father speak. While it is true that he was already well used to teaching, it was the first time he had to make a political speech. So he began in a timid voice, visibly moved by having to speak to a crowd of listeners about something other than scientific matters. Paul Langevin said of the mutineers: “... To understand and judge their actions, we must make the effort of intelligent thought of which I spoke earlier. One must know these admirable naval people, as I have had the opportunity to do personally87, to know what intelligence and devotion they bring, at all levels of the hierarchy, to the execution of orders that they are always ready to understand, under leaders that they are always ready to love. “You know what their lives were like during the war, a life of fatigue and danger endured to the end without a day of weakness. “Three months after the armistice, when they could consider their superhuman work accomplished, instead of returning home like their comrades in the trenches, they left for the East, in particularly harsh material conditions that were admitted by their leaders, who alone could have been kind to them... “The moral situation was even worse. The war was over and no legal reason could be given for sending them to fight in a country in which events were taking place that were not well known, and which appeared to many of them as an obscured dawn, and perhaps all the more beautiful for that, of a long-awaited new day. “Who of us does not remember the emotions of the beginning of the Russian Revolution, that first realization of the hope of universal liberation, for which so many young men had already freely and almost joyfully consented to die, that first collapse of political despotism?” The campaign thus launched in front of a packed Salle Wagram was not to be long in producing its effects. A first, a very partial amnesty, 87 Paul Langevin relates here the impressions he received during his years of work at the Toulon Arsenal.

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voted in October 1919, affected those sentenced to military imprisonment (thus without degradation). Great debates on the amnesty took place in the Chamber. Anatole France, always ready to defend just causes, entered the struggle with his open letter, published in L’Humanité, in which he said, among other things: “Ah, blind rulers who do not know how to govern, your cruel and stupid obstinacy has produced its necessary effect.” The result of this strengthened campaign was the passing of the amnesty law of 12 July 1922, which led to the release of all Black Sea convicts except André Marty, who was not released from prison until 17 July 1923. Paul Langevin’s action aroused the anger of his political opponents: Admiral Schwerer, a man of Action française, intervened to try to have him dismissed from his post as examiner at the Naval School; the reactionaries of the Academy of Sciences defeated his candidacy for the Institute in June and November 1923. The Black Sea campaign sealed his break with the bourgeoisie, which would never forgive him for his loyalty to his working-class origins. This same loyalty inspired my father to make a long effort for solidarity with the Russia of the Soviets, and the resumption of relations with the Republic of the Soviets.

For Franco-Soviet friendship Paul Langevin was among the first members of the association Les Amitiés franco-russes, founded in September 1924. He gave at least one lecture on “Intellectual Activity and Russia”. I have found, in his own handwriting, the preliminary notes for and the incomplete text of this lecture. In 1955, I paid tribute to it at the Academy of Sciences of the USSR. My father said: “... It is certain that from the intellectual point of view, Europe is not itself when Russia is absent, and that Russian collaboration, which began two centuries ago, is becoming increasingly important as the unlimited resources of this young country, which is eagerly learning, are better exploited. “We need it as it needs us. Of all the foreign writers, it is those from Russia who for 50 years have exercised the greatest influence here, both from a literary and a moral point of view. You also know how much new inspiration modern music and theatre owe to its admirable artists. The services rendered by Russian science in general are less well known, and I would like to emphasise them by seeking to highlight the essential features of a few examples.

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“We find here the same qualities that are so brilliantly displayed in the fields of literature and art. “The Russian spirit possesses above all those youthful characteristics of freshness, spontaneity and richness of imagination. It gives it the power to go outside itself, as it were, to create and bring to life a world like that of Tolstoy’s novels, in an atmosphere so very human. This primordial quality creates originality, which leads to the discovery of new forms of thought and expression... “It also makes great scholars, by enabling them to free themselves from formulaic learning, as Lobachevsky did, in order to prepare or construct a more faithful image of reality, to glimpse, as Metchnikoff did, the essential and simple facts beyond the complex appearance, and to reach, as we see for Mendeleyev or Lomonossov, truly prophetic intuitions...” The work that Paul Langevin had begun within the Society of FrancoRussian Friendship, he continued stubbornly within the Cercle de la Russie neuve. He was one of its founders with Madame Duchêne and a few others, such as his friend the painter Grandjouan. He also fought very effectively for the resumption of scientific relations with Soviet Russia within the French Physical Society and the ChemicalPhysical Society. Thanks to the action of the French Committee for Scientific Relations with the Union of Soviet Republics, of which he was the leader with Sylvain Lévi and André Mazon88, he arranged an invitation from the Collège de France for a mission of Soviet scientists to come and give lectures in France. He was especially involved in organising two of these meetings, which took place in November 1929, those of Vladimir Mitkevich and Alexander Froumkin. Paul Langevin was in contact with many other Soviet scientists. The following letter from the famous physicist Landau, known in particular for his theoretical work on the properties of helium II, and one of his colleagues at the USSR Academy of Sciences, Georg Reimer, is worth mentioning. The letter dates from the time of the Spanish Civil War: “Academy of Sciences of the USSR Section for Mathematical and Natural Sciences Physics group “Professor, “We have written a small book together on the theory of relativity and we wish to publish it in French. We are sending you, together with this letter,

88 André Mazon wrote to me much later, in 1951: “How often I heard your father protest against the pharisaism of intellectuals who profess to abstain from action! The men of our generation will recognise him as a rare example of high intellect mixed with steadfast action and of action raised to the level of ideas.

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the English manuscript of this book. We would be very grateful if you could help us with the publication. If our book is published in French, we would like all the fees received by us to be donated to a public organisation supporting the Spanish Republic. Yours very truly: L. Landau – Georg Reimer” Through Giovanni Malfitano, Paul Langevin had become friends with a very sympathetic Soviet biologist, Leon Tarasyevich, who we saw very often at my father’s house. This man was very cultured and understood and spoke French very well. His correspondence attests to the concern he had for my father’s health. The following letter is proof of this: “In a carriage on the way to Moscow       28 January 1926 “Dear Mr Langevin, “The journey is over, very interesting and pleasant, but also very tiring. Before leaving Berlin, I saw Kraus (a premier German clinician). He found some cardiovascular and nervous phenomena, but his prescription is interesting and I am passing it on to you because it would also do you good (I remember our visit to Teissier): change your way of life (this is what is generally necessary, according to him, for men from 55 years of age onwards, and for the overworked especially). Every day before dinner or after lunch, a one-hour rest. Every week, an English weekend. Every year, take two four-week (better still six) holidays and a complete rest, seeing as few people as possible, etc. Without this, you risk losing the ability and the taste for work, and your morale too. Excuse me for writing this, but I believe that something like this would at least be very useful to you. “My best compliments to all your family. Yours faithfully,” L. Tarasyevich “Excuse my writing, the train is moving and shaking my hand.” At this time my father also became acquainted with Lazarev, a physicist known for his work on the nuclear paramagnetism of solid hydrogen, and with Ioffé, another leading physicist, renowned for his studies of magnetic anomalies and his research on semiconductors before 1912. I will leave out Kapitza, for whom Paul Langevin had a very deep friendship, but who he had already known when the Russian scientist was working in England before going to direct his Research Institute of Applied Physics at the USSR Academy of Sciences in Moscow. In 1965, Piotr L. Kapitza published a booklet in homage to four physicists he considered to be the greatest: Lomonossov, Franklin, Rutherford and Langevin89. The chapter of this booklet devoted to Paul Langevin is 89 P. L. Kapitza: Life for Science (Lomonossov, Franklin, Rutherford, Langevin), pp. 54-62, State Publishing House “Naouka”, Moscow, 1965.

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entitled: “Physicist and man of social action: Paul Langevin”. The author says in particular: “Paul Langevin was not only a remarkable physicist, but also an important man of progressive social action, and a great friend of the Soviet Union... “His fundamental works are related to theoretical physics. The most important of these are related to magnetism, which are still of importance today and are considered to be classics. In acoustics, Langevin discovered the elegant method that allowed the emission of ultrasonic waves and was the first to propose the use of the piezoelectric phenomenon for the production of short-wave acoustic waves. An entire field of science and technology was immediately born on this basis. “It must be said that Paul Langevin’s influence on the development of world physics was very strong and was not limited to these two fields. Langevin was also a great teacher, he had many pupils among whom two were universally known: one of them is Louis de Broglie, the other Frédéric Joliot-Curie. “Although Langevin published relatively little, he was a very generous scientist, giving out ideas and inspiring and supporting his students. In this respect, his influence on French physics, if it is possible to quantify – and unfortunately this cannot be done exactly – is surely no less, and possibly more important than the influence of the works he published. “I met Langevin often enough and was lucky enough to win his friendship, and today I remember him extremely fondly. “We can characterise Langevin’s behaviour, I think, by a single word: he was a man entirely at the service of progress, he was progressive in science, progressive in his political opinions, progressive in his philosophical conceptions, progressive in his social activity. This progressive character is the central theme that runs through his whole life. “The culture of mankind is growing, science is advancing, social struggle is developing, our understanding of man’s relationship with the material world is deepening. Whether we like it or not, everything is always moving forward. People can be divided into three categories: some go forward and expend all their strength to advance science, culture and humanity, these are the men of progress. Others, and this is the majority, stand on the sidelines of progress, without preventing it or helping it; finally, there are men who stand back and hold back culture, these are the conservatives and also the timid without imagination. “To those who go forward, all difficulties fall away, they open new paths to progress, on them fate rains all kinds of trials. Such was Paul Langevin, and fate assigned him a series of difficult trials... He fought for progress until the end of his life, and the older he got, the more ardently he fought.

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It was this unusual trait that always amazed me about him and that aroused in me a deep sympathy and admiration.” 90

Receptions – Family life Whenever a Soviet scientist came to a congress in Paris, or simply passed through the city, my father invited him to lunch or dinner at his home, 10 bis boulevard de Port-Royal, or later, in his beautiful apartment as director of the School of Physics, at 10 rue Vauquelin. My father liked to have his whole family round him, so we often dined with the Soviet guests, my wife, my brothers and sisters and myself. We called ourselves ironically “the extras”. When the guest was not one of these Soviet scientists, but Einstein, or Blackett, or Bernal, or some other, the result was the same: all of Paul Langevin’s children dined on rue Vauquelin. If any of us declined to come, my father was upset, because he was only truly happy when he had us all around him. The dinners in the rue Vauquelin were famous for the quality of the food, the liveliness of the discussions and the liberal spirit that presided over them. Often there were English scholars at these dinners, some of whom were teetotal and most of whom did not like fermented cheese. But my father found it difficult to imagine anyone not appreciating a good bottle of Burgundy or a well-made Camembert. Naturally, he would offer foreigners a good glass of old Burgundy with the roast and a dry white wine with the hors d’oeuvres, and champagne with the ice cream. Generally, the foreigner, out of politeness, did not dare to refuse, even if he was teetotal, and he raised the glass to his lips with a hardly concealed grimace. The English scholars often did not dare to refuse the well-made cheese that my father so kindly offered, but you could see that they had difficulty in swallowing a mouthful. At these gala dinners the maid, who was trained well by my mother, followed the rules of protocol: that is, she presented the dishes first to the ladies in the places of honour, then to the other ladies. When she had finished going round the guests, she would attend to the important foreigners on my mother’s right and left, and then go round the table in order. But, preoccupied with serving the guests in the proper order, this young lady who

90 Many other tributes have been paid to Paul Langevin in the USSR. Mrs O. A. Starosselskaya-Nikitina devoted a large part of her work as a historian of science to exalting the scientific, educational and political work of my father. She spent about 10 years of her life studying the life and work of Paul Langevin, so passionate was she about his subject. We cannot list here her numerous publications from 1954 onwards, culminating in her large collection of Œuvres choisies de Paul Langevin (750 pages) and her 1962 biography Paul Langevin (316 pages).

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served very often, one might even say regularly, forgot my father, who was obliged to protest discreetly as she took the dish back to the kitchen. Paul Langevin could not think of anything that was too good or too beautiful for his children and he was ready to make any sacrifice for them. As soon as we wanted to buy something, he would “open a credit account”, as he put it, without even knowing if the expense would be too great. When my brother Jean became engaged to Edwige Grandjouan in July 1925, my father organised a large reception in Montmartre for all the friends, in a painter’s studio. At my own wedding on 16  July 1925, he made a point of organising as delicious a lunch as possible to please us. He chose the Lapérouse restaurant, which he considered to be the best in Paris. With his usual conscientiousness, he carefully studied the seating plan, seeking, as he always did in similar cases, to reconcile protocol and personal sensitivities. The meal was memorable: the menu was marked simply “Hors d’oeuvres”, but in fact twelve different hors d’oeuvres were served, changing plates after each one. The bride was so confused that she thought the meal was fully served and wanted to leave the table after the hors d’oeuvres were finished! When he was in a good mood, which was usual except before his lectures at the Collège de France, and when he was with family or good friends, my father liked to sing a song or recite verses during dessert. When he was with his family, his repertoire consisted mainly of student songs like Les quatre z’étudiants or pieces from the repertoire of Aristide Bruant and Yvette Guilbert. Often, in the evening after dinner, Paul Langevin would stay up with us until half past ten chatting and resting in a green velvet armchair from the 1900s, in which, after a hard day’s work, he would very often doze off. When we asked him if he was sleeping, he replied: “Je réfléchissais (I was thinking)”, which, however extraordinary it may seem, was not untrue, since he told us many times that in his youth he often solved problems while sleeping. Sometimes he would fall asleep without having solved a difficult problem that he had thought about for a long time and, he said, he would wake up the next morning with the solution in his mind. Paul Langevin’s tenderness for his children always made him alert and quick to alarm. When, during my military service in Tours, I was afflicted with rheumatic fever, he was very worried, as his letter of 24 January 1925 testifies. This is also valuable for the information it gives about his heavy workload: “My darling André, “We are sorry not to see you again tonight or tomorrow, and that you are unwell and bored in the infirmary or barracks. If your condition does not improve and the rheumatic pains continue, I will try to get you sent to hospital and from there to convalescence, so that we can treat you seriously here.

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“Your mother went to see Dr Janin, who is going to approach the major who is treating you through a friend he has in Tours. If that is not enough, I will find another way. It is not possible for you to hang around indefinitely between the infirmary and the company, where conditions are bad for a complete recovery. Are you better these days? Can we expect to see you next Sunday? If not, your mother and I will go to Tours... “I have to go now to rue Vauquelin, where I have called a meeting of educationalists. Tomorrow I am going to Compiègne, where I have to speak to the local section of the League of Human Rights. All this takes up a lot of my time, not to mention the hours when I am disturbed on rue Vauquelin, as I have been these days, by Americans from the north and the south, who come to visit our schools and who are terribly talkative and curious, and friendly besides. Then this morning I had a Russian Admiral, Mr. Kryloff, who has just returned from Bizerte, where he went to see the condition of Wrangel’s fleet before the Russian Government took possession of it91. He is very interested in ultrasound and I spent a large part of the morning talking to him about it. “All this doesn’t leave me much time to think and I’m getting out of the habit. I’ll have to go on strike and lock myself in, until all my work and papers are in order. “I kiss you very tenderly, my darling. Paul Langevin”

Paul Langevin in the USSR With the resumption and expansion of Franco-Soviet contacts, of which my father had been one of the main architects, the Cercle de la Russie neuve soon became the Association pour l’étude de la culture soviétique. Under Paul Langevin’s and Henri Wallon’s direction, this association played an important role in the dissemination of Marxism among French scholars. Naturally, Paul Langevin did not limit himself to gathering documentation from books on intellectual developments in the USSR. Mrs StarosselskayaNikitina included some interesting details92: “Paul Langevin’s work was well appreciated in the USSR; that is why he became a member of the Academy of Sciences in the U.S.S.R. 10 years earlier than in his homeland. Paul Langevin was elected a corresponding member of the Russian Academy of Sciences during the general assembly 91 * After the French Government recognised the USSR in 1924, this Black sea fleet was returned to the Soviets. Alexei Kryloff’s technical commission found that it was could not be repaired and it was sold as scrap. 92 Starosselskaya-Nikitina: Paul Langevin, pp. 192-196.

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of the Academy on 6 December 1924, on the proposal of A.F. Ioffé and P. P. Lazareff.” Paul Langevin was obliged to decline an invitation to Moscow in November 1926 to take part in the Congress of the Association of Russian Physicists, because he had to be in Cambridge, where physicists were meeting to celebrate the seventieth birthday of J.  J.  Thomson. He was invited again in May 1928, and this time he accepted. On 23 May, under the chairmanship of A. P. Karpinski, a meeting was held at which Paul Langevin gave a talk on the subject of radiated energy and its relation to mass. That is, he spoke on the very subject that was soon to become one of the most disturbing of our time, nuclear energy. On 24  May the following short statement by my father appeared in Pravda: “I was very happy to accept the kind invitation of the USSR Academy of Sciences, which enabled me to visit some of the universities of the USSR and to establish contact with my colleagues, several of whom are already my friends, and also to become better acquainted with the excellent scientific work which is being done here in all fields... We, in France, appreciate the work of Soviet scientists very much, as well as the favourable conditions for their work.” The Soviet scientist E.F. Gross recalls his meeting with P. Langevin: “I remember with pleasure my meeting with P. Langevin when he came to Leningrad; my teacher D. S. Rojdestvenski showed him around the Optics and Physics Institutes. During this visit they came to my laboratory. Langevin charmed me with his simplicity, his benevolence and his deep intelligence. When D. S. Rojdestvenski wanted to talk to him about my work, he stopped him and asked me to explain the subject of my experiments myself. I was very pleased with the interest that he showed in my young collaborator, who had just finished his studies at the university. One could immediately sense the strength of his intelligence and his profound understanding of physics, from the ease with which he grasped the essence of everything I explained to him that related to a field of physics that was not his own.” On 29  May, Paul Langevin gave a presentation to the Council of the Physics Section of the Association of Soviet Physical Chemists. He then visited large laboratories in both Moscow and Kharkov. Paul Langevin’s trip to the U.S.S.R. was a truly courageous act, it occurred the day after Marshal Foch had given an interview to the English Sunday newspaper ‘The Referee’ to say that armed intervention against Soviet Russia was essential. The following year, Paul Langevin accepted an invitation to visit the USSR to receive the title of honorary member of the Academy of Sciences on

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the occasion of the Academy’s 220th anniversary93. Again this time, he travelled extensively throughout the country. He even went as far as Georgia, crossing the Caucasus using the military route. He was very warmly received at the University of Tbilisi (Tiflis) and on 22 November he gave two lectures, one on “the structure of atoms and the origin of solar heat” and the other on “ultra-sound vibrations and their applications”. The rector of the University of Tbilisi offered, according to tradition, a magnificent banquet, which went on well into the night, with many toasts. After the rector’s kind words of welcome at dessert, Paul Langevin responded by expressing his admiration for the beautiful organisation of scientific work in Georgia and for the splendour of the country’s sites. Amusingly, in the course of his speech, Paul Langevin made the imprudent mistake of mentioning the strong impression that the beauty of Georgian women had made on him. Then the rector replied: “If you are willing to extend your stay in Georgia by just 24 hours, we will ensure that you have more intimate contact with the beautiful Georgian women.” Alas! Paul Langevin left Georgia on time. My father stayed in Moscow again in January 1932 on his return from China on the Trans-Siberian Railway. He again had time to make contact with his Soviet colleagues and friends. Testimonies abound on Paul Langevin’s extraordinary popularity in the U.S.S.R. Jacques Nicolle spoke about this at the ceremony for my father’s seventy-third birthday:

FIGURE 6. Paul Langevin during a trip to Moscow (winter 1932). © Bibliothèque de l’ESPCI.

“As we were talking one day with Mr. Langevin about the USSR, I told him the following anecdote: Coming from Leningrad in the summer of 1937, I arrived at midnight at the Latvian border. Two Red Guards invited me to tea and asked me for my identity papers. As I handed them my passport, there was a letter that M. Langevin had given me as an introduction

93 The list of all the foreign distinctions that were granted to my father would be long. We will limit ourselves here to the main ones: member of the Academy of Lincei in Rome (1921), member of the Academy of Sciences in Prague (1926), member of the Academy of Sciences in Bologna (1926), member of the Royal Society in London (1928), honorary member of the Academy of Sciences in Buenos Aires (1928), member of the Academy of Sciences in Copenhagen (1929). There are innumerable foreign universities that made my father an honorary doctor.

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to Professor Ioffé, a member of the USSR Academy of Sciences. As I said Langevin’s name, the two guards saluted, saying, “Professor Langevin, a very great French scientist. “I had the opportunity to talk about our great physicist with some Soviet sailors one evening on the Sevastopol express train, with a Moscow car driver and a lumberjack from the north of Russia. They all knew who he was and all had a great admiration for him. Last week, a young Soviet lieutenant, who is probably in the room tonight, told me that as a philosophy student at Moscow University he had often heard Langevin’s name mentioned, and that it would be a great joy for him to be able to tell his comrades on his return to the U.S.S.R. that he had been able to see Professor Langevin...” On that same evening of the seventy-third anniversary, my father received many congratulatory telegrams. The most moving were undoubtedly the telegrams from Moscow. The following came from Vladimir Komarov, President of the Academy of Sciences of the USSR: “The Academy of Sciences of the USSR, proud to be able to count among its members the great scientist and intrepid man that you are, sends you on the occasion of your seventy-third birthday its fraternal congratulations and warmest wishes. Stop. It is a great joy for all Soviet scientists to know that, after having given such a fine example of civic-mindedness and intellectual probity in the Resistance, you have been able to resume your scientific activity in a France that is once again free. Stop. The pact of December 10, which unites our two peoples in the common struggle to free humanity from the fascist danger, promises a fruitful collaboration of French and Soviet scientists for the progress of science and the happiness of future generations. Let us unite our efforts for the conquest of the future.” Understandably, the telegram referred mainly to Paul Langevin’s activities during the war. But his involvement with the USSR occurred much earlier than that. In 1937, Paul Langevin was a member of the Patronage Committee of the Journées françaises pour la paix et l’amitié avec l’U.R.S.S. He had agreed to present the report on the physical and chemical sciences in the USSR during these events, which took place on 23 and 24 October in the large amphitheatre of the Sorbonne. Unfortunately, my father was unable to give this talk because of his health. We shall see later that his feelings towards the USSR never changed and that his sympathy for the land of socialism only increased until his death.

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Exuberant activity To be as complete as possible and not forget anything that might seem important, we must point out that in 1927, Paul Langevin chaired the congress of the Association for the Advancement of Science in Constantine. He had insisted that all his close family accompany him. So, we left for Constantine in force. There were seven of us: my father, my mother, my brother Jean, my sister-in-law Edwige, my sister Hélène, who incidentally met her future husband, Jacques Solomon, my wife Luce and me. At the inaugural session on 13 April, Paul Langevin succeeded in transforming the opening address into a fine lecture on the history of science and a plea for peace, for the peaceful use of the results of science. Following the Congress, various excursions took place, the main one being a walk to Touggourt. All the Langevin family had chosen this excursion. We all thought that it was a must, during a trip to Algeria, to see the Sahara and its oases. So, we set off on this little desert train that looked more like a child’s toy than a trans-Saharan. We spent the night in Biskra, which already smelled of desert with the arid steppes surrounding it. The next day, in Touggourt, we were not welcomed by officials but by a beautiful sandstorm. As Luce had stayed to hand out coins to the little Arabs who were holding out their hands, we completely lost sight of the main body of visitors. We then tried to catch up with them, running in all directions. In desperation, passing a rather dodgy-looking house with a sign “To the Desert Orphans”, we had the idea of looking through a small, eye level window. Through this opening we saw my father’s moustaches, watching the “desert orphans” doing the belly dance. We were wrong to be astonished. The visitors and their president were obliged to pay a visit to the Ouled-Naïl people according to their custom... In August 1927, and again in July 1928, Paul Langevin made two lecture tours of South America, including Brazil, Argentina and Chile. From Buenos Aires, he wrote to my wife on 12 September 1928: “… I am leading a terribly hectic life here. Between Argentina, Chile and Brazil, I will have given thirty-eight lectures in two months and lunched or dined or banqueted countless times. My stomach suffers a little, but I think I’ve done some useful work, even with the chatter. “I am looking forward to seeing you all again; I have a meeting later on and I have to leave you…” Leaving for Argentina in 1927 took place under rather picturesque conditions. Initially, my father was only supposed to take my sister Madeleine, but when the “Aurigny” left for Bordeaux, my sister Hélène burst into tears and was desperate to be on the trip. The case seemed suspicious: why was a fifty-five-year-old gentleman insisting that papers for Buenos Aires be drawn

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up urgently for a sixteen-year-old girl who had no passport? The papers were obtained with great difficulty... Most of the French academics who went to lecture in South America brought back handsome fees, which the Argentinians rather disapproved of. My father used all his to invite all the academic and scientific personalities of Buenos Aires, about forty in all, to the most chic restaurant in town, the Jockey Club! The popularity of Paul Langevin in Argentina is confirmed by Henriette Psichari, who knew him in Buenos Aires and who wrote94: “… Of all the Frenchmen I met there, Paul Langevin was the one who had the most success with his lectures and especially with his personality. He dealt with scientific subjects and yet surprisingly there were crowds of people to hear him; he had a philosophical mind, and the Argentinians do not have that at all; in an elegant land he was dressed like some gentleman and he was not a socialite... It must be said that there is, in spontaneous expression, a force that breaks all prejudices, for Paul Langevin was by far the most admired in this distant land...” In December 1929, my father again gave lectures in Lisbon, Coïmbra and Porto. In January 1934, he gave a lecture at the Faculty of Sciences of the University of Madrid95. Of course, routine work was required at the ESPCI, the Collège de France, the Société de physique, the Société des ingénieurs civils, and various congresses (courses, conferences and communications) in addition to all this effort. Publications multiplied under my father’s name, in spite of his modesty and self-criticism: he never thought his works were sufficiently perfect. His “enemies”, his persecutors, were those who wanted to force him to write, those who demanded articles from him. He slowed down publication of his writings or lectures with all his might. I found in his papers a number of sheets that he had deliberately not sent back to the publishers after proofreading. The most persistent of his tormentors was Xavier Léon, who edited the Revue de métaphysique et de morale and who harassed my father with requests for articles, vehemently reminding him of his promises first by letter, then by telegram – an insistence that was often ineffective, since my father, knowing what it was all about, did not even open the envelopes! In 1937, my father helped to organise of the International Exhibition in Paris. He was particularly involved in the creation of the Palais de la découverte, which was partly his idea. This exuberant activity caused great tiredness that worried his relatives. There was hardly any time left for family life. 94 Henriette Psichari: Des jours et des hommes, Grasset, Paris, 1962, p. 104. 95 This lecture was published in the Annals of the Spanish Society of Physics and Chemistry (with some notes and additions by Dr. J. Palacios), vol. 32, p. 5, 1934.

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Participation in the Solvay Councils from 1920 to 1939 Those of us who followed his work closely knew that one of the tasks that absorbed him most was the activity of the Solvay International Institute of Physics and the holding of its periodic Councils. After the war, it took some time for international scientific relations to resume. It was only in 1927, for the fifth Council, that my father succeeded in getting Einstein and four other German scientists invited. Previously, the third Council (1921) was devoted to atoms, electrons and radiation, and the fourth Council (1924) studied “The mechanism of metallic conductivity and certain groups of phenomena which may serve to elucidate this mechanism”. It was thanks to my father that the Russian Ioffé had been invited to present a report. Paul Langevin greatly appreciated the qualities of this scientist, who had been the first to study the properties of semiconductors, whose properties, as we know, has led to the creation of transistors. Paul Langevin’s influence on the Fifth Council (1927) was very important. It had a significant affect not only on the appointment of the participants but also on the choice of the programme. Paul Langevin had to fight vigorously against the physicists and mathematicians who still did not accept the abandonment of classical physics. He had Einstein at his side in this struggle, which was to last another 10 years until the final victory of the new physics. In addition to the rapporteurs, – W.  L.  Bragg, Compton, C.  T. R.  Wilson, L.  de Broglie, Einstein and Ehrenfest, Heisenberg and Max Born, there were also world-renowned scientists such as P. A. M. Dirac, Niels Bohr, H. A. Kramers, E. Schrödinger, Langmuir and so on. Hence the enormous interest in the discussions that followed the reports, in which materialist physicists, who refused to abandon determinism as the basis of all science (led by P. Langevin), clashed with the supporters of indeterminism, based on Heisenberg’s principle of indeterminacy (led by Heisenberg himself ). At this Council, Paul Langevin had won a victory over the proponents of classical physics by getting his very brilliant student Louis de Broglie invited. He had just published his original thesis on wave mechanics. There was strong opposition to Louis de Broglie’s ideas, just as there was strong opposition to the Einsteinian conception of the photon (particle of light), which had served as the basis for these ideas. Lorentz died in 1928, shortly after this historic Council, at the age of seventy-five, leaving a great void among the theorists of electromagnetism. His death struck my father very hard, so much so that he spoke to us about it at length in the family. He greatly admired the scientist in him, and moreover

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he had a deep friendship for the man, a friendship that Lorentz reciprocated, as their correspondence shows. It was therefore necessary to find a replacement for President Lorentz. All the members of the Scientific Committee decided in favour of Paul Langevin, who was informed on 20 August 1928. Professor Eugène Guye, of Geneva, expressed the general opinion when he wrote: “Not only has M. Langevin attended the Physics Councils from the beginning, – better than anyone else he is therefore in a position to get inside the fine tradition introduced by Lorentz and to continue it, – but he is also a progressive scientist, very much aware of the difficulties and problems of modern physics. He is also a remarkably clear, precise and quick thinker, even in the explanation and analysis of the most difficult questions. These are very necessary and even indispensable qualities for a president.” Immediately after his election, Paul Langevin took on his work as President with great awareness and enthusiasm. He began by reorganising the activities of the Scientific Committee, which he ensured operated in a very democratic manner, doing nothing without consulting his colleagues. It is difficult to appreciate the enormous amount of work involved in organising a committee, and especially for a Solvay Council. For the Committee meetings, it was not only necessary to prepare the proposals for the topics to be discussed, in agreement with all the specialists, it was also necessary to be sure of having the agreement of everyone in advance, or at least of the majority of the members. It was also essential to obtain the agreement of all the personalities for the choice of the rapporteurs. The least difficult thing was to agree on the names of the scholars who would be invited to the Council. If one considers that not only were personal sensitivities involved, but that national prejudices further complicated the choice, one can imagine what diplomatic problems arose, and also what practical work had to be done to write the letters and convince so many people. This secretarial work, I know, had to be done for the most part by my father himself. At the beginning he had neither a secretary nor a typewriter. Later on he considered it rude to send a typed letter (all the photocopies and original letters I have had from him are handwritten, and show his beautiful, regular handwriting). But that was not all: another task was to get the rapporteurs to send their texts in on time. This was the hardest part of the job, as many reminder letters were needed! Instead of simplifying his task, on the contrary Paul Langevin had systematically complicated it. As soon as he took over the presidency, he gave considerable thought to the problem of better organising the discussions of the Solvay Council. The proposals he made at the meeting of the Scientific Committee on 19  May 1929 were revolutionary. They aimed to make the

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discussions more efficient and more complete by saving the time spent presenting reports. His innovation was to have all reports copied well in advance of the Council meeting so that all participants could study them beforehand. But it is easy to see what extra work this progress represented. Before the Council, Paul Langevin read all the reports, scrutinised them, criticised them and asked the authors for any changes he felt were necessary, as the letters I have found show. Paul Langevin retained this method until the end of his presidency, when war started in 1939. The Council was to be held from 19 to 25 October 1930. For a first try, it was a master stroke. Paul Langevin succeeded in bringing together all the great specialists in magnetism from around the world. They all responded enthusiastically to the invitation. After a short respite, Paul Langevin resumed his hard work to organise a new Solvay Council in 1933. Having considered the current topics, he thought it would be reasonable to focus the discussions on “the structure and properties of atomic nuclei”, given the latest discoveries in physics, which were mostly related to the atom and radioactivity (the discovery of the neutrino in 1931 by Pauli, the discovery of the positron in cosmic rays by Anderson in 1932 and, above all, the discovery of artificial radioactivity by Frédéric and Irène Joliot-Curie in 1933). The correspondence necessary to set the date of the meeting democratically, the list of reports and the names of the rapporteurs took a lot of my father’s time. In addition, he had introduced another innovation, and this was to translate all the reports into two languages before sending them out. This was a huge extra work for him, as he felt obliged to do the difficult work of translation himself to make sure that no mistakes were made. In addition to my father’s efforts on the occasion of the Council, he was obliged to give the opening and closing speeches of the debates, not forgetting the traditional speech at the dinner given by the King and Queen of the Belgians. Such overwork could not help but cause Paul Langevin’s health to deteriorate. The consequences soon became apparent. The eighth Council was to be held from 26 to 31 October 1936, with the theme: “Cosmic rays and atomic physics”. But various setbacks got in the way. First of all, Paul Langevin began to feel the first manifestations of the inexorable disease which was to take him away in December 1946, – a consequence, unfortunately inevitable, of the use of X-rays at a time when their harmfulness was not yet known (the work for his thesis and assistance given to the treatment of the wounded of the 1914 war, who were operated on under the X-ray screen). However, this was not the only cause of the delay. Nazism and Fascism forced many scientists, Paul Langevin in particular, to engage energetically in the civic struggle. These worries and actions no longer allowed him to work with the necessary serenity and intensity to prepare a great Council in time, given the increasingly rapid progress of science.

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FIGURE 7. 7th Solvay Physics Congress. Brussels 22-29  October 1933. Left to right. Back row. E.  Henrio, F.  Perrin, F.  Joliot, W.  Heisenberg, H.A.  Kramers, E. Stahel, E. Fermi, E.T.S. Walton, P.A.M. Dirac, P. Debye, N.F. Mott, B. Cabrera, G. Gamow, M.S. Rosenblum, W. Bothe, P. Blackett, J. Errera, Ed. Bauer, W. Pauli, J.E. Verschaffelt, M. Cosyns, E. Hertzen, J.D. Cockroft, C.D. Ellis, R. Peierls, Aug. Piccard, E.O.  Lawrence, R.  Rosenfeld. Front row. E.  Schrödinger, Mme  I.  Joliot, N.  Bohr, A.  Joffe, Mme Curie, P.  Langevin, O.W.  Richardson, Lord Rutherford, Th.  De  Donder, M.  de Broglie, L.  de Broglie, Mlle L.  Meitner, J.  Chadwick. © Bibliothèque de l’ESPCI.

Moreover, in his activity at the Solvay International Institute of Physics, Paul Langevin had made the mistake (the only one, I believe) of not asking for the appointment of one or more vice-presidents who could have assisted him in his work. This failure can be explained by the fact that he loved perfection in his work too much not to want to do all the work himself, even the practical work. Contrary to a widespread habit in higher education, he never made any of his students or collaborators work for him. More often than not, not only did he involve them in the publications, but he even did the work of others himself or let others publish his own work. It is my duty to recall these truths. The Solvay Council did not take place in 1936. Paul Langevin proposed to the Administrative Commission to hold the Council without him, but the Commission preferred to postpone the eighth Council to 1939. On 11 March 1939, he sent out the invitation for the meeting, which was to take place from 22 to 29 October. Unfortunately, the Second World War

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broke out. He decided to publish the prepared reports anyway, but these reports were not actually printed until 1948, after my father’s death, at the end of the eighth Council, which was chaired by Joliot-Curie. Thus ended the enormous effort that Paul Langevin made during all this time for the development of the new physics.

Anti-fascist leader: educationalist: philosopher The fight against war and against fascism From an early age, Paul Langevin, Jaurès’ disciple, took a stand against nationalism and for peace. He fought all his life for just causes. In January 1898, during his internship at Cambridge, he signed a petition for Dreyfus that had been sent to him by Péguy. Within the newly created Ligue des droits de l’homme et du citoyen, he campaigned alongside his friends Émile Borel, Jean Perrin and especially Jacques Hadamard. He realised, then, that the scientist had to be involved in politics. My father had a possibly somewhat naive faith in the scientific mind. He thought that scientific thought could animate and renew political debate. He often said that the Republic needs scientists. As an activist in the Ligue des droits de l’homme, he knew and supported Ferdinand Buisson, Guernut and especially Victor Basch, of whom I have heard him speak very highly. There too, he took his work very seriously; he studied many cases very conscientiously to bring justice to good people. Beyond the injustices affecting individuals, Paul Langevin fought, through the league, against collective injustices, against everything that infringes human rights, meaning also against war. It is largely thanks to his action that, in the aftermath of the First World War, the league assisted in the fight for peace. He was very disappointed by the Treaty of Versailles, which he felt was not a treaty for peace between peoples, but a treaty for the sharing of the world’s wealth between certain imperialists. Initially enthused by the basic principles of the League of Nations and by the idea of collective security that it was supposed to establish, he soon came to believe that these principles were only a façade, behind which the interests of the most powerful capitalists were in fact at work. His ideas on the organisation of peace were expressed many times during this period, in the course of various conferences and meetings. This view

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was expressed notably in Médan, on 2  October 1932, on the occasion of the commemorative ceremony in honour of Émile Zola. The text was published as La science et la paix in the Cahiers des Droits de l’homme, 32(28) 10 November 1932: “… He (Zola) had understood that in order to derive beneficial action from an understanding of the world, in order to pass from science to justice, it is necessary to love humanity and have faith in the effectiveness of human effort, and that the propagation of this faith is no less necessary than the teaching of science... “… I would like to try to fulfil this duty for the most important, the most distressing of current problems, that of organising peace and an international form of justice, a human creation like science and like pity. “What Zola’s action and the crisis of the Dreyfus Affair have done to create a collective and living consciousness of individual justice, we must transpose to the international level and realise the new creation that a peaceful world must become. “Here too, by extracting truth and clarity from the facts, we must try to understand and then create and propagate the faith necessary to move from the idea to the voluntary and fruitful action.” But Paul Langevin did not limit himself to fighting for peace within the League; he pushed his dedication to the point of giving numerous talks in France and abroad. In particular, there were five meetings in Berlin in 1923, where Paul Langevin participated at Einstein’s request.

FIGURE 8. In Berlin, before the pacifist meeting (1923). © Bibliothèque de l’ESPCI.

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Paul Langevin went to Berlin. But, as soon as he arrived, the Prefect of Police told him he was not allowed to speak. In fact, he and Einstein were only able to have a short statement read out at the meeting. German nationalism had already regained its dominant influence. In Paris, the newspaper Le Temps, which was later to play such a big role in the betrayal of the nation, joined forces with the Berlin bosses and perfidiously criticised Langevin, this clear-sighted and courageous patriot. In the same year, 1923, Paul Langevin travelled to Vienna as a leader of the League of Human Rights, with the task of presiding over the foundation of the Austrian League of Human Rights. He took advantage of this trip to pick up my brother and me in Venice, where we were tourists, to take us to Vienna. Leaving Venice on 21 August, we went up Lake Garda by boat to Bolzano, where we stayed on 22 August. The next day we went to Austria via the Brenner, visited the Tyrol and finally arrived in Vienna on 27 August. While my father was on his mission, my brother and I took the opportunity to visit Vienna and travel by boat up the Danube to Linz. Naturally, Paul Langevin turned his mission into a crusade for peace. He first had meetings with the future President of the Austrian League, Prince Karl Anton von Rohan. This authentic prince was a democratic man. But of course, he was anti-Bolshevik, although he supported peace with the Soviets. Paul Langevin also wrote letters. His correspondence with German physicists about the struggle for peace is not without interest. This is Planck’s reply to a letter from my father, who had submitted a draft appeal to him. “Berlin-Grünewald, 24.9.1925 “Honoured colleague, “I found your valued letter of the 22nd of last month when I returned from a trip to Russia, and I sincerely regret having left you so long without an answer, perhaps arousing the impression that I was not showing any appreciable interest in the question raised by you. You may be sure of the contrary. I would gladly do everything in my power to act in the direction of the reconciliation of the existing opposing views as you desire. “But it seems to me more than doubtful whether a declaration by a few people to be published in the newspapers can serve to bring people closer to this noble goal. I have never yet seen that a measure of this kind, however well intended and however skilfully set in motion, has produced any real good; often I have seen exactly the opposite. Always a contradiction arises, the declaration is misunderstood, misinterpreted or used to slander the signatories and thus actually produces a renewal of mistrust instead of the desired rapprochement. “It is my conviction that a lasting improvement in the international situation and thus a reduction in the danger of war cannot be brought about by any regulation from above, but can only be brought about quite gradually,

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silently, from below, with the help of individual activity from one person to another, so that like-minded people come together until the time is ripe for action on a broader basis. This is what I have always tried to do and what I hope to continue doing. “Yours sincerely M. Planck” The second answer is no more encouraging. It comes from the well-known German physicist Wilhelm Ostwald: “Gross-Bothen (Saxony), 4 August 1925 “Honoured colleague, “I thank you very much for the confidence you have placed in me on the important question of future world peace. Unfortunately, I am not in a position to fulfil your wish. I live isolated in the country, without any connection with the German press and without any influence on public opinion, so that I could not be of any use to you. On the other hand, my experiences before 1914 have taught me that statements of this kind have no practical effect. It would therefore be a pointless waste of energy to try this ineffective means again, and in accordance with the requirements of energy (sic), I cannot bring myself to link my name with this enterprise. “Europe’s present state is undoubtedly dangerous and there is only too much reason to be concerned about our future. Although the state of affairs cannot be described as unstable in the strict sense, its stability is nevertheless very weak, comparable to the state of a long cylinder that is placed upright on its base: relatively small disturbances can make it fall. If the possibility of future wars is to be limited or excluded, stability must be increased by widening the base. “Reflecting on this problem, which can be studied scientifically as a complicated problem of statics, I arrived at the following possible solution. Among the states of the European continent, Switzerland has proved to be the most stable in the last century, because it has best achieved the balance between autonomy and centralisation. It is therefore desirable to extend its organisation and in particular to generalise its successful solution of the problem of languages and nationalities. “This could be done if Alsace-Lorraine, Luxembourg and Belgium were to join politically with Switzerland to form a confederation, whose neutrality would be covered by an international guarantee. Internally, this would produce a lasting solution to the linguistic rivalries of these hybrid regions. Externally, there would be a large neutral zone between Germany and France, and the elimination of direct contact between the two countries would remove the causes, or even the possibility, of future wars between them. “I cannot develop this idea further in a letter. If you take it further, you will recognise that it represents an applicable method for the creation of

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the United States of the European continent, which would make future wars impossible, at least for Europe. The larger and more influential this central and centralising body becomes, the sooner it will be joined by new regions. “I have no doubt that this plan can be improved in many respects. That is why I have come to ask you to take it into consideration and to give it to the public with your corrections. “Please give my best wishes to Professor Richer. “Yours faithfully Wilhelm Ostwald” Like Ostwald’s pedantic letter or Planck’s evasive reply, Haber’s suave reply did not give my father any hope. Here it is: “Sils-Maria, Engadine, “My dear colleague, “Your kind correspondence and the draft appeal attached to it reached me here after considerable delay. I agree with you in sincerely wishing for a lasting consolidation of peace, especially in Western Europe and very particularly between our two countries. But I caution against spreading your appeal in Germany. We have arbitration arrangements in the League of Nations, which Germany is in the process of joining. We have entered into obligations which go beyond that in the Locarno Treaty. The educated people of Germany are very keenly interested in the development of international political life under the influence of this new situation. They ask whether your appeal intends to go beyond Geneva and Locarno or whether it says the same thing that we have already agreed about. If the former, it will be seen as premature; if the latter, as superfluous. To get to know each other personally more closely, to become personally certain of each other’s sincere desire for understanding and peace, is the attitude which seems for the moment right and important to reasonable and conciliatory Germans. But one recoils from appeals conceived in general terms, because they are of no use for the moment in the relations between nations, while in the internal political situation of Germany they are unfortunately misinterpreted and harm the aspirations that you are trying to serve. From the German point of view we are forced to avoid all documents for the time being and to work by personal meetings to bring together reasonable, friendly and important men from all specialities and groups. I hope to have the opportunity to see and speak to you personally soon. “Please accept my highest consideration and friendly greetings. F. Haber 15 August 1926 ”

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Present and active in all areas After 1923, the year of the advent of fascism in Italy, Paul Langevin no longer trusted international organisations to organise peace; he increasingly put his hopes in mass action against the war. This is why he participated in popular demonstrations such as the peaceful demonstration on Sunday 3 April 1927 in Levallois-Perret where he was invited to speak at the inauguration of the war memorial. On 23 February 1927, Paul Langevin presided over the great anti-fascist meeting in Paris, under the honorary presidency of Albert Einstein, Henri Barbusse and Romain Rolland. This meeting was a resounding success, I remember very well that the Salle Bullier was full to bursting. Eight thousand workers were present. My father took the floor with Paul VaillantCouturier, Marc Sangnier, Guido Miglioli, Giuseppe Di Vittorio and other speakers. But Paul Langevin did not limit his anti-war propaganda to popular circles. Not only did he campaign among academics, which goes without saying, but he also managed to address the most aristocratic circles, since he was received at the homes of Madame de Noailles, the Countess Greffülhe, Madame de Vilmorin and Madame Singer-Polignac. Queen Elisabeth of Belgium was a great friend of his. As he said after the liberation, paying tribute to the memory of the old democracy activist assassinated by the Milice96, he had met Victor Basch for the first time

FIGURE 9. Paul Langevin and Marc Sangnier, Salle Bullier (February 1927). All rights reserved.

“at the home of Madame Ménard-Dorian, this woman adorned with all the gifts of beauty, intelligence, wealth and kindness, whose house, welcoming and full of art, brought together men of the left, both French and foreign”.

I found a clipping from the newspaper L’Œuvre of 25  January 1931 in the “Propaganda against the war” file compiled by my father. It is an article by Henri Simoni entitled: “Science and war – A pathetic cry of alarm from Professor Langevin”, where we read: “For the first appearance of its activity, the Comité d’accueil et de propagande des amitiés internationales offered yesterday to an elite audience,

96 * A French political paramilitary organisation created in January 1943 by the Vichy government to help fight the Resistance Movement.

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in the historic Hôtel de Cavoye and under the presidency of Madame la Baronne d’Estournelles de Constant and Paul Landowski, a lecture by Mr. Paul Langevin, professor at the Collège de France... “In spite of the magnificent residence, the high woodwork with its fine gilt, the elegant and perfumed atmosphere, Paul Langevin does not attend a social gathering, and often the audience felt the shiver of atrocious death conjured up by the speaker, whose words are soft and serious...” In 1932, Paul Langevin, as President of the French Pedagogical Society, addressed the Teachers’ Congress in Clermont-Ferrand. He publicly denounced the powerlessness of the League of Nations in the face of the threatening war and highlighted the inadequacy of the measures it proposed. Following this robust stance, he received the following letter from Edouard Herriot: “Paris, 4 August 1932 “My dear Langevin, “I was saddened, if I may say so, by your criticism of the Geneva conference. I think that if you were fully informed, you would be less harsh on those who have to react against both those who think that too much is being done and those who think that not enough is being done. “Sincerely. Herriot” Paul Langevin immediately replied to Herriot, from Nice, where he was chairing the Congress of the International League for New Education. I have found the draft of this letter written on the letterhead of the Lycée annexe du Parc impérial: “My dear President, “... I thought it my duty to express opinions which I beg you to believe have matured over many years by reading numerous books and official documents and participating in many discussions. I am more and more convinced that there is no possible compromise with war. If the madness of men allows it to break out, it will be total, because that alone is the absurd logic of force. A nation which mobilises itself entirely for defence will necessarily be entirely exposed to attack to the full extent of the technical means available... “... Do you really believe that words can ward off such terrible realities, and do you believe that people are so doomed that they have already been reduced by the morphine of conventions? “For my part, I still have enough confidence in humanity’s good sense and will to live to believe that they are capable of reacting to the stimulating view of the facts.

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“If you think it useful to hear my reasons at greater length, I would be only too happy to talk with you, whenever and wherever possible...” The help he gave to the Romanian democrats in their struggle against the dictatorship of the “Iron Guards” should also be included in his action for peace and against fascism. As early as 1926, he took a clear stand against the crimes of the Romanian Sigurantza97: “Because it knows that this continues and because it must stop, the Ligue française des droits de l’homme wants to affirm human solidarity in the face of such events and to contribute, to the best of its ability, in making them known and to seeking their causes and remedies. “The notes which precede Mr. Costa-Foru’s presentation, and which were sent to us by a Frenchman who has spent long periods in Romania, throw much light on the origin of the evil. Apart from the historical reasons that are particular to the country, there is also the very many symptoms of the general recession caused by the war, a prolonged echo of the immense outburst of ancient bestiality. Finally, there is an example of the stupid and tenacious illusion that ideas considered subversive or dangerous can be overcome by violence, when all that is achieved is to make them more alive and more human as a result of all the suffering endured for them. “The old methods of persecution and dictatorship, which are presented today with new names, suffer from a deep flaw which is certainly instinctive, which forms the origin of the horror they inspire; the idea is strengthened by appearing to fear it, by compressing it, by surrounding it with mystery and by giving it martyrs, instead of giving it a correct value by confronting it freely with other ideas or with the facts. In individual or collective psychology, as in biology, the most toxic fermentations are not those that take place in the open air and in broad daylight. The shattering force of a thought, like that of an explosive, is never greater than in a vacuum…” And to criticize “the shameful procedures of the police and the government”, Paul Langevin took part in meetings for the release of Professor Constantinesco-Iasi and other imprisoned Romanian militants. He wrote a preface to the booklet written by his friend Henri Mineur, an astronomer at the Paris Observatory, relating the martyrdom of the heroes imprisoned in the Doftana prison.

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* Security.

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President of the World Committee against War and Fascism From 1932 onwards, Paul Langevin completely associated his fight against the war with the fight against fascism. Following the energetic appeal written by Romain Rolland, he immediately joined Albert Einstein, Heinrich Mann, Maxime Gorki, Theodor Dreyser, John Dos Passos, Upton Sinclair, Madame Sun Yat Sen and Sen Katayama, who all formed the Initiative Committee for a World Congress against the imperialist war, the Amsterdam Congress. Later, John D. Bernal would report some revealing words from my father during this period: “... Addressing an audience of workers, he explained his views on a social problem using the scientific method. I then took him back to the School of Physics and Chemistry. On the way, he said to me: “Ah, perhaps I’d better continue doing research in the laboratory” – he was talking to himself, in front of a witness – and, without waiting for an answer, he said: No, no. I think I’m right, I don’t want to go back to the laboratory. I think I’m right, I’m certainly right, because you will be in a better position to develop science without any ulterior motive, having a clear conscience if these ideas spread and if the worker in his laboratory only has to worry about scientific questions.” Soon my father agreed to become president of the World Committee Against War and Fascism, together with Henri Barbusse and Romain Rolland. Our friend Georges Cogniot was one of the secretaries of the Committee. As always, my father carried out his duties with unbounded commitment and, despite all his other activities, to find the time to increase his representations in Geneva or to governments. This activity naturally led him to become involved in the International Committee for the Liberation and Amnesty of Anti-Fascists Imprisoned in Germany, of which he shared the presidency with André Malraux, while Romain Rolland and André Gide were honorary presidents. My father was especially committed to the Committee for the liberation of Thaelmann and the imprisoned German anti-fascists98, which organised, with the help of its 32 affiliated groups, a large assembly under the title of “People’s Tribunal” on 9  May 1934. With the participation of Paul Signac, Henri Wallon, Jean-Richard Bloch, Francis Jourdain, Madame Lahy-Hollebecque, Miss Zévaès, Miss Willard and Paul Vaillant-Couturier, anti-fascists from all walks of life and all classes gathered there to protest against the lynching justice of the Hitlerites. Many scholars, writers, artists and lawyers eagerly followed the speakers’ indictment against Nazism. Paul Langevin, greeted by a huge ovation, read out, in the name of the intellectuals, a motion of support 98

The address of this Committee was 10, rue Notre-Dame de Lorette, Paris 9.

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for the struggle for the liberation of Thaelmann and all the anti-fascists. He spoke of the pain of the men of his generation, who thought they had made definitive progress and were witnessing the return of barbarism. On 10 March 1935, once again at the Salle Bullier, my father presided over an enthusiastic assembly convened in particular for the defence of trade union rights; he was accompanied by Henri Raynaud and Jacques Duclos. On 10 May of the same year, Marcel Cachin, Marcel Willard, Racamond and Vaillant-Couturier were with him at a big meeting where he proclaimed the solidarity between manual and intellectual workers. Paul Langevin was enthusiastic about the magnificent courage of the hero of Leipzig, Georges Dimitrov, who victoriously stood up to Goering and transformed himself from an accused to an accuser of the Nazi executioners. He was active on behalf of Dimitrov alongside his friend, the lawyer Marcel Willard. It would take too long to recount all my father’s interventions in those years of fevered struggle. He played an essential role in the campaign to wrest Georges Dimitrov from Hitler and Goering. Until 1939, Paul Langevin was the most active and hard-working president of the Amsterdam-Pleyel Movement, the French movement for unity against war and fascism. Among Langevin’s speeches during this period, the words pronounced at the funeral of the great Henri Barbusse on 7 September 1935 are quite remarkable. My father praises the fighter who put his pen and his word at the service of truth and peace: “... Gifted more than any of us, equivalently in thought, expression and action, he was better than a leader, the inspired guide who, after having deeply felt the horror of war in his body, heart and mind, understood the baseness and stupidity of its causes. For 20 years he devoted himself, in a continuous effort, made heroic by his state of health, to the double task of stating by pen or spoken word, and of defending by action what he knew to be the truth...” My father’s 65th birthday was celebrated by the Amsterdam-Pleyel Movement on 24 January 1937. A big banquet at the Pignarre restaurant, rue d’Alésia, was presided over jointly by Francis Jourdain, president of Paix et liberté, and by the man being honoured. Most of his friends were present, along with Maurice Thorez, Jeannette Vermeersch, Marguerite Cachin, Professor Hadamard, and Elena Stassova, president of the World Committee of Women Against War and Fascism. Among all the letters my father received on this day of celebration was one from Romain Rolland: “Villeneuve, 24 January 1937 “Dear great friend and companion, “I have never regretted so much as this evening that I am prevented by my health from coming to Paris, I would have liked to have added my

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fraternal tribute of affection and respect to all those who will be present with you this evening. “Others will say as well as I could, but none feels it more, about the generosity of your social commitment, which takes you away from your own work of brilliant scientific research to carry the heavy load – heavy with responsibility and danger – of those powerful social movements of which you are one of the lights. You give us an admirable example of selflessness and faith in humanity, for which reason must be the guide. “We are proud to have you as our leader and adviser. Nothing is greater, nothing is more exhilarating, nothing is more conducive to strengthening our confidence in victory than to see the masters of science, like you, marching at the head of the innumerable masses of the people, who, at this hour, are defending Justice, which has been savagely attacked, and who are conquering unlimited free progress. “We express our deep gratitude and affectionate admiration. Romain Rolland” For his part, Francis Jourdain wrote in his greeting to my father: “I do not believe enough in justice to be sure that all admired men are admirable; and I willingly hold to be suspicious the veneration with which a kind of conformism has quickly surrounded such and such a hero, past or present. “The glory of Langevin has never inspired the slightest distrust in me. I know what pure elements, what solid materials this glory is made of. I have long respected this great scientist and this great citizen, but I am deeply happy that fate has allowed me to leave the climate of respect – salubrious, but a little rough and cold – to gain the warmer climate of affection...” Francis Jourdain’s greeting was entitled Le bon Langevin. In these years, within the World Committee, “the good Langevin” participated energetically in action for republican Spain. He was a speaker at all the meetings for Spain, a signatory to all the appeals, and an organiser of all the delegations whose aim was to safeguard collective security and peace. Together with Victor Basch, he was President of the International Co-ordination and Information Committee for Aid to Republican Spain  – an organisation that was recognised by the Negrin government as the only official one. His documented and convincing articles mobilised opinion in France and abroad. He devoted himself to the Universal Rally for Peace alongside Lord Cecil and went to the London conference for aid to Spain.

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FIGURE 10. Meeting for Spain: Jacques Duclos, Paul Langevin and La Passionaria. All rights reserved.

Among the meetings chaired by Paul Langevin in this period, we can mention the one held on 26 May 1937 at the Salle de la Mutualité “to stop the massacres in Spain, for the withdrawal of German and Italian troops, to save peace”. My father was assisted at the podium by Francis Jourdain and Henri Raynaud. Before giving the floor to Racamond, Longuet, Duclos, Bayet and Malraux, he said in lofty terms what a disgrace it was to allow impunity to criminal aggressors. It is interesting to see how my father’s work was judged by his collaborators on the World Committee. They wrote in January 1938 in the Committee’s newsletter: “Our President Paul Langevin has recently been promoted to the rank of Grand Officer of the Legion d’honneur. In this just tribute to the genius of a universally revered scientist, we wish to take the opportunity to reiterate to our faithful and glorious friend the expression of our affectionate respect and our profound gratitude. It seems superfluous to us to repeat here the reasons we have had to admire Paul Langevin’s higher conscience for so long, with what devotion he placed the most judicious activity and the most clear-sighted courage at the service of all great causes. “In addition to the congratulations we are pleased to offer him, we would like to add our heartfelt thanks for the valuable collaboration he has always given us, for the sympathy he has always shown us, for the interest with which he has agreed to direct our work. “In assuring him of our grateful association, we ask our dear and affectionate President to accept our wishes for his forthcoming and complete recovery, momentarily compromised by the overwork due to the load of carrying out his double task of scholar and citizen: the health of our great friend will soon allow him to give us the benefit of his experience and advice. Let him not doubt our joy; neither let him doubt the pride that is caused by the trusting friendship with which he honours us.”

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The Vigilance Committee of Anti-Fascist Intellectuals Paul Langevin had personally participated shoulder to shoulder with Victor Basch in the march of 12 February on the Place de la Nation (I was by his side that day). On 5  March 1934, after the fascist riots, he launched the manifesto which constituted the birth of the Comité de vigilance, in agreement with two other academics, Paul Rivet, a socialist activist, and Alain, a radical writer. “United above all differences, before the spectacle of the fascist riots in Paris and the popular resistance which alone faced them, we come to declare to all the workers, our comrades, our resolution to fight with them against a fascist dictatorship to save the people’s hard-won rights and public liberties. We are ready to sacrifice everything to prevent France from being subjected to a regime of oppression and warlike misery... “We will not allow the financial oligarchy to exploit, as it has in Germany, the discontent of the crowds troubled or ruined by it. “Comrades, under the guise of national revolution, a new Middle Ages is being prepared for us. We do not have to preserve the present world, we have to transform it, to free the state from the tutelage of big capital, in close liaison with the workers. “Our first act was to form a Vigilance Committee which is at the disposal of workers’ organisations. Let those who subscribe to our ideas make themselves known.” This appeal answered a need. It was a rapid success: 3500  members joined in July 1934; then, following a new personal appeal by Paul Langevin dated 15 October 1935, the number of members rose to 5000 by the end of the same year. Paul Langevin’s persuasive words were heard at many meetings, large and small. I remember in particular the evening when Luce invited thirty teachers from the Lycée Fénelon to our living room to listen to my father, who spoke to them of the frightening dangers and called them to action. The French Comité de vigilance was able, under my father’s influence, to help found a similar group “for intellectual freedom” in Britain. As we have already said, Paul Langevin often travelled to England and received many British visitors. According to Bernal, “He encouraged, advised and inspired. He represented the advanced element of political consciousness, and this is a primary aspect of his influence on British intellectuals.” However, within the Vigilance Committee, as within the Popular Front, the union did not withstand profound differences on foreign policy. As Claude Willard rightly said in his 1966 report on “French intellectuals and

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the Popular Front” 99, two tendencies clashed. The first wanted energetic, determined resistance to all threats and aggression from the fascist powers. “The second tendency asserts that in order to preserve peace, which is the supreme good, it is necessary to multiply concessions, to soften the aggressors. Peace at any price! In 1935, the first stirrings within the Committee were caused by the signing of the Franco-Soviet assistance pact. The following year, the remilitarisation of the Rhineland and above all the Spanish war consummated the split; only the integral pacifists remained in the Comité de vigilance. Several of them (such as André Delmas and Félicien Challage), at the extreme but logical end of their bleating pacifism, collaborated with Hitler’s occupiers.” As the reader may well imagine, Paul Langevin, despite his deep hatred of the war, could not be in favour of concessions to fascism. He belonged to the first tendency, as his speech of 12 June 1938 at the “Peace and freedom” Congress in the Paris region proves: “... In the very interests of peace, the horror of violence, common to all men of good will, must not be allowed to prevail over the love of justice and freedom, over the necessary feelings of human solidarity towards the heroic and ever-increasing victims of international fascism. I believe that it is impossible to overemphasise, in these critical times, the profound connection between the defence of peace and the defence of freedom, the impossibility of separating them in our thinking and in our action. It is a matter of elementary common sense, confirmed by history, that bowing to force can only lead to the ever more brutal reign of force: each of the tragic days in which we live would prove this to us, if it were still necessary. “From this comes the danger of an attitude of so-called integral pacifism, which, having its intellectual origin, […] threatens to become deeper and has certainly recently influenced the attitude of democratic governments to justify the policy of so-called non-intervention. It is becoming more and more obvious that this policy favours the blackmail of war, constantly increases the audacity and cynicism of the violent, and would inevitably lead us to war under the worst conditions, even if we were to go as far as the last renunciation by adopting the selfish and cowardly formula: “Rather servitude than death! ”, which is singularly similar in terms of inaction to the other formula often heard and which expresses another form of selfishness: Rather Hitler than the Revolution!” The most violent clashes between the two groups in the bureau meetings of the Comité de vigilance occurred in early 1936. To give an idea of Paul 99 Claude Willard’s report was published in “Les cahiers de l’Institut Maurice Thorez”, 3-4, p. 121. Institut Maurice Thorez, 64 boulevard Blanqui, Paris 13e.

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Langevin’s work during this period, it is sufficient to reproduce here JeanRichard Bloch’s impressions, noted from day to day in letters to Romain Rolland: “15 February 1936 “My dear friend, “A word in great haste this evening to bring you up to date on two sets of facts: “1. A very painful session at Vigilance (“the bureau”) on the 8th, following which (or rather during which) Walter (P.  Gérôme) 100 ended up giving his resignation after a violent scene. But Rivet had already resigned and did not even attend the meeting of the bureau. The “Alexandre-Walter ” tendency was counting on this to force Langevin (against whom they had raged) to convene a meeting without delay. For perish Vigilance, perish the Popular Front, provided that communism loses all audience, all credit, all influence. A provisional bureau was therefore elected and Langevin worked hard to get Rivet, who was playing a great nervous and flirtatious game, to withdraw his (nth) resignation. Finally, Langevin telephoned me the evening before last that his efforts, combined with those of Bayet and Delmas, had overcome the resistance. Rivet remains... “2. Vigilance is temporarily saved from Trotskyism, from total pacifism, from doctrinaire outrages... I assure you that Langevin, Baby, Maublanc, Wallon, myself and a few others breathe a sigh of relief...” In July 1936, Vigilance split. Here is what J.R. Bloch wrote to Romain Rolland: “26th July 1936 “Dear friend, “... Did you know that Magdeleine Paz and Rivet have founded a socialist “House of Culture” to try to torpedo the one in rue Navarin? “The Trotskyite spirit (always Magdeleine Paz) has entered the Vigilance office, in place of the Prenant, Bayet, Wallon, Joliot-Curie, Langevin and Chamson, all of whom had been cleaned out beforehand to make room for the integral pacifists (In Feuilles libres, by Émery-Alexandre, we find statements that Doriot would sign today).

100 Walter belonged to the group of more or less ‘integral’ pacifists.

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The Front populaire Despite all these dramas, Vigilance had initially helped to facilitate the achievements of the “Popular Front”. The Front populaire was born on 14  July 1935, at the huge rally prepared by an organising committee which was composed of the following ten groups: Comité de vigilance des intellectuels; Ligue des droits de l’homme; Mouvement Amsterdam-Pleyel; Parti radical-socialiste; Parti S.F.I.O.; Parti communiste; Intergroupe des partis socialistes; C.G.T.; C.G.T.U.; Mouvement d’action combattante. As president of the Amsterdam-Pleyel Movement, Paul Langevin must be regarded FIGURE 11. 14 July 1935, as one of the organisers of the 14 July rally, during the National Parade and above all as one of its moral patrons. He at the Bastille, with rode on the roof of one of the two taxis at Paul Rivet and Pierre Cot. the head of the procession, one carrying a red © Bibliothèque de l’ESPCI. flag, the other a tricolour. Maurice Thorez, Henri Barbusse, Paul Rivet and Pierre Cot could be seen at his side. This parade took place in the midst of indescribable popular excitement, with an estimated 500,000 people participating. I can still remember the enthusiasm that Paul Rivet’s Popular Front candidacy in the 5th  arrondissement generated during a supplementary municipal election on 12  May 1935. At the electoral meetings, chaired by my father, notably on rue du Cardinal Lemoine and rue de Poissy, there was delirium. All the left-wing intellectuals like Janets, Maurice Lacroix, Georges Fournier and so many others collaborated in Rivet’s propaganda. Pierre Cot wrote to characterise this period101: “The Popular Front was a wake-up call for the soul of the people. It was a magnificent period of generosity, confidence and enthusiasm. It opened the doors to a better future. It showed the French people an ideal to achieve. There is no nobler period in the history of the Third Republic than the years of struggle of the Popular Front.” Naturally, Paul Langevin was in full agreement with the protest movement of June 1936. These strikes with factory occupations were carried out with great happiness and in a good mood.

101 Pierre Cot: The trial of the republic, New York, 1944, pp. 180-181.

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My father went himself to speak to the strikers. My brother-in-law Jacques Solomon, my wife and my sisters, already militants of the Communist Party, were also going to spread the word. This is how Luce spoke to the salesmen and women of the Galeries Lafayette, from the top of the central staircase. It was curious to observe, as I did, the horrified reactions of the bourgeoisie to the influx of “paid holidays” in the summer of 1936. They found it scandalous that workers could enjoy holidays at the sea or in the mountains like themselves.

Support for German refugees and Albert Einstein During all these years, Paul Langevin’s personal efforts to help refugees from Germany did not slacken for a moment. He actively intervened on behalf of his colleague and friend Albert Einstein. He had had to leave Berlin in 1932 to take refuge in Holland at Ehrenfest’s house and later in Belgium at Coq-sur-Mer. On 9  April 1933, my father wrote to the great German scientist the following letter, in which he invited him to accept a chair at the Collège de France. This was the result of numerous approaches to the administrator, the professors and the Minister himself: “My dear friend, “I have just received your address and I am happy to be able to contact you again. I am looking forward to seeing you and would have already left for Coq if I were not held up here by an overwhelming task, and in particular by the celebration of the fiftieth anniversary of my School of Physics and Chemistry (where Curie and his wife discovered radium). The meetings will take place in about ten days’ time and I would be very happy if you could be here then. Won’t you come and bring the answer to

FIGURE 12. With the great German chemist Fritz-Haber, during the ceremonies of the 400th anniversary of the Collège de France (1930). Jacques Solomon, Mme P. Langevin and their two daughters, Madeleine and Hélène. © Bibliothèque de l’ESPCI.

FIGURE 13. With Ehrenfest in Holland (from left to right), standing: Ehrenfest, Einstein, Paul Langevin, seated: Kamerlingh Onnes and Weiss. © Bibliothèque de l’ESPCI.

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the offer that we were happy to see succeed? Do I need to tell you how welcome you would be at the Collège de France and that your place is already reserved in the new physics institute that we are starting to build? “A friend, Mr. Wertheimer, asks me to pass on to you the offer contained in the attached letter. It concerns a villa in Maisons-Lafitte, on the outskirts of Paris, very close to the Seine and the forest, in one of the most pleasant places. This would fit in very well with the College. I also enclose a note from Jean Perrin. “I send you my best wishes and hope to see you soon, either here or in Belgium. P. Langevin” After receiving a personal letter from Minister de Monzie in April, Einstein replied that he accepted. But on 5  May he reversed his decision and sent my father a letter that revealed the difficulties he was facing: “Le Coq-sur-Mer, 5 May 1933 “Dear friend, “Since our last meeting in Antwerp, great events have taken place which make the situation seem truly disturbing. As during the war, one has become accustomed every day to reading accounts of horrors and violence with the feeling that the greater part of this violence is not made public. In Germany, an armed rabble has completely silenced the men who knew of their responsibility. It is a kind of invasion from below, which will soon destroy all that is finest in them. “What is today a threat to culture will soon become a serious threat from the military point of view, if all the countries that are still governed by parliaments do not pull themselves together to take energetic action, which could still today be of a purely economic nature. Unfortunately, it appears that either we are not sufficiently aware of the threat of the present situation, or else, despite the existence of the threat, we cannot pull ourselves together to take energetic action, even though the last terrible lesson ended only 15 years ago. I am convinced that even today the German danger could be completely averted and that the whole system would collapse through a severe economic blockade. “The wonderful behaviour of the French government and my French colleagues towards me has brought me great joy. I have not yet been able to thank you officially, because I have not yet had official communication of my election to the Collège de France. I would be very grateful if you would let M. de Monzie know so that he will not be surprised by my silence. “I am now in an awkward situation, exactly the opposite of that of my compatriots who were driven out of Germany. Indeed, I am engaged for the whole winter (five to six months) at the Abraham Flexner Research

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Institute in Princeton. Then I was invited for five years, one month each year, to Christchurch College, Oxford. Apart from that, Spain has offered me a teaching position (as a professor) at the University of Madrid and I  have promised to go there next April. I had committed myself before I received the French offer. I have no official communication on the process with regard to the Collège de France. “This year, moreover, I have accepted lectures at the Franqui Foundation in Brussels for this month, so that I am not free for the whole month of May. “As the summer holidays cannot be taken into consideration, I do not know when or for how long I will be able to come to Paris. “I find myself in a painful quandary and I really don’t know where to turn. I explained all this clearly to the French consul when he first told me about the project to create a chair at the request of the French minister. “You may think that it would have been my duty to refuse politely both the Spanish and the French offer, since what I can give is not in proportion to what is expected of me. But such a refusal in the present circumstances would have been misinterpreted, for both proposals have, at least in part, the character of a political demonstration, and what is most important in the immediate future is that these gestures succeed. You will then understand that I cannot accept Mr. Wertheimer’s wonderful offer for the time being. In any case, please convey my heartfelt thanks to him. In the present circumstances, one never knows how things will turn out, for in these times everything is uncertain. “Maybe one day we will be able to live near each other and for me that would be a great joy. “I saw in a newspaper that you had celebrated your fiftieth birthday102. I would like to take this opportunity to express my wish that you may still experience true joy. This desire is not without selfishness, since what is a joy for you is also a joy for me. “I greet you cordially. Your: A. Einstein P.S. I also send friendly greetings and thanks to Mr. Perrin.” These facts contradict many omissions or even inaccuracies found in books about Einstein. For example, in Albert Einstein et la relativité by

102 It is obviously P. Langevin’s sixtieth birthday. Whether this is a typo or a mistake on Einstein’s part is difficult to know, as his letter is typed.

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Hilaire Cuny, one can read, pages 132 and 133 (collection Savants du monde entier, Seghers): “For our conservative academics, it was necessary to transform the chair of Germanism, which had become vacant following the death of Andler, into a chair of physics. This was theoretically simple. However, the chauvinism of some and the intrigues of others, who could not bear to be overshadowed in their mediocrity by the brilliance of Einstein’s knowledge, caused this beautiful project to fail. De Monzie then asked the Finance Committee of the Chamber to create a chair of physics. The attempt met with similar difficulties, and eventually Einstein was ousted from the so-called land of the free.” Hilaire Cuny is not the only one to make this mistake about the position of professor at the Collège de France. In his book Le drame d’Albert Einstein, published in the collection Chefs-d’oeuvre d’hier et d’aujourd’hui by the Book of the Month Club, a book which otherwise contains a host of interesting and well-written things, Antonina Vallentin says (pp. 179 and 180): “The chair at the Collège de France never materialized. In fact, de Monzie had moved too quickly and too far. Joseph Bédier had also, it seems, been too hasty in committing himself, without asking the Collège de France for its full agreement. The Collège refused to transform Andler’s chair into a scientific chair; de Monzie asked the Finance Committee of the Chamber to create a chair, but it was refused. I have long had a bitter memory of this absurd step...” What Antonina Vallentin says is incorrect. The information taken by my brother from the administration papers of the Collège de France itself shows this, apart from the letters from Langevin and Einstein himself. It turns out 1) that Einstein’s chair was created and available until 1946 when it was given to Francis Perrin; 2) that de Monzie had committed himself neither too quickly nor too far because, thanks to his persistent efforts, according to the Journal officiel, Einstein’s chair was created by article 27 of the finance law of 31 May 1933. The project had been defended before the Chamber by de Monzie at the session of 12 April 1933, the report of which appears in the Official Gazette of 13 April 1933. The vacancy was renewed every year (even during the occupation, as the name of the candidate for whom the chair was created was not mentioned in the law) until 1946, so that in principle, Einstein could have come to occupy it until then. It is obvious that Einstein appreciated the military danger that threatened France. Einstein was not the only victim of fascism who Paul Langevin wanted to help. I was able to find in his papers a letter from Schrödinger, which shows that he too had difficulties with the Hitlerites and had taken refuge in Italy: “Malcesine, Lago di Garda, Pension Scherl, 16-9-1933 “Dear Mr Langevin, “Thank you very much for your kind letter of 8  September and for the official invitation to the Solvay Council, which I received at the same time.

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“My address will be for a week, from today, the one on the letterhead (Malcesine, etc.). Towards the end of the month I shall be in Zurich (C/o Prof. Boer, Bergstrasse  27). I would like to take the opportunity, Mr. Langevin, to tell you something about myself, although it is still a little confidential at the moment (I would not like it to appear in the newspapers). I am leaving my position at B103 and will be going to Oxford in the autumn. I don’t want to detail the reasons for this here. I hope to have the great pleasure of talking to you in a few weeks in Brussels. “Believe me, dear Mr Langevin, your most sincere devotee. Schrödinger” No doubt Schrödinger, who was thinking of taking refuge in Oxford, wanted to ask Paul Langevin for an introduction to the many English scholars he knew. Paul Langevin gave him all the assistance he required. But Langevin did even more: he welcomed a certain number of German political refugees into his laboratories at the École de physique et chimie and the Collège de France. I knew several of them, such as Ernst Baumgart, who wrote a remarkable bibliography on ultrasound in the Revue d’acoustique. But with his usual benevolence and lack of distrust, Paul Langevin also welcomed into his laboratory a man called Engeland, who claimed to be a democrat victim of the Nazis, but who was most probably an informer introduced into France by the Hitlerites. During the occupation he once threatened me with the rigours of the Gestapo under the pretext that he was not given everything he needed to work and that he was not being treated kindly enough.

Paul Langevin’s educational activities Langevin was also involved in a thousand different things and his activities were not limited to those we have just described. As a humanist, he was interested in the problems of general culture, and therefore of education. This is why he joined the Société française de pédagogie. He first chaired the Science Section of this Society, and then, in 26 October 1922, the Board of Directors of the Society elected him President. Within this group, he gave a remarkable talk on “The educational value of the history of science” in December 1926104. What he proposed in this lecture was to “highlight all that scientific teaching loses by being solely dogmatic, by neglecting the historical point of view... Dogmatic teaching is cold, 103 It should read here: Berlin, since Schrödinger was a professor at the Institute of Physics at the University of Berlin. 104 This lecture was published at least four times. A first time in the bulletin of the Société de pédagogie, 22, December 1926; a second time in the Revue de synthèse (VI(1) April 1933), a third time in 1950, in the book La pensée et l’action de Paul Langevin, pp. 193-208; and finally a fourth time in a booklet published by the Association Paul Langevin, 10, rue Vauquelin, Paris 5.

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static, and leads to the absolutely false impression that science is a dead and definitive thing. Personally, if I had remained with the impressions I had from my first science lessons, given by teachers of whom nevertheless I have the most grateful memories, and if I had not come into contact with outside reality or from a different angle, I might have thought that science was finished and that there was nothing left to be discovered, whereas we are barely in the first stammerings of knowledge about the external world. To believe that there are only consequences to be drawn from principles that have been definitively acquired is an absolutely erroneous idea, which risks losing all the educational value of scientific teaching. “In order to contribute to general culture and to draw from the teaching of science all that it can give for the formation of the mind, nothing can replace the history of past efforts, brought to life by contact with the life of great scholars and the slow evolution of ideas.” This statement of the educational value of the history of science led my father logically towards a new study on “The contribution of the teaching of the physical sciences to general culture”, which he presented on 11 June 1931 at the Musée pédagogique under the auspices of the Société de pédagogie: “... It is essential to give a dynamic meaning to culture: education can only give a beginning to culture, which enables the individual to desire and to savour it. He must, for himself and throughout his life, maintain the contact prepared by the school and must be able to find the necessary time... “If we examine what has been done up to now to initiate the child and the adolescent to this activity, to this penetration of external reality by the mind, which is properly the work of science, we are obliged to observe that scientific teaching has not achieved true culture in the way that it has been presented up to now, and that it has been limited almost solely to information, to a utilitarian acquisition of knowledge. “This limitation ignores the true nature of the physical sciences, which aim essentially at an increasingly perfect understanding of reality. The activity of the physicist (by which we mean the person who seeks to adapt his mind to the material world) develops in three stages: the observation of facts, the elaboration of laws allowing for prediction, and finally understanding and explanation...” This profound criticism of the usual teaching system naturally determined Langevin to try to remedy its defects and to take an interest in the study of new methods of education. This is why he joined the Groupe français d’éducation nouvelle, of which he was honorary president from 1928 to 1946.

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At the same time, he campaigned for l’école unique, which would make it possible to establish “justice in the school”. As early as 1925, he joined the organisation of the Compagnons de l’Université nouvelle, of which he became president in 1930. The Ministerial Commission for l’école unique, of which he had been appointed a member in 1925, held numerous meetings. Nothing decisive came out of them. Primary education remained separate from secondary education. The main difference remained between upper primary education on the one hand and the lycées on the other. Paul Langevin was perfectly aware of the inadequacies of this outcome since he was able to say at a meeting given by Edouard Herriot on 6 December 1945 on L’école unique et la troisième République: “The Third Republic did not have the time to complete the work that had been started to create a single school, the work of introducing and achieving justice in schools, and it is by relying on what was done at that time that we hope to achieve this development of all the human resources of our country. It is in this way that it can be the greatest, by putting each of the individuals at its disposal in their place, for the greater good of each and everyone. The presence of both of us here symbolises, in a way, the union of what the Third Republic wanted and what the Fourth hopes to achieve by building on what was done by the Third...” On 8 December 1926, my father was appointed chairman of the commission to study and propose the reorganisation of the Musée pédagogique. He did not consider this appointment as an honorary distinction, but on the contrary participated very actively in the work. I found in his papers the report of the commission written entirely in his own hand, in his beautiful, clear and regular handwriting, with hardly any corrections. He insisted that the new Institute should have sufficient resources for information and research, a library and a library of records.

The mission to China In 1931, the Chinese government asked the Council of the League of Nations for the collaboration of the Geneva technical agencies in the preparation and implementation of a reconstruction plan, including the dispatch of “advisers to help improve the Chinese educational system and to facilitate exchanges between intellectual centres in China and in foreign countries”. The designated mission consisted of five members, including my father, the only representative of France105.

105 * The other members were Carl Becker (Germany) Marian Falski (Poland) Richard Tawney (UK) and Frank Walters (League of Nations)

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The members of the mission left Cherbourg on the “Bremen” on 31 August 1931 and arrived in Toronto on 7 September. Langevin stayed at the Royal York Hotel, from where he wrote the following letter: “Toronto, 7 September 1931 “My beloved children, “After the excellent crossing and the exhaustion, for which the sleep cure I took on the boat repaired the annoying fatigue of the weeks before departure, here is the first quiet moment when I can write to you to tell you both once again how sorry I was not to be able to kiss you a little peacefully. I have made a great effort and have only been able to complete a small part of what I should have done and, for the rest, I have sacrificed another part and continue to gradually dispatch those of mes remordes (remorses)106 which are merciless and which I still drag after me. “The first few days, despite the exceptional comfort of my cabin on the Bremen and the solace of the surroundings, including the ocean, and despite the sleep from which I am beginning to benefit, have been very sad and I did not expect to feel so much nostalgia for this sudden separation. Since New York, I am more taken up with the work that is coming, following the conversations we had there with knowledgeable people... “We arrived here this morning, after an overnight train ride from New York, and are spending the day here as we await the departure of the train to Vancouver, which will take us tonight for four nights and four days past the lakes and across the Rocky Mountains. “I will write to you again from Vancouver with a photograph of our group taken on the Bremen. I will take the opportunity to introduce you to my fellow travellers, with whom contact has been excellent. “The night comes to darken the pure and tender blue of the sky, which promises us a beautiful journey. Before undertaking it, I send you my best thoughts and many kisses for Michel107 and for both of you, my dears. Paul Langevin” In fact, we did not get a letter from Vancouver, but a simple postcard: “The train that brought us here in four days and nights from Toronto arrived late because of a rock fall on the track in the Rocky Mountains. This gave us a twelve hours’ halt and an admirable excursion, but leaves

106 Langevin called the letters he had not found time to answer “my remorse”. He also called the articles he had agreed to write and which he had not yet had time to write ‘remorse’. 107 My son.

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me no time to write to you at greater length before the ship leaves. I shall do so at sea. All is well. A thousand kisses to all three of you. P. Langevin.” The promised photograph was received in a letter written on the Empress of Canada on 12 September: “My beloved children, “Our ship, after Vancouver, is stopping for a while this afternoon at Victoria to receive the King of Siam. I am taking the opportunity to complete this morning’s card by sending you a photograph of our missionary group, taken on the “Bremen” (September 4). You will find the names on the back and the comments in my next letter108, written on this ship at leisure. Also, a map of Lake Louise where we had such a beautiful walk the day before yesterday. “Write to me in Nanking, by the Trans-Siberian Railway. I will still be able to receive your letter there before leaving for Peking. A thousand kisses. P. Langevin” The ship taking my father to Japan and China stopped on 18 September in Honolulu, from where we received a new card: “Before leaving this enchanted island I am sending you a card which will only give you a very inaccurate idea109 of what I have done here. A letter from Yokohama will be more explicit. I hope to hear from you there. Many kisses. P. Langevin”. It seems that Paul Langevin did not send the letters he intended to write to us, but then we received a letter from Peking dated 29  October 1931, which reached us on 16 November: “My beloved children, “Peking, 29 October 1931 “Luce’s lovely letter arrived only a few days ago, in the midst of the great excitement and admiration in which I live. After a fortnight spent here without a moment’s freedom, we have just made a four days’ journey to Tingchow, an old town surrounded by an immense wall, 2000 years old, with gates of impressive strength and an admirable pagoda. There we saw villages of peasants, who received us in a touching way, during a day spent on horseback and from which I was left somewhat bruised, but delighted. This morning we had to get up at two o’clock to go by “rickshaw”, on a night with a wonderful full moon, to catch a train which was to arrive at three o’clock and which came quietly the following morning, so that we got here at two o’clock and left at five o’clock for Shanghai, where we will arrive on the 31st in the evening. From there we will go to Hangchow in Chekiang, then to 108 Unfortunately I have not been able to find this letter. 109 The coloured card depicts a half-naked Hawaiian dancer with a necklace of flowers around her neck, and could really have given a false impression of Langevin’s activity in Honolulu.

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Canton, and then up to Shanghai, then to Nanking. Instead of returning by sea, as I had intended, I am giving in to the desire to see Peking again, which has completely seduced me, and to the insistence of the physicists, who have asked me for a series of lectures, and I shall return by the Trans-Siberian Railway after spending three or four weeks here, going on into the New Year. “Write to me at the Nanking address until 1 December, as I shall be there until the 15th, and then to the French Legation in Peiping (now Peking), where I shall arrive on the 16th or 17th. “I am anxious to know how you are all doing, Luce in particular. I will write to you these days, on the train, if it is not too hectic, about André’s work. “I give you three and soon all four of you a thousand tender kisses110. Paul Langevin” There are details about this trip in a letter that my father wrote to me on 15 November from Nanking: “... To avoid any delay, here is our programme as it is currently planned from the moment you receive this letter, i.e. at the beginning of December. Until the 8th, when we return from Canton, pass through Nanking (Weishenshu, Nanking) from the 8th to the 12th we will be in Shanghai (Cathay Hotel) and in Peking (French Legation, Peiping), if nothing prevents me from doing so on the political side... “Since I am talking to you about my journey, I will first settle what concerns me, by telling you that I am very well, thanks to all the care with which we are surrounded, and that I am seeing many beautiful and interesting things, of which I will speak to you at greater length on my return. Above all, two delightful cities, Peking, where we spent a fortnight, and Hangchow, in Chekiang, where the first week of November was admirable in every way for us, weather, things and people. Since we came up from Hangchow to Shanghai, eight days ago, the weather has changed and the rain does not stop; it is the same November weather as in Paris, not cold, but rather sad. We spent four days in Shanghai and two in Wusih, on the railway that leads here, and we arrived yesterday evening to stay here for ten days before leaving for Canton, which can only be reached by sea, via Shanghai and Hong Kong, going there and coming back... “I hope to find a moment’s respite during the Canton trip, though I am not sure, as I am overwhelmed with requests for lectures to be given in December and January in Shanghai and Peking, and I am not sure when I can prepare them.”

110 My daughter Aline was supposed to be born on 15 December, but in fact she was born on 23 December.

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We then received a letter dated 16 December: “French Legation in China     Peking, 16 December 1931. “My beloved children, “For the first time since our arrival in China, I have a little peace and quiet in the very fine accommodation provided by the hospitality of the French Minister. But I must prepare for the series of lectures that I will begin next Tuesday, in less than a week. Having arrived yesterday evening after a fifty-hour journey from the hustle and bustle of Shanghai, I am spending my first few hours of rest thinking about all of you, looking out over the sunny terrace on which the five large French windows of my room open, and beyond that the blue winter sky of Peking, cut by the tracery of the tall trees that adorn the lovely city. I have just come from a bookshop, where I have bought some picture books with which I will try to give you an idea of it all, and also the wish to come there. Although it is hardly suitable at my age, I am already making plans to return to this China, which I infinitely regret not having known previously and remain very guiltily in ignorance about. I hope to take you there, the journey is easy and time passes so quickly amidst so many new impressions... “Good luck, my children, I expect good news from you every day and send you my best kisses. P. Langevin” Then came the departure from Peking after a dozen meetings. In a letter he sent us on 10 January 1932, my father tells us the date of his return to Paris and gives his last impressions: “Peking, 10 January 1932. “My beloved children, “I am still waiting impatiently for a letter giving me some details of the birth of the little girl whose name I do not yet know, but I am not surprised that I do not have it yet, since it was only the day before yesterday that I received the letter that you two and Kiki111 wrote to me on 17 December. “I am looking forward to meeting this fourteenth member of my immediate family. It will be, I expect, on February 3, when the train is due to bring me to the Gare du Nord at 6:43 a.m... “I am made to work hard here, but they are so very grateful that I can refuse nothing, and in spite of a little weariness I continue to do well in the admirable Peking winter. The weather is very settled with a fine sun every day, generally without wind, so that the cold, sometimes very sharp, is very easily borne. I hope that this will last until my departure to 111 It is my son Michel.

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ensure an easy crossing between Tientsin and Daïren. In any case, I am equipping myself with furs to cross Siberia at the coldest time of the year. “Your mother will tell you that I have found a trace of my uncle here112, my mother’s brother who settled here and whose memory is venerated in a temple where I unfortunately do not have time to go. Perhaps I will make the pilgrimage with you. “In the meantime, I’m trying to find out if we still have little Chinese cousins. “I kiss all four of you very tenderly. P. Langevin” Paul Langevin left Peking on 11  January 1932  and was on the TransSiberian Railway on his sixtieth birthday. Three Chinese friends, Tcheng Yen-Tsio and Tchao-Tsen-Lon, and Madame Renée Tchao, who were with him, were kind enough to celebrate his birthday very cordially by offering him the gifts and wishes that are in accordance with Chinese politeness. He was very well received by his Soviet colleagues in Moscow and returned home delighted with his trip. He told us of his enthusiasm and spoke of taking all his family and friends to China on a special train, a project that he never carried out.

FIGURE 14. Left: Paul Langevin with his granddaughter Aline. © Bibliothèque de l’ESPCI. Right: On holiday with Mrs Dubas-Hermin in Merlemont-Warluis (Oise). With his grandchildren Michel and Aline (1932). All rights reserved.

112 The reader has already been given details of this uncle above.

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The spirit of education and the role of science My father remarked that the problems posed by the reorganisation of education in China were, in essence, the same problems that Western nations had to face and, for the most part, had yet to resolve. On both sides, the difficulties had the same origin, the same principles to be applied in the search for solutions and the same ethos to be introduced. The differences were only in degree, the task being more difficult in China: “Until our Renaissance, the conception of education in Europe remained what it was in China some 30 years ago: dominated exclusively by social or religious concerns, it reserved instruction based on the knowledge of texts for an elite of clerics or scholars, who were preparing to teach the masses in temporal or moral matters just through an oral tradition, and whose peasant or artisan activity was based on techniques simple enough to require no schooling. “The new fact which has most powerfully contributed to the transformation of this state of affairs, stable in itself, is the ever more rapid development of technology, itself fertilised by experimental science. Intellectual activity, hitherto concerned with the relationships between men, or of men with gods, or closed in on itself in abstract speculation, has now turned towards investigating nature. Recovering, like Antaeus, new strength in this intimate contact with reality, subjected to the harsh and healthy discipline of the unceasing control of experiment in the construction of an adequate image of the world, the human spirit has become self-aware and self-confident as it measured its strength with the extra power it has gained. Hitherto honed in a preliminary and perhaps necessary way within the closed circle of scholasticism, the ability to understand has shown itself to be the most powerful tool available to men for the rich harvest of natural laws and the technical progress associated with their possession. The discovery of this new power, still very recent in the life of our species, a blind instrument in itself, liberating as well as destructive, requires an adaptation of the social system to the new conditions of life that it represents. The problems which this necessity has posed in the West for nearly two centuries and, by an increasingly direct repercussion due to technical progress, to the whole of humanity, are still far from being solved. It even seems that their solution has never been more urgent or more difficult than at the present time.” My father’s report on educational reform in China, rich in general considerations, was a real introduction to the educational reform that he would later propose for France in 1945–46.

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From the Workers’ University to the French Encyclopaedia In 1932, my father realised his dream of intellectual contact with the people through the foundation of the Workers’ University, which was established in the annex of the Maison des Syndicats, 8, avenue Mathurin-Moreau. With three other intellectuals of great esteem, Romain Rolland, Henri Barbusse and Francis Jourdain, helped by my brother-in-law Jacques Solomon, Georges Politzer, Georges Cogniot, Paul Labérenne, Paul Bouthonnier and many other militant intellectuals, Langevin undertook to bring to reality the project of bringing to the masses the joy of free access to science, philosophy and the other high forms of human thought. At the same time as he was involved in the Workers’ University, my father was actively involved in the French Encyclopaedia, launched in 1933 and placed under the general direction of Lucien Febvre. The best way to describe this is to refer to Lucien Febvre’s foreword, written in 1956 for Volume II of the French Encyclopaedia: “When I drew up the plan for the “French Encyclopaedia” around 1933, I asked Paul Langevin to “set up” the volume devoted to the problems of modern physics. And this great mind, this powerful inventor of ideas, did the Encyclopaedia the honour of accepting my request. I, the incompetent, the ignorant, will never forget that afternoon when, around 1938, in my office in the rue du Four113, the whole brigade of French physics at that time met. This, fortunately, still remains to a large extent the brigade of French physics today: the two Perrins, the two de Broglies, the two Joliot-Curies and a few others of the same calibre. Langevin set out his plan, assigning a field to each of his collaborators. The most critical could only bow to the logical power, the solidness, let us say the mastery of such a work which mobilised with such singular force all the ideas and all the hypotheses of these men. They were so simple in appearance, but they surrounded the leader of this orchestra of creators, imagining and forming a new reality. “Years have passed. Langevin has left us. The Encyclopaedia went into a state of dormancy. It is reborn today and it is Louis de Broglie who does him the great honour of assuming, in the place occupied by his predecessor who was so often hailed as a teacher, the difficult task of explaining the state of the science in which he himself has made such great advances...”

113 The offices of the French Encyclopaedia were at that time located at 13 rue du Four.

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The 50th anniversary of the School of Physics The same year, 1933, celebrated the fiftieth anniversary of the ESPCI. On Thursday 27  April, at 9 p.m., a solemn session took place in the large amphitheatre of the Sorbonne. Paul Langevin gave the first speech, which he had all the more reason to prepare and deliver because he had just suffered a very cruel loss: on the same day, his brother Julien had been buried. My father expressed himself as follows: “The troubled period during which we have both the pain and the honour of living will be marked in history by the essential fact of the marvellous development of the applications of science, by the discovery of the unlimited power of action which comes to us from the penetration of the world by the mind, sometimes for good, sometimes for evil. The movement which has been going on for scarcely more than a 100 years, bringing science and technology together for their reciprocal fertilisation, has grown steadily until, especially during the last half-century, it has achieved the closest and most intimate union... “The field and the factory are thus increasingly transformed into an extension of the laboratory. Science takes on its full human significance because the scientist can no longer remain isolated, but must be linked to the increasingly educated peasant and worker by a continuous chain of intermediaries and interpreters, represented by engineers and technicians at the various levels of their skills and training. “The need soon became apparent to reinforce this link by creating the organisations necessary for the preparation of men to be not only informed of the results of science, but also and above all to be imbued with the method, learning how science is done through direct and prolonged contact with experimentation and through a serious initiation into laboratory techniques, how provisional and animated it is, and what degree of confidence one can have in its results, which are all too often taught in a way that is dogmatic, definitive and dead. “... The growing solidarity between science and technology that I have mentioned above means that the same training must meet the needs of both: a strong introduction to the experimental method based on sound theoretical ideas. Given the common thread of both practice and intellect, it is in the interest of both the training of future scientists and technicians that they should not specialise prematurely and to rely on individual aptitudes to determine their futures. For the young engineer, the details of a particular practical technique will be learnt all the more quickly if the general ideas are clearer and the method of research and observation more familiar. Here, as always, the spirit is worth more than the letter...

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“On the other hand, the young scientist, however abstract his later preoccupations may be, must not prematurely lose contact with the facts or the concrete sense of material or human realities. Only on this condition will the obscurity of formulae, which science extends more and more between our minds and reality as the degree of abstraction and the generality of our representations increase, remain open. “In fact, experience shows, and we must always bow to it, that an increasing number of scientists, and among the best in all countries, have passed through technical training. Examples abound, but I would like to cite only one, that of our great Einstein, whose exceptional faculty of concrete vision through the most abstract theory and his paramount ability of not paying lip service are not unrelated to the fact that he passed through the Polytechnic School of Zurich and published his first works on Brownian motion and relativity while he was still an engineer at the Federal Patent Office in Berne. And many other similar cases are also important...” What Paul Langevin wanted to do with the teaching in the ESPCI challenged his colleagues. One of the teachers, Marcel Tournier, wrote in 1958 in Physique et chimie: “To characterise the gulf that separated his teaching from others, I feel clearly today that Langevin was the only one to understand that the adjective “industrial” which was attached to physics and chemistry in the title of the place needed to be interpreted. “It was not meant to mean incomplete, distorted and vulgarised sciences for the use of a youth that could only have pragmatic and self-interested concerns in his adulthood. On the contrary, he felt that in the future, the school should always be at the forefront of new knowledge; that, in order to achieve ‘industrial’ success, it should be capable of creating something new. “In order to enable these hard-working young people to act effectively in the direction they wanted to go, it was necessary first and foremost to provide them with in-depth knowledge.” Rich lessons, still valid, but still ignored nowadays by those in charge who, for example, wanted to turn the University Institutes of Technology into mere factories of intellectual labourers!

Election to the Academy of Sciences In 1934, my father had to do a rather thankless job. As a candidate for the Academy of Sciences, he had to completely rewrite his CV that he had prepared in 1908 when he applied for a full professorship at the Collège de France. This was a major task, because in 1934 he had a host of new

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publications to his credit. The (new) volume was 95 pages long instead of 45, and the format was twice the size. Paul Langevin was presented in the first group and obtained 30 votes out of 56 in the first round. He was therefore proclaimed elected on 25 June.

Philosophical activity To show the extreme diversity of my father’s work, I think it is worth mentioning the training he received at the ancient abbey of Pontigny. Paul Desjardins who, like Paul Langevin, was a teacher at the École normale de Sèvres, had tried many times to persuade him to join in the discussions at Pontigny; he finally brought him to a ten-day summer school, from 1 to 10 September 1929. The title of the talks was rather oddly formulated: “The Universe without form, and the courage to live”. Thanks to the kindness of my friend G. Durup, I was able to reconstruct the list of participants. There were, among others, Léon Brunschwieg, Jean Schlumberger, André Spire, Martin Buber, Gaston Bachelard and Alexandre Koyré. Langevin was one of the stars was; the other was Léon Brunschwieg. Durup writes: “Langevin was absolutely dominant. His words provoked in me an extreme admiration. For a long time as a student, I had had some excellent teachers: Montel (mechanics) and Caullery (biology) with perfect clarity, Fabry (physics) and Dumas (psychology), more lively. But they only had to explain their degree courses without any unforeseen difficulties. Langevin had to improvise on various questions. He did so calmly, taking time to think, to seek the best term, and the brief wait only enhanced the listener’s appreciation of clarity and precision. Then the wriggling Brunschwieg wanted to say his word, to add on a ‘philosophical’ conclusion. Langevin listened patiently, then politely explained again. The feeling of perfection was then revived in me, its exhilarating pleasure consoling me because of the absence of a radical critique of metaphysical language, that specious verbiage which Langevin implicitly brushed aside, but which I would have liked to be able to denounce, to ridicule on the spot.” The handwritten notes I have found show that my father had read a lot of philosophy. There are passages on Democritus, Aristotle, medieval philosophers such as Saint Anselm and Albert of Saxony, Descartes, Spinoza, Leibniz and Hegel. In the 1930s he read Marx, Engels and Lenin and discussed them with Jacques Solomon. His anti-idealist convictions were known to all his friends and colleagues. On 12  August 1934, Professor Etienne Rabaud sent him an article from Saint-Afrique which had appeared the day before in Le temps and was

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entitled “The crisis of indeterminism”. The author praised Bergson for having said “that movement is the only reality” and that “by assigning mobility a support, the scientist is only making a concession to the habits of our visual imagination”. And Rabaud, ironic about “the eminent metaphysician Bergson”, jokingly noted: “These people are dangerous, although they are not Bolsheviks...”. My father shared this view. In his office at the ESPCI, the Groupe d’études matérialistes (Group of Materialist Studies) had been meeting since the beginning of the 1930s with scientists such as Henri Wallon, Georges Fournier, Professor Lahy, Paul Labérenne, René Maublanc and others. My father’s deep knowledge of Marx’s works and of dialectical materialism is demonstated in particular by the remarkable article he wrote in 1939 in La Pensée on Modern Physics and Determinism. It is shown even more clearly by the speech he gave on 10 June 1945 on the occasion of the inaugural session, at the Palais de Chaillot, about the “Encyclopaedia of the French Renaissance”, an undertaking which was unfortunately subsequently abandoned. My father expressed his opposition to positivism on many occasions; in this respect, we find the following in his speech at the meeting organised in Warsaw from 30 May to 3 June 1938 by the International Union of Physics and the Polish Commission for Intellectual Cooperation114: “Despite its successes, I would like to emphasise that this positivist doctrine is rather narrow. There is in the overly clear-cut statements of present-day positivism too much direct reference to immediate experience, which confines us to the present. “This doctrine, with its more precise and narrower position, denies history, because it has no possibility of going back into the past in the sense of immediate experience. What for us is history, it will be said, is the immediate experience that can be derived from the facts of the past. “The difficulties that exist looking backwards also exist looking towards the future, and as Reichenbach, one of its most outstanding theorists, has rightly pointed out, positivism is obliged to give a special role to induction, hence contains a source of great difficulty. “The proof that this doctrine, left to itself, willingly closes the future and is a static doctrine, is that its first author, Auguste Comte, was not afraid to set limits to the possibilities of the experimental chain; he considered that we could never know what happens in the stars. Very soon afterwards he was disproved by the discovery of spectroscopy; and this very morning we were able to hear Sir Arthur Eddington speak of the temperature, the state of disintegration of atoms, and the nuclear chemistry in the interior of stars. 114 Published by the International Institute for Intellectual Cooperation. Paris, 1939, pp. 235-237. Reproduced in La pensée et l’action, pp. 124-127.

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“It is certain that by translating the assertions of scientific laws into the language of experience, that is, into the language where the senses play an essential role, this doctrine willingly takes an attitude opposed to realism115. I believe that physicists would be binding themselves in a very narrow and awkward way if they renounced the word reality, and so you will feel that I personally am a realist. I believe that it is difficult to be an experimental physicist without believing in reality, not only of other physicists, but also of the world. And if one considers as meaningless any assertion concerning the reality of the external world, if one considers the essentially collective character of our science as resulting from our common contact, from our realities and communications, in which we postulate our shared existence, if one speaks of an inter-subjectivity, I confess that I do see subjectivities, but I don’t see how we can speak of inter-subjectivities, because then each of us is locked into the role of subject, feeling and thinking, but feeling without incentive to act, since there is no external reality on which we are prompted to act. “This attitude is therefore essentially critical, analytical and static; it is more suited to taking stock of acquired knowledge, to clearly formulating the structure and content of this knowledge than to showing the way to extend or renew it, more suited to pointing out difficulties than to solving them. It allows the elimination of concepts or theories, the denunciation of problems and empty statements, but it does not allow the formulation of indications for the construction of new concepts or theories. “This critical attitude is therefore valuable in preparing the way for the constructive attitude, but it is insufficient in itself, and it even seems that some physicists I have talked to about these questions consider that they are being somewhat insulted by saying that they are only concerned with tautology. “Mathematicians are aware that the notions they work on are also evolving; it is certain that the notion of number, from the beginning and passing through the different stages of the continuous and the discontinuous, the theory of sets, etc., represents something that involves a real construction, expressible in the language of logic and mathematics, and in which the contribution of the mathematician seems to play a considerable role. “The positivist or the logistician may well dissect the content of a doctrine, but he does not have in his very conception of mathematics the means to develop, to construct, to make true syntheses within this doctrine.”

115 In Paul Langevin’s mind, this ‘realism’ is materialism.

Paul Langevin, symbol of the resistance Against Munich – The Gennevilliers Declaration and the founding of La Pensée Paul Langevin had already condemned the capitulation to Hitler and Mussolini when the Munich Agreement was signed in September 1938. No one could have been more aware than he of the fatal consequences of this betrayal: servitude and war. He declared himself against Munich from the outset. For years, my father had been moving towards communist policy, which seemed to him the only right one, and towards the Communist Party itself.

FIGURE 15. National Conference of the French Communist Party, Montreuil (January 1937). With Maurice Thorez, Pierre Sémard, Jacques Duclos and Arthur Ramette. All rights reserved.

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He expressed his gratitude to the Communist Party and to Marxism in his statement to the National Party Conference in Gennevilliers on 26 December 1938. This statement was entirely improvised. My father had come with Luce and was sitting in the seats left free at the top of the galleries when he was called to the chair, and it was then that he said spontaneously: “It is the honour of your party to closely unite thought and action. “It has been said that a communist should always be educated; but I want to tell you that the more educated I am, the more communist I feel. “In this great doctrine, illustrated by Marx, Engels and Lenin, I found the clarification of things that I would never have understood in my own science. “Lenin, as well as Marx and Engels, were steeped in the thinking of those who founded the French Revolution. “Your party is the only one with clear ideas: it is a kind of extension of the French Revolution, just as the doctrine of Marx, Engels and Lenin is an extension of the ideas of the great French thinkers of the eighteenth century.” Paul Langevin and Georges Cogniot soon created the publication La Pensée (Thought), whose appearance marked a turning point in the history of intellectual life in France. Maurice Thorez and Georges Dimitrov welcomed this initiative. La Pensée was intended to defend and propagate Marxist philosophy, but, as René Maublanc would later recall, my father wanted it to be called Revue du rationalisme moderne because he saw dialectical materialism as the heir to the great tradition of the Enlightenment and the modern form of advanced thinking. In the first issue of the new journal, whose first editorial secretary was André Parreaux, Paul Langevin gave his key article entitled: La physique moderne et le déterminisme. It was an open fight against indeterminism and against the position of certain theorists who had fallen into idealism.

La drôle de guerre116 A few months later, as Paul Langevin had foreseen, the Second World War broke out. Maurice Thorez’s declaration to the Communist parliamentary group on 25 August 1939, calling for resistance if Hitler persevered in his plans for aggression, was systematically suppressed. The ruling class wanted to sow disarray in the progressive camp by any means, in order to be able to strike more surely at the communists. Pressure was exerted by radicals such as 116 * This French expression refers to the first months of the war, when nothing happened on the front. It is said to have its origins in a comment from a soldier to a journalist meaning ‘its a funny war’. Equivalently, this time was referred to as “the phoney war” in America and in Britain.

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Albert Bayet and socialists to make Paul Langevin turn, but their efforts were in vain, thanks to my father’s solid convictions and the influence of the communist militants in his family, mainly that of his son-in-law Jacques Solomon, who was not shaken for a moment, and that of Luce and my sisters, who had already joined the Communist Party at that time and were active militants. At the beginning of October, the repression even reached the World Committee against War and Fascism. Paul Langevin addressed a very dignified and firm protest to Daladier against these measures.

FIGURE 16. At a banquet of activists. Seated at the back, from left to right: Racamond, Prenant, Mme J.-R. Bloch, Henri Wallon, Mme P. Langevin, G. Cogniot, P. Langevin, Mme H. Wallon, J.-R. Bloch. In the foreground, from left to right: Billiet, Laberenne, Mme Laberenne, Marcel Cohen. © Bibliothèque de l’ESPCI.

At the trial of the communist deputies My father’s civic sense was such that he knowingly agreed to testify at the trial of the communist deputies on 29 March 1940. Florimond Bonte has this to say on the subject117: “The presence of witnesses was, in itself, an admirable act of civic courage. Each statement could, in fact, earn each witness prosecution, conviction, detention in a camp or prison. To dare to come before a 117 Florimond Bonte: Le chemin de l’honneur, Éditions Hier et Aujourd’hui, 1949, pp. 308-312.

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military tribunal in March 1940 and pay tribute to the honesty, probity, devotion and sincerity of the communists was to be entered in the book of suspects, and was considered by the government, its judges and its police to be an offence and a crime punishable by the most severe penalties... “Many men and women had this courage. They knew, when they came to the courthouse, that they might never go home. And they did not give in to fear. They were there, in front of us, and every word they spoke, expressing their greatness of spirit, moved us to the depths of our hearts with infinite gratitude. They were there from all walks of life, scholars, professors, writers, artists, teachers, priests, doctors, lawyers, social workers, housewives, shopkeepers, craftsmen, workers and peasants.” We first heard from old militants such as Daniel Renoult, already held in the Bayet camp, and Marcel Cachin, the founder of the Communist Party, then aged seventy-one. Then came the testimony of René Maublanc, a philosophy teacher at the Lycée Henri IV, that of Jean-Richard Bloch, a talented writer and director of the newspaper Ce soir, which had been banned by the government in August 1939, and that of a professor at the Collège de France, Dr. Henri Wallon. Here is the text of my father’s deposition. He had learned it by heart and, in our country house in Samoreau, he came to test it against my wife Luce’s judgement, the only member of the illegal Communist Party with whom he had contact at the time. He told her about it during a long walk in the paths of our garden. Nothing was more moving than my father’s concern to serve the cause that was dear to him as effectively as possible: “I know most of these men, all of whom are qualified representatives of an important part of our country by their personal value. I have had many opportunities to meet them and to appreciate their ability to guide ideas as well as in action: political action in the meetings of the People’s Rally, where I represented the League of Human Rights or the Committees of Intellectuals, action of popular education in the meetings of the Workers’ University, or daily action linked to the exercise of their legislative or municipal mandate. “I have seen the highest moral value in all of them, the most absolute devotion, disinterestedness and integrity, and I am happy to be able to pay them this tribute. These men only wanted to keep the minimum strictly necessary for themselves out of the parliamentary allowances. I have always found them concerned with the public good, with the development of educational works from the most elementary level to the highest culture, with public health and with the solution of the many problems concerning children. “I met with them to discuss ideas, sharing their ideal of social justice and their desire to implement this ideal through a human effort to transform the world materially and morally. To achieve this, they put their trust

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in the possibility of an unlimited enhancement of science and human awareness: science of nature, the applications of which, truly placed at the service of all, would make it possible to ensure their liberation from now on, to overcome misery, ignorance and the suffering they entail; science of the development of human societies, making it possible to understand the laws of their evolution, highlighting the predominant role of the conditions of work and of the social organisation that they involve. Reaching justice through science seems to me to be the formula that best summarises their doctrine. “It can only appeal to those who, like me, believe that the tragedy of our time comes from an imbalance between the power to act that science gives us and the insufficient development of social or international justice, from the delay of justice behind science. “The present social organisation means that the new means of production, instead of improving the well-being of all, only exaggerates inequalities by increasing without limit the wealth and power of some, while creating unemployment and misery for others. The absence of international justice means that the unlimited increase in our means of destruction results in an outburst of violence, which endangers the future of our species and its civilisation. “There is only one remedy for these evils that result from the lag of justice behind science: put science at the service of justice. And in this way we are in line with communist thinking. “Another aspect of this doctrine, which is essential for me, is that it is based on a dialectical philosophy, which extends the great line of human thought from the Greeks and, through Descartes, our eighteenth-century philosophers, Kant and Hegel, to Marx, Engels and Lenin. I owe this philosophy the tribute of my gratitude, because it has allowed me to understand better my own science and the history of its development as a particular case, of the fact that all life, material, intellectual, moral or social, unfolds through a dialectical series of contradictions, overcome by syntheses, mutations as the biologists say, towards increasingly higher forms of understanding or action. “Since I have been following the deeds of these men, I have always seen them dominated by the ideas I have just mentioned, directed towards high goals and ends of general interest. I have always seen them fighting passionately against misery and injustice, striving to expose their ideas and to convince others. “Obviously, since the questions raised were not specific to our country, this action was linked to parallel actions undertaken abroad, supported by an international organisation whose aims and means were perfectly clear and lawful. They did not receive their directives from abroad any more

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than I receive mine when, because of the international character of science, I teach or propagate in France discoveries made abroad and which belong to international scientific organisations. “Besides the Justice International and the Science International, many other international organisations exist for the defence of material or spiritual interests, and their more or less hidden watchwords, especially as regards economic or financial organisations, are not always in conformity with what we, one or other, may consider to be the national interest. “And do we not all agree that, in the present state of the world, given the ever-accelerating development of our means to spread, produce and destroy, it is necessary to go beyond the national point of view and to prepare, in the time of war, an international economic and legal organisation which will singularly strengthen the bonds of solidarity between nations? “I leave it to the defenders to show you the futility of the proceedings here, and the fact that the acts complained of are perfectly in accordance with the rights and duties of representatives of the people; but I can affirm, having seen them at work, that the action of these men has always been directed, in their minds, to the good of the nation, and has never been directed against it. “I confess, for my part, that I do not understand how, in a democracy where all political life, even in times of war, consists of an incessant confrontation of opinions, the fact, especially for a parliamentarian, of having a conception of the national interest different from that of the majority, of defending or propagating it, can give rise to prosecution, disqualification without possibility of defence, or dismissal. “It is impossible, without profoundly disturbing collective life and without creating injustice, whether in relations between individuals or between individuals and the community, to oppose ideas with anything other than ideas, arguments with anything other than arguments. To proceed otherwise, to oppose ideas with violence, to impose silence on opinion, to employ revolting totalitarian methods, is to give proof of weakness, of self-distrust, and to harm the cause which one defends, especially when this cause has the claim to be that of freedom and human progress. “Does the regime under which we live have such a bad conscience that it cannot bear free discussion, in Parliament or before public opinion, of anything that does not concern the secrecy of military operations? Is it not the best way to sustain the morale of a free country to treat it as an adult and give it the impression that the truth is nothing to fear? “It is in a closed environment, in the absence of light and open air, that the most toxic fermentations develop. “Let us leave to our opponents the solutions of laziness and the use of violence against the spirit. Let us look instead to Great Britain, where, perhaps

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because of a longer tradition of liberalism than ours, prosecutions such as these would be quite impossible, and where, as I was able to ascertain on a recent trip, public opinion is infinitely better informed than ours. Wider information and better organisation of radio broadcasts in response to enemy propaganda would serve our cause better, both internally and externally, than trials behind closed doors against those who are only accused of not thinking like the majority, of committing the crime of non-conformism. “However, we must not forget that conformity is fatal to civilisation, that all human progress is the work of individuals or minorities acting through persuasion. “Violence can do nothing for or against an idea: either it is fruitful and triumphs over all persecution – any effort to stifle it only increases its explosive power – or it is without human value and falls into oblivion by itself. “At the start of all great innovations, of all the great movements of religious, scientific or social thought, there is resistance to the established order, difficulties or persecutions on which the future is not slow to pass a very severe judgment. Let us not write pages in the glorious history of our country of which we could be ashamed tomorrow118.” Paul Langevin was not immediately concerned after his deposition. But it was not long before Pétain took charge, in collusion with the Hitlerites, as we shall see later.

Transfer to Toulouse As soon as war was declared, my father put all his research resources at the service of national defence, as he had done in 1914. At the time of mobilisation he was head of Group IV of the C.N.R.S.119, which included all the laboratories of the ESPCI and his laboratory at the Collège de France. Before 25 May, the government decided to evacuate the C.N.R.S. laboratories from Paris. My father was ordered to leave urgently for Toulouse and on 28 May he received two ‘mission orders’, for himself and for one of his assistants, with the specific aim of going there to prepare a fallback position for all his laboratories. He appointed me as his assistant to accompany him to Toulouse. My father and I took the train to Toulouse on 29  May. After an uneventful journey, we went to see the rector of the Academy of Toulouse, M. Deltheil, who received us very kindly. He promised to make available the Faculty buildings located in Allées Jules Guesde. On 10 June, with German

118 In the versions of this deposition published so far, the last sentence was missing, since the last page of the manuscript was considered lost. It was recently found. 119 * The Centre national de la recherche scientifique was formed on 19 October 1939.

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troops about to occupy Paris, my father received the final order to remove his laboratories and staff to the banks of the Garonne. My father’s assistant at the Collège de France, Jacques Nicolle, has faithfully described the activity that Paul Langevin had to deploy at that time: “… All those who, like me, lived closely with him during those tragic weeks will remember with emotion the devotion of our leader for all those men and women from France and other countries, who joined us and for whom he gave himself without counting the cost. “How many visits did he make to overcrowded Toulouse, in this stormy heat and in the midst of general anguish! I was often woken in the middle of the night by worried people, who asked me for his address so that they could go to him immediately to bring their complaints. The universal influence of Paul Langevin, both on the scientific and human levels, was apparent in the midst of the turmoil, and all those who were distraught, all the uprooted people, turned to him. The French army was in retreat; the city was receiving more and more refugees every day, and in certain circles real panic was emerging, and there was only talk of leaving for Spain or even for England or America. Paul Langevin remained calm, but one could feel that all these people who were spreading alarming news were obviously irritating him. Finally, one day at noon, while we were having lunch in the canteen of the Faculty of Science, news suddenly reached us: we got up and went to the concierge’s lodge, where the radio set was; we all stood up and heard a mumbling voice announcing that an armistice was going to be concluded with the Germans with dignity and honour. And to top it all off, the speech ended with la Marseillaise! Langevin, visibly moved, but keeping his sarcasm, whispered in my ear: They would have done better to play the Chant du départ.” Paul Langevin had heard from friends, also in Toulouse, that Jean Perrin had decided to go to America to help with the war effort there; he was urged to join Perrin in Bordeaux. But he decided that leaving the country and leaving the field open to the invader was not the best solution. Instead, in July, he took one of the first trains that were made up in Toulouse back to Paris, where he arrived safely. Paul Langevin’s priority was to get back into the ESPCI, as he said in a letter he wrote to my wife on 30 July.

The arrest In September, we returned to Paris. Then nothing important happened until the evening of the All Saints’ holiday on Wednesday 30 October. That day, while I was busy with my students at the Laboratoire d’électrotechnique et de radiotechnique at ESPCI, two Gestapo cars arrived at

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10 rue Vauquelin and immediately the Germans barred all the exits from the administration building, so that I was unable to witness either my father’s arrest or the search of his office. After getting him into one of the cars, the Germans left, taking him to an unknown destination. They went as far as the Porte d’Orléans, appearing to be heading for Fresnes, then reached Longjumeau, from where they turned round abruptly and returned to Paris, passing the Porte d’Orléans again and taking my father to the Santé prison. There he was treated like a common prisoner, and he was spared nothing. He was searched, his personal belongings were confiscated, including his pen, tie, braces and shoelaces. He was in the Santé prison just a stone’s throw from us but, for three days, in spite of all that we did and the telephone calls we tried, we remained deeply worried without any news.

From thwarted interrogations to charcoal calculations We couldn’t see him until several days after he had been interrogated on 25 November by Colonel Boehmelburg, one of the heads of the Gestapo in Paris. During this interrogation, this policeman told him that the Hitlerites considered him to be as dangerous to National Socialism as the philosophers of the eighteenth century were to royalty and the ancien régime, which made my father quite proud, as might be imagined. These policemen questioned him about his relations with politicians, Maurice Thorez, Jacques Duclos, and so on, with scientists such as Einstein, about his views about Germany and about his participation in the alleged campaign to incite the war. On his position towards Germany, Langevin told Colonel Boehmelburg: “I have always had great admiration for the German contribution to the work of civilisation in the various fields of science, literature, art and technical progress. I wanted to get to know Germany and to show it to my children120 by going there on my own initiative for two holidays, in 1903 to Göttingen and in 1912 to Heilbronn. I was invited, in a scientific capacity, to participate in three Naturförscher congresses in 1903 in Cassel, in 1911 in Karlsruhe and in 1912 in Heidelberg. “I was also invited in 1924  and 1925 to lecture at the University of Hamburg on the ultrasound technique that I had created.

120 Paul Langevin was not boasting. On the contrary, he left out that he had given us a German governess, whose name was Fraülein Heichhorn and who came from Heilbronn.

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“I have always considered collaboration between France and Germany in all fields, whether economic or intellectual, to be highly desirable. I have endeavoured to contribute to this, not only through the relations that I have established with German scientists during the congresses in which I have participated, but also by taking the initiative, some 10 years ago, of a collaboration between the Journal de physique which I edit and the Physikalische Berichte, which was then directed by Dr. Scheed, with whom I met several times and who had shown himself to be in agreement with my project. “I only consider that this collaboration, to be effective and lasting, must be based on the rules of all human morality, applicable to nations as well as to individuals: respect for personality and the duty of solidarity towards all.” At the end of his interrogation, Paul Langevin made this important statement: “My action has always been: “1. Only at a human level; I do not place myself in any race, sect, or political party; “2. Only in the field of ideas; for the defence of those who are dear to me; for individual and collective justice, freedom and peace. I have always sought to convince those I have spoken to of the dangers and absurdity of war and of the possibility of putting an end to it by an organisation of international justice and police and above all by the development of collaboration between peoples. I have never at any time said or written a word in the sense of a provocation to war; “3. Absolutely in the open; on the basis of public exposure and discussion of ideas and facts. I have never participated in any hidden action of any kind; “4. Absolutely disinterested: I have devoted almost everything I had to it and have no assets or resources other than my salary or possible retirement pension.” A few days later, the Nazis allowed us to visit our father at La Santé. His walk, with his trousers falling down for lack of braces, with his discomfort when walking for lack of shoelaces, with his shirt without a tie, made him look 10 years older. Fortunately, his morale remained high. In his cell, he managed to work despite the lack of books, paper and pen. In the corridors and in the prison yard he had picked up burnt bits of matches, with which he managed to write with great difficulty by dipping them in the activated charcoal that the doctor at La Santé had given him to treat his digestive problems. With admirable patience, he managed, during the thirty-eight days he spent in cell number 112 at La Santé, to work and write calculations on a roll of toilet paper. We have kept this roll; it contains exactly

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sixty pages of calculations, the date is written on it and we know that the calculations were made from 21 November to 2 December 1940. I am certain that it is, on the one hand, a complete calculation of an oscillating circuit with capacitance and self-induction consisting of a coil with a core of “permalloy” (a special iron-nickel alloy), and on the other hand, complementary calculations for the thesis of Ouang Té-tchao (one of the scientific workers of the research laboratory of the ESPCI) whose subject was related to the ionisation of gases. But Paul Langevin did not only distract himself with intellectual occupations, he also worked with his hands: he made a soap dish for his cell, with bits of wire that he had collected here and there; we have carefully kept this DIY soap dish. On 8 November 1940, as soon as Langevin’s imprisonment became known, protest demonstrations were organised. As Lablénie said, my father’s arrest was the beginning of the university resistance. The first demonstration was organised by communist students on 8 November in front of the Collège de France. Frédéric Joliot demonstratively refused to appear in his laboratory and to collaborate with the German soldiers who had been imposed on him as researchers. On 11 November, the students staged a demonstration at the Étoile, which was, as we know, ferociously repressed. Langevin’s arrest also gave rise to the publication of the university resistance, L’Université libre, edited by Jacques Decourdemanche, Georges Politzer and Jacques Solomon. The first issue of L’Université libre, dated November 1940, was almost entirely devoted to Paul Langevin.

Courageous friends I would like to pay tribute here, on behalf of Paul Langevin’s family, to the men of dignity and courage who were not afraid to send messages of friendship to my father at La Santé. Among them were Élie Cartan, Paul Montel, Paul Le Rolland, Jean Cassou, Maurice Curie, Émile Labeyrie and many others. Noble letters came from Marguerite Borel, Jacqueline Hadamard, and the students of Sèvres. My father’s assistant at the Collège de France, Jacques Nicolle, wrote to him on 11  November, in hushed tones, that he was very happy with his “work” and gave to understand that the protest movement was developing: “I am eagerly occupied with rational mechanics”. Francis Perrin wrote on 16 November: “My dear master, “I am one of those who feel most deeply affected by all that has happened to you; you must know this, but I cannot forgive myself for not having written to you before. The emotion caused by your arrest continues to grow among your friends, your followers, and all those innumerable

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people who, from near or far, have benefited from the radiance of your universal soul. You are suffering for all of us and we are sorry that you have to bear alone the rigours that have befallen you…” Here is the beautiful letter from Jean-Richard Bloch, dated 7 November: “My dear and great friend, “When my wife and I returned from our trip, we were shocked to learn of the measure of which you had just been the victim. “I do not know the reasons given by the authorities for taking and continuing such a decision. I hope that it will not be continued beyond the time necessary for them to be convinced of the tremendous mistake they have made in striking one of the most representative personalities of world science and the perfect honest man who universal opinion salutes in you. “Your wife, who my wife went to see at once, and your children, who I also met, are courageous, confident and full of calm in their pain. All your friends share the feelings that I am expressing. The emotion is widespread and deep; people are talking about it in the streets, in the metro and in the Latin Quarter. “See you soon, I hope, my dear friend. Soon the joy of embracing you in freedom. Yours faithfully and affectionately Jean-R. Bloch”

The solidarity of Einstein and Kapitza The news of Paul Langevin’s imprisonment also aroused strong feelings throughout the world. It should be remembered that Albert Einstein, who had left for America, had in September 1940 already taken steps secure to a refuge for his friend in the United States. Immediately after Langevin’s arrest, Einstein intervened again to obtain his release. The proof is provided by his letter of 7  November 1940 to Ambassador Bullitt: “My dear Bullitt, “The contents of the dispatch from Geneva concerning the news of Professor Langevin’s arrest, which you enclosed with your letter, cause me the greatest concern. I think you must have met this great man of science and this admirable personality. Feeling absolutely incompetent in this matter, I take the liberty of asking your opinion as to the possibilities of helping him. Would it be wise to publish his case in the American press?

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“Happy with the re-election of President Roosevelt and with my sincere admiration for your good political activity, I remain your sincerely devoted servant, Professor Albert Einstein” Two days later, on 9  November, Bullitt sent the following reply to Einstein: “Department of State, Washington. 9 November 1940. “My dear Professor Einstein, “I thank you very much for your letter of 7 November. “I am sending a telegram today to my office in Paris to see what can be done for Professor Langevin. I have also asked the Embassy to do everything possible to help Professor Langevin. “With all my best wishes and friendly sentiments, I remain sincerely yours. William C. Bullitt Professor Albert Einstein The Institute for Advanced Study School of Mathematics, Princeton, New Jersey” A little later, on 28 January 1941, Einstein went back to the chairman of the Advisory Committee for Political Refugees, George Warren. For his part, in the USSR, Kapitza approached the leaders of his country to intervene on Langevin’s behalf. I was able to obtain photocopies of the letters he sent at the time to Vychinski and Lozovski. Kapitza’s requests were effective. The Academy of Sciences of the USSR invited Paul Langevin to come and work in Moscow through the consular agent in our country. But my father wanted his situation as a civil servant in France to be settled first. That is why he postponed his departure. While the invitation from the USSR Academy of Sciences was taking shape, Langevin’s imprisonment in La Santé had led to a worldwide wave of protests, the most effective actions naturally coming from those countries that were not at war with Germany at the time (Switzerland, USSR and the United States).

Deported to Troyes and revoked Without accepting my father’s departure for a foreign country, the Germans considered it more appropriate to back down somewhat in the face of general indignation. From 9 December 1940, Paul Langevin was placed under house arrest, in the flat of a Jewish man, Jean Blum, 18 rue Raymond-Poincaré in

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Troyes, which had been requisitioned by the Gestapo. The Gestapo forbade him to leave the city of Troyes and, every two days, he had to sign on a special register at one of the two Commandants’ offices in the city121. Before his release from prison, where he had spent 38 days, from 30  October to 7  December, he was removed from his academic post and placed into retirement. Thanks to the kindness of the administration of the Collège de France and especially of Madame Brucker, who I would like to thank here, we were able to have all the details on how this happened. It was indeed on the orders of the German authorities that the dismissal took place. Here is the official letter that the French government delegate received from the military commander in France: “Letter dated 31 October 1940 “The military commander in France. “(Administrative Staff – Administrative Section, ref. V.  Kult.) to the French Government Delegate to the Military Commander in France – 127, rue de Grenelle, Paris “The activity of professors Ernest Tonnelat, Paul Langevin and Henri Wallon at the Collège de France was incompatible with the interests and prestige of the occupying authorities. “I would ask you to take the necessary steps and inform me of the measures you have taken in this regard...” Naturally, the Vichy government was quick to comply, and the official journal of 20 November 1940, page 5748, published the following order: “Withdrawal of functions “The Secretary of State for Public Education, “With regard to the law of 17 July 1940 concerning magistrates and civil and military officials relieved of their duties; “With regard to the law of 23 October 1940 extending and amending the provisions of the law of 17 July 1940; “With regard to the law of 27 July 1940 on the form of individual acts, “ARREST “Article  1. – Mr. Langevin (Paul), professor at the Collège de France, director of studies at the École pratique des hautes études122, is placed in 121 Before taking him to Troyes on his release from La Santé, the Gestapo officer who accompanied him took him to his home at 10, rue Vauquelin, so that he could prepare his luggage. He was left to dine and sleep at home; the family then telephoned close friends that Paul Langevin had been released, that he could see his friends in the evening, but that he would be taken to Troyes the next morning. There was a crowd of friends in the rue Vauquelin that evening, all of them happy about his release from prison, but worried about the future. 122 It seems that the Vichy authorities confused the director of studies at the École des hautes études with the director of the École de physique et chimie.

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this double capacity in the position provided for by article 1 of the law of 17 July 1940, modified by the law of 23 October 1940. “Article 2. – The Director of Higher Education is responsible for the execution of this order. “Done at Vichy, 19 November 1940 Georges Ripert” Out of curiosity, I investigated the text of the “law” of 17  July 1940, under which my father was relieved of his duties. It is a curious document, which places an official stamp on the purest arbitrariness: “Law of 17 July 1940, concerning magistrates and civil or military officials relieved of their duties. “Article 1. – For a period ending on 31 October 1940, magistrates, civil servants and civil or military agents of the State may be relieved of their duties notwithstanding any legislative or regulatory provision to the contrary. The decision will be taken by decree, on the sole report of the competent minister and without further formalities. “Article 2. –... For a period of three months, salary plus allowances shall be received... “Article 3. – A subsequent decree will determine either the reclassification or the admission to retirement... “Article  5. – This decree shall be published in the Official Gazette and executed as a State law. Vichy, 17 July 1940 Philippe Pétain Raphaël Alibert, Keeper of the Seals, Minister of State for Justice Adrien Marquet, Minister of State for the Interior Yves Bouthillier, Minister of State for Finance General Weygand, Minister of State for National Defence”

The meeting with Marguerite Flavien Paul Langevin was imprisoned like a common criminal, then relieved of his duties and placed under house arrest in a small provincial town with no science faculty. The Nazis intended to isolate him from his friends. Their calculation turned out to be wrong. He became a symbol of intellectual resistance. Near Troyes, in Voué, my father found another exemplary activist and a woman of great heart, whom he had known when she was a student at the École normale supérieure de Sèvres. This was Madame Marguerite Flavien,

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née Buffard. Paul Langevin himself recounted, in a moving tribute123 to her memory, how he had met her in 1932 and how he saw her again in Troyes: “I met her at the École de Sèvres, where I used to come and work with the science students. Already preoccupied with philanthropic actions, she came to ask me for my signature to a protest against Italian barbarism in Ethiopia. I did not see her for more than 5 years, and then under quite unexpected circumstances. A few days after my arrival in Troyes, on my way up to the flat where the Gestapo had lodged me, I came across a young woman on the stairs who I seemed to recognise, but who, as she told me later, “ran away like a child” out of shyness, after having placed a precious offering of butter and eggs in front of my door.” Paul Langevin saw her again later at the home of mutual friends: “I used to help her,” he says, “to get books, which she read avidly during the season when she was less busy in the fields.” She, in turn, helped Paul Langevin magnificently at a time when supplies were especially difficult (1941 and 1942). She brought him magnificent parcels until the Germans had her arrested in the field by Frenchmen in the pay of the Gestapo in September 1942. Imprisoned in Troyes in a filthy room, she was shortly afterwards transferred to the Indre-et-Loire camp in Monts, as a “preventive measure”. This treatment was due to her pre-war militant activity. Miss J. Streicher, in her obituary, gives us details of her conduct in this camp: “You can’t imagine,” said one of her fellow prisoners, “how much she did for us in the camp. Morale was rather low; when she arrived, she immediately cheered everyone up... She immediately organised a gymnastics class, a choir, she gave lectures, French lessons, and she made us do dictations. And above all, she brought us together to do the “news commentary”: she would take the newspaper and read it back to us, giving courage and cheerfulness to the most desperate. And she tidied up, cleaned, looked after us; few received parcels, so she gave away almost everything, receiving herself some very nice packages.” But the odyssey that followed is worth telling again, because it is extraordinary. Here again, we quote Miss Streicher: “On 25 July 1943, the inmates, hearing of the capture of Kharkov, sang: “La victoire en chantant nous ouvre la barrière...” Eight days later, the Hitlerites surrounded the camp and took the “strong leaders” to the camp 123 Extract from the booklet published in homage to the memory of Marguerite Flavien by the Troyes section of the Union of French Women (reproduced in the obituary by Miss J. Streicher, which was published in the brochure À la mémoire des Sévriennes mortes pour la France (1939-1945), p. 78).

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at Mérignac, Marguerite Flavien was among them... Sensing the threat of deportation, she managed to escape.” At night, in a London-like fog, after working for one and a half hours, she managed to get through the barbed wire. She arrived in Paris on 19 December 1943. But she still did not feel that she had done all her duty. Instead of staying in hiding, she joined an insurance company under a false name. Soon she asked to be admitted into the FTP124 as an intelligence officer. She joined as head of department. It was while carrying out her dangerous work that she was arrested by the Militia, interrogated and tortured. She did not betray, but died on 13 June 1944, horribly injured, in the courtyard of the Milice building in Lyon. But, one may ask, how did a brilliant student of the École normale supérieure de Sèvres come to be a simple farmer in Voué during Paul Langevin’s forced residence in Troyes? After leaving college, Marguerite Buffard had taught and been very active in Colmar, Caen and Troyes, setting an example of dedication and self-sacrifice everywhere. In 1939, she met a farmer and communist section secretary, Jean Flavien, who lived in the village of Voué, 20  km north of Troyes. The two militants got married on 12 August 1939. Flavien was mobilised, and his wife was suspended from her duties at the Lycée in Troyes on 18 December. After trying to work in a hosiery factory, she decided to replace her husband on the farm and live with her parents-in-law. She learned to plough, sow, harrow and look after the animals. Her exhausting work on the land enabled her to contribute, as she said, to “feeding the cities”, that is, first of all the militants among whom she included Paul Langevin. One can understand the admiration my father had for this heroic woman. But he also had the pleasure of finding in Troyes one of his first pupils from Sèvres, a scientist this time, Miss Hélène Leschot, who was headmistress of the girls’ high school and who also contributed greatly to softening his isolation in an unknown city.

The group of friends in Troyes He also made friends with a fair number of the citizens of Troyes that he had not known before. He came into contact with practically all the intellectual resistance fighters in the city. Mr. Casati125, who was the first of these to

124 * FTP: The French resistance organisation Francs-tireurs et partisans. 125 Mr Casati was at that time the headmaster of the Lycée de garçons in Troyes.

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meet Paul Langevin, recounted how he got to know him, in a speech given on 6 March 1949 in the town hall of Troyes in memory of my father: “It was at the beginning of December 1940, when one of the two Commandants that we were “lucky” enough to have, had been installed in a room opposite the station. For some reason – I think it was to avert a threat of requisition on the very modest premises where the school had taken refuge – I went to these offices. As I was leaving, a young French interpreter who knew me came up to me and, pointing to a gentleman who was there, said: That is Monsieur Langevin. “I did not know him, except, of course, by reputation, and went up to him and introduced myself. He had just made his first visit to the Commandant and had for the first time signed the register, which was to receive so many other signatures later on. Together, we went out and I accompanied him to his home through rue Thiers and the Place de l’Hôtel de Ville. The Germans had sent him to Troyes, he said, assuring me that in that town he would have a laboratory in which he could work. And he asked me if there was a possibility for him to continue his research in one of the senior schools. I couldn’t help smiling as I gave him a negative answer, because at that time the two senior schools were unable to provide any Langevin with equipment or rooms. We were living in makeshift premises and the cramped students were continuing their studies as best they could, thanks to the dedication of the staff and the goodwill of all. Our experimental equipment had been looted or destroyed. The promise made to Mr. Langevin was another example of German bad faith which he recorded and which led him to tell me about the weeks he had just spent at La Santé, his life in the cell, his arrival in prison holding his trousers in one hand because his braces had been taken away from him, and dragging his shoes whose laces had been removed. All this he said simply, without acrimony, and without raising his voice, like a scientist who notes the habits of a species he is observing. It was the first time I had been in the presence of someone who had been imprisoned by the Germans, and that someone was Langevin, a professor at the Collège de France and a member of the Académie des Sciences. His account confirmed that the feelings were quite correct that I had already formed about our occupiers, who had been praised for their kindness to us in Troyes by someone I had believed to be authoritative. We parted at the bottom of the staircase of 18, rue Raymond-Poincaré and, in the days that followed, I tried to let a few people know that Paul Langevin was in Troyes…”

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FIGURE 17. Troyes, 27 February 1944. Paul Langevin, Pierre Bourgeois, Mme Paul Langevin. All rights reserved.

Immediately, a small group helped my father and surrounded him with active support. This group was described by Mr. Robert Vassart126: “Miss Dibon, headmistress of the Troyes girls’ college; Miss Feiss, bursar of the same college; Miss Cabane, headmistress of the Sainte-Savine girls’ teacher training college; Miss Claude, science teacher at the same college; Mr & Mrs Zévaco, inspector of the Academy, Mr & Mrs Vassart, public prosecutor; Mr Casati, headmaster of the high school; then two Troyes industrialists, Mr & Mrs Robert Stains and Mr & Mrs Sauer, the former managing a wood company and the latter a hosiery company. This small group made Professor Langevin’s stay in Troyes a little easier, but what is certain is that he brought memories into our lives that we will never forget. He spent his wealth of goodness and knowledge with us without counting the cost, and from which we drew greedily. He had a clear and prophetic view of events and when something unfavourable occurred to the Allied cause, he knew how to give it its exact value and we left the rue Raymond-Poincaré comforted...” To this list, we should add the names of Mr. Cuttoli, prefect of l’Aube, who helped my father escape in 1944, of Mr. Cuynet, general secretary of

126 M. Robert Vassart, procureur de la République in Troyes, was one of the intellectuals who developed a friendly relationship with P. Langevin. He wrote a memoir of the time entitled: Paul Langevin à Troyes (1941-1944), which was published in “La Pensée” 12, May-June 1947, pp. 68-76.

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the town hall, who so helpfully arranged the furniture necessary for his accommodation, and of Dr. Hurez, who treated him. May all these friends of my father be thanked here for the material and moral help they generously gave him during his forced stay in their city! At the Liberation, Dr. Henri Mondor, speaking to my father, said to him: “In these last years, I happened to receive patients from Troyes from time to time, where the enemy, not content with putting you in mourning, put you in captivity. I did not yet have the honour of knowing you, but you were for me, as for so many others, one of the signs of the French conscience. Each time I would say to your compatriots there: “How is Mr Langevin? ” and they would let me know that your presence ennobled their little town.” Mr. Vassart, in his memories of the time, also recounted the circumstances in which he met my father: “One winter evening in the early months of 1941. Like so many French towns, Troyes, crushed under the occupation, lives in slow motion. We bump into silhouettes suddenly emerging from the dark night and we hear, before we see them, the sound of their boots, those who have created this night: the German soldiers. I leave the Palais de Justice and head for 18 rue Raymond Poincaré. The public prosecutor is going to pay his respects to Professor Langevin, whom the Germans have just placed under house arrest in Troyes, after having imprisoned him for more than a month at la Santé. I am moved and worried. How will the professor receive me? “I arrive in front of 18 rue Poincaré. It is a modern seven-storey house with a lift. On the second floor, on the door, a square of paper stamped with the German eagle indicates that the flat is requisitioned by the occupying authority. I ring the bell. The door opens and reveals the famous face that I had often seen in photographs or at meetings, but to whom I had never spoken. “I was immediately struck by the simplicity and cordiality of Professor Langevin’s welcome. In his warm voice, a little slow, as if he was always trying to find the right word, the professor told me about his arrest in Paris and his life in Troyes. He allowed me to come back and see him. I left the room in a bit of a daze, wondering if I had just been dreaming and if it was real that one of the most powerful minds of our time had just spoken to me in such a simple manner. I also wondered anxiously if it was possible that my respectful admiration for him might have been matched by some sympathy for me. But Professor Langevin’s generosity was boundless, and in this respect too he filled me with satisfaction. “He allowed me to climb the two floors in the rue Raymond-Poincaré at least once a week; he often came to Saint-Julien-les-Villas (six kilometres from Troyes), where I lived in a bungalow.

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“I can still see him in 1941-1942 walking the twelve kilometres there and back without fatigue. He was already 70 years old and yet he seemed to be in full possession of all his intellectual and physical powers.” Soon, my mother was able to come and stay with my father in Troyes, but having all her children and grandchildren in Paris, she had to commute between the two cities. In any case, my father wasn’t alone; his secretary and former student, Éliane Montel, surrounded him with her care, and he also received visits from many friends.

The vitality of a young man But the reader will rightly ask, what could Paul Langevin be doing in a flat emptied by the Gestapo of all the amenities of life, without his library, without the memorabilia of his travels, without his usual comforts, outside the framework of his intellectual activities? With a vitality worthy of a young man, Paul Langevin adapted to his new way of life at the age of seventy. This is what he said about it to his friend Vassart127: “What an astonishing thing it is that my life, that before the war was so full of scientific work and political activity, has become these years of semi-rest, of slow life in a provincial town! Unfortunately, I lack a lot of things to work on, but there is always an old problem which one waited to have time to think about, and I do not lack time now.” I believe that one of the first tasks he undertook in Troyes was to reread and revise my thesis, which, because of his multiple occupations, had been awaiting correction for nearly 10 years. This work had been done in his own laboratory and under his direction. In a letter dated 17 December 1940, he told me: “My darling, “I have just finished reading your manuscript so that your mother can bring it back to you. It is generally good and I have only made minor comments... “Finally, I see Foch as rapporteur, if he accepts, Mauguin and Debierne or Cotton. Mauguin could preside if Cotton can’t, as he is, I believe, more senior than Debierne as a professor at the Faculty. See Maurain on this topic and give him my best wishes. I hope to come a bit at the end of the

127 Article in La Pensée No. 12, already cited, p. 72.

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month for the holidays and will be very happy to be able to embrace you all128. “In the meantime, good kisses to Luce and the children. P. Langevin” A little later, my father, who was very interested in the defence of my thesis, wrote me the following letter: “Troyes, 24 February 1941 “My darling, “I am very glad to know that the difficulty concerning the registration has been removed and that you can now defend the thesis at short notice. When you come on Sunday at eight (or rather Saturday), please bring your manuscript to show me the corrections that Foch has asked for. We’ll talk about your defence presentation. “Hélène brought a cake for you yesterday, which you will eat to my health, which is good, as you can see. “I kiss all four of you very tenderly. Paul Langevin” At the beginning of May 1941, Paul Langevin received a negative answer to his request for leave, which he confirmed in a letter to my wife, at the same time still indicating his interest in my thesis: “Jean Blum129 Trade in horses and cattle R.C. Troyes 3866 8 May 1941 “My dear little Luce, “I am sorry that your commitments will not allow you to come before next month; the second Saturday in June, the 14th, would be the most convenient for me. “You have heard of the negative answer I received to my request to leave. Perhaps it is only temporary, but I will certainly remain in Troyes for a long time to come. I hope for some mitigation of my situation, and in particular the possibility of going to Paris to embrace you as a result of the visit I received on Tuesday from an officer of the Kreiskommandantur who came to inquire about my situation and my wishes and who seemed

128 My father still had illusions: in fact the German authorities never allowed him to leave Troyes. 129 If my father used Mr Jean Blum’s stationery, it was because he had given him formal written permission to do so.

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to ignore the refusal of my request for leave before Malfitano died130. I will try to be optimistic about this improved outlook. I am particularly looking forward to seeing the children again: tell Aline and Michel that I am very interested in their journal and that I am looking forward to the fourth issue. “I don’t have a note to send you for the cheeses131, I am only too happy to be able to alleviate your supply difficulties. The situation here is in danger of becoming worse in this respect. Cheese without vouchers, which has helped me a great deal, seems to be disappearing and meat is also becoming scarce. “I apologise to you for having kept Thibaud’s book132, whose figures are useful to me at the moment for my lectures at the École normale. I will return it to you when you come next month, but do not be afraid to get another copy if you have a little more freedom to work by then. “I have not yet seen any of the students’ scripts following my lectures: they are certainly very busy with last-minute preparation for the higher certificate examinations which begin on 23 June. I will probably only have their texts during the holidays. “The subject133 that André has chosen for his second thesis pleases me very much and will certainly teach him many things. He will tell me about it, I hope, when you come: tell him to bring his papers. It is a question I know little about and I would be happy to learn more. “How could he arrange with Freyman for the printing? “I kiss you very tenderly. P. Langevin”

Volunteer teacher This letter reveals that Paul Langevin’s second occupation in Troyes was to give lectures in physics134 to the young girls of the École normale de SainteSavine. In La Pensée number 12, under the title “The passion to teach” Vassart specifies how things were done: “M. Zévaco, the academy inspector, asked the professor if he would agree to give a course in physics to a group of students from the École normale 130 * Giovanni Malfitano (1872-1941) died in Paris on 6 April 1941. 131 This cheese story is quite simple: at that time, one could no longer find cheese in Paris, even with the voucher; but one could still find some in Troyes. So we arranged for my father hold the vouchers and he sent us the corresponding cheeses. 132 The book by Jean Thibaud referred to here is: Vie et transmutation des atomes, Albin Michel, Paris. 133 The subject was electron optics. 134 According to a letter from my father dated 7 February 1941, these weekly lessons began on 8 February and ended, as Mr Vassart says, on 25 July 1941.

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de Sainte-Savine. Langevin, who had a passion for teaching, accepted wholeheartedly. Every week until 25 July 1941, he gave the girls a physics lesson lasting an hour and a half...” I have a lot of details about the programme of lessons that my father presented. It is a complete course in modern physics, including the latest discoveries. As Mr Casati said, it took the headmistress, Miss Cabane, and the inspector of the academy, Mr Zévaco, a great deal of courage to encourage or authorise, in an occupied town, teaching by someone who had been dismissed from all his posts by Vichy. A group of doctors, engineers and others from Troyes asked my father, through Dr. Hurez, to give a course on the latest discoveries in physics and the latest theories. This course took place over two months on Sunday mornings in one of the rooms of the Lycée de Troyes, rue du Cloître-SaintÉtienne, at the beginning of 1944. M. Casati said later, “I attended two of these lessons where, in a familiar voice, sometimes serious, sometimes playful, not denying the fun of an anecdote or even a joke, the professor transmitted to his listeners some of the treasures he possessed in such abundance. I had a split sensation: one came from his slow speech, giving the impression that I was assisting in the very formation of the ideas that Langevin wanted to express and for which he found the right word which made an image and illuminated the idea, and there was also one that came from this certainty that I felt I was, colloquially, in front of a well of science, which I could dip into so as to draw out invaluable riches. “In Langevin’s presence I was sure that there was no question in any field that he could not answer, and no problem, however difficult or complicated, that he could not make clear and comprehensible even to the most uninformed man and one most distant from these things. It has happened to me, by chance of the circumstances of my career, to meet a good number of eminent men or those reputed to be so, and those who have a reputation. I could easily count on the fingers of one hand the rare ones who have given me, as M. Langevin did to such a high degree, that impression of depth, universality and lucidity by which one recognises what is called genius. And, above all, an inexpressible charm, a simplicity which made him enter fully into the concerns of his questioner, with whom he was sincerely interested as if they were his own. “Nothing was foreign to him and nothing seemed unworthy of his attention. Did I not hear him one day discussing the latest research with Villard, a teacher at the boys’ high school, whom he also often saw, and then, almost in the same breath, telling an anecdote about Bergson, who he had known at the Collège de France and then, moving on, thinking about housewives concerned about the shortage of salt, to a very concrete

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and practical explanation of how you could make a white salt capable of satisfying even the most difficult of people, from the abominable earthy salt that we were being sold at the time? These culinary and practical questions seemed to be of particular interest to him and I do not remember drinking any coffee at his house that was not made by his hand, just as I remember on Shrove Tuesday in 1946, a few months before his death, tasting the excellent wonders that he himself had made. Would one find, I beg you to tell me, many men so complete, so diverse, to whom none of those who lived near him would think of applying the term “pontiff ”, which we are too often obliged to attribute to certain characters? “The inhabitants of Troyes, the shopkeepers of the neighbourhood who soon became accustomed to his characteristic profile, with his soft black hat and his pilgrim’s coat, the people who found this old man near them in the milk and horsemeat queues, this old man, whose moustache and white imperial coat gave him a very special look, had no idea, if they didn’t know, that the man they were sitting next to was a prince of science of world renown. We ourselves, who knew it, tended to forget it, so great were his simplicity and modesty. If, at the beginning of his stay, he was for some “Monsieur le professeur”, he soon became “Monsieur Langevin”, the man who was greater than the eminent scientist. “No doubt the secret of this charm and the key that opened this nature to us was his passionate love of life. He never turned up his nose over a good meal, he enjoyed a good cigar, and he took his share of the joys that life offered. He attracted sympathy, and no one who approached him in Troyes failed to show it in an active way, to help him to bear the great material difficulties that, like all of us, M. Langevin experienced during those years of scarcity...”

Continuing scientific activity But Langevin’s activity in Troyes was not limited to teaching. In spite of the absence of a laboratory and working facilities, he used his leisure time to reflect on theoretical problems which had already preoccupied him for a long time and which could be solved by simple calculations135. He corresponded on this subject with Paul Montel, with Miss Ramard and with other scientists. Thanks to René Lucas, he learned of the activities of the ESPCI. His relations with the Academy of Sciences were uninterrupted and fruitful; the Academy was keen to maintain permanent and friendly links

135 * During his time at Troyes he carried out theoretical work on ion transport, and on neutron collisions.

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with him, despite his arrest. Several meetings were held in Troyes on the subject of the election of Frédéric Joliot-Curie, and my father was responsible for writing the notice presenting the young scientist. However, scientific activity did not occupy all of Langevin’s leisure time. Robert Vassart136 gives us details of his literary distractions: “Miss Dussane had organised a series of lectures on French poetry in Troyes with Dr. Pierre, an art lover and bibliophile. Langevin assiduously followed these talks in the town hall. During a lunch with Miss Dussane at Dr. Pierre’s, he spoke about his favourite poets. He loved Villon, the classics, the romantics, Rimbaud and Verlaine (whose strange silhouette he had seen in his youth in the Latin Quarter). He could comment on Valéry’s Cimetière marin, whose delicate images he appreciated, but he preferred less abstruse language, which touched his particularly vivid sensitivity more directly. “Lapierre, general secretary of the International Federation of Teachers, who has since died in Buchenwald, had brought him a work in progress: linguistic and etymological research on the origin of Champagne words. He had been very interested and sometimes spoke about it with the inspector of the academy, Zévaco, a distinguished linguist. One day he brought me the “Memoirs of Madame de Rémusat”, which he used to enjoy during his insomnia. And he set out on the historical trail, travelling it with his infallible surety of judgement. He was also very pleased with some finds made in the municipal library: old books by physicists, whose explanations made his hair stand on end, but served to strengthen his ties with his dear Evariste Galois, a brilliant mathematician... and revolutionary.”

Passionate about the battles of the Eastern Front My father was naturally passionate about the course of military operations, and Robert Vassart also tells us so: “…  What never disappeared from the rue Raymond-Poincaré and what we frequently came to consult, were the maps of the various fronts, in particular the magnificent maps of Russia that could not be found anywhere else. They were pinned to the wall and on them we followed the vicissitudes of the battle. Mr. Langevin would explain and comment on the movements of the troops, and sometimes, at times when many would have given in to discouragement, his unshakeable confidence and vigorous optimism would restore hope and strengthen the certainty we had, with him, of the inevitable defeat of the forces of evil. All his visitors knew this, children and parents, friends and colleagues and former students. 136 Robert Vassart : Paul Langevin à Troyes, « La pensée «, 12, 71-72.

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“I also see him in his office, in his dinner jacket, slowly moving a screen he had made to eliminate the interference on Radio London on his wireless set and helping me to make an identical screen for myself.” Paul Langevin’s keenness in studying the events of the Eastern Front are demonstrated in this letter that he wrote to my son Michel on 3 February 1943: “My dear little Michel, “I have received your beautiful card137 and I thank you for it. It is in my dining room in the place of honour, next to those which Mr Nicolle had brought me eight days before, and it complements them very pleasantly. It enables me to follow in detail events which are far away, but which nevertheless touch us very closely. “I am very interested in what you tell me about your work. I hope that we will soon be able to talk about it together and that I will be able to help you to enter this world of physics and chemistry which is expanding so rapidly, or that of mathematics, whose peaks are sometimes difficult to climb, but from which one has a very beautiful and extensive view and where the atmosphere is very pure. “Give my love to your parents and Aline. Love from your grandfather: P. Langevin” Paul Langevin’s firm stance stimulated the scientists who were opposed to Hitler in France and outside France. John D. Bernal later testified to this: “I remember the messages he was able to send us by illegal means during the occupation. The knowledge that he remained steadfast, even though imprisoned by the Nazis and despite the false trappings of a new order proclaimed by a victorious Hitler, gave us the assurance that in time France would be restored to its integrity and resume its glorious task in the service of humanity.”

The “culinary laboratory” With more than half of his financial resources gone, my father had to cut back on his lifestyle and the restrictions forced him to take a personal interest in culinary matters. Naturally, there are traces of his new concerns in his letters. On 30 January 1941 he wrote: “My dear little Luce, “I thank you for your good letter and for the shipment that was enclosed. The mustard, which I cannot find here, gave me great pleasure and it tasted very strong with an andouillette sausage that the butcher’s shop across the street

137 Of course this was not a postcard but a colour map of Russia.

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sent to me. The salt and the curry, which I will use according to your instructions, fortunately complete my “arsenal”, or rather my “culinary laboratory”... “I give you all a big hug, big and small. Tell Michel and Aline that I look forward to seeing them again. Your Father Langevin” When Luce and I went to Troyes, we found my father in his sparsely furnished flat. With just the essential small kitchen utensils, in this tiny room that he pompously called his “culinary laboratory”, he busily cooked his meal or prepared his coffee. He had discovered a rather original coffee maker in the Troyes market, and he praised the advantages of it to us. The main part consisted of an aluminium disc, on which was fixed a piece of thick fabric a little like a short sock. Naturally, the sock had to be cleaned often enough to prevent the coffee from tasting bad. It was a shame to see an eminent man of science like my father reduced to doing his own cooking, making coffee and shopping for supplies. A few months before his second arrest in Troyes, at the beginning of July 1941, we had made a plan to come and see him during the long holidays, which we spent in our house in Samoreau near Fontainebleau. We had planned that all the family, that is to say Luce, my son Michel, my daughter Aline and me, would leave by bicycle from Samoreau to reach Troyes, a journey of 114 km for the outward journey and as much for the return. This journey was a bit long for the children, who were at that time, Michel, 15 years old, and Aline, barely ten, but my father was so keen to see them that the four of us took the road anyway. Riding bicycles loaded with the produce of our garden, we took circuitous routes to avoid any check by the military police. On arrival in Troyes, my father was particularly happy to hug his grandchildren, who he had not seen for nearly a year.

The rest of the ordeal Unfortunately, in the autumn of 1941, my father’s ordeal was still far from over. One morning in early October, probably on 10 October (and not in January 1942, as Robert Vassart mistakenly states in his article in La Pensée), the German military police came to arrest him again. The lack of liaison between the Gestapo and the military police meant that for two or three days my father thought he had been taken as a hostage, to be shot the next day. It was indeed the time when armed resistance against the Germans had begun and hostage executions were on the increase. Deeply moved by the fact of Paul Langevin being questioned at the Troyes Commandant station, my mother and my sister rapidly brought the news to all their friends so that they could try to get my father out of his

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perilous situation. When they arrived at the academy inspector Zévaco’s house, they were invited in for dinner. Before the guests had sat down, a bell rang: it was my father arriving, his heavy suitcase in his hand. He had finally been released and when he found no one at home he guessed that his family were at Zévaco’s. This is what Robert Vassart says about this arrest138: “This new blow seemed to have worn the professor out; he lost weight, his features had changed, but he still had his fine smile and said: “I will soon be able to write ‘My prisons’ like Silvio Pellico”. “Before interrogating me,” he continued, “they made me wait for a very long time in a room that was completely bare, except for a huge portrait of Hitler. As I walked back and forth, the eyes of this madman followed me everywhere. I had plenty of time to express my feelings for him. “The interrogation by the Germans was very quaint. They asked me if I had been able to get my school certificate. “The Troyes service was completely unaware of my situation as a prisoner of the Paris Gestapo, and they thought they had been successful in catching me. “I was locked in a cell with a veterinary dealer, who kept in touch with his wife, who was incarcerated in the upper cell, by means of papers hidden in a thermos bottle. The bottle was raised and lowered by means of a string. “There were also bicycle thieves from the Lécorché gang in my cell (as a public prosecutor I was familiar with this case). One of them suffered from scabies and he scratched himself vigorously, carefully maintaining his scabies in order to go to the hospital, where one was better treated and from where one might escape. I worked in order to keep up my morale and my health. I was able to get paper and a pencil. Sometimes the mangy chap would lean over my shoulder, look longingly at the numbers I was writing down, and then return to his comrades, repeating with a resigned air: I did look at it, but it doesn’t make sense.” My father had hardly recovered when he wrote the following letter to my wife, in which he clearly shows that he no longer believes that his situation will improve: “Troyes, 27 November 1941 “My dear little Luce, “If there is no change before the end of the year, I will have to wait until then to see you, since my constraint does not seem to be ending.

138 Robert Vassart, same article in La Pensée, 12, p. 74.

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“It will be most convenient for me if you come at the end of the holiday you will have at that time. If, as I hope, you do not return before Monday the 5th, come on 1 January or the second. If things do not work out well for you, let me know so that I can look for another arrangement. “Nothing new here and no prospect of me going to Paris any time soon. Time is beginning to drag. “Good luck, my dear. Give my love to André and the children. My best wishes to your parents and your sister. All the best, your P. Langevin” So, we visited my father during the Christmas holidays and then went back to see him on his birthday, 23 January 1942; it was his seventieth. On 7 March 1942 we received the following letter: “My beloved children, “I should have written to you a long time ago to thank you, big and small, for the part you played in the celebration of a birthday that I am not very proud of because it seems to have come so quickly and which I might have pretended to ignore if the outside world had not so kindly reminded me of it, including a radio, which fortunately says many more important things. “The cigars received on this occasion have not yet gone completely up in smoke, but André’s port was so good that it has suffered the ephemeral fate of all good things, – and it is no longer available! “I am glad that Luce has sent me back the notes of my last year’s lectures, and having finished other writing, I am going to start on this one, some of which I hope to be able to present to you when you come to see me. Please make sure, by coming soon, that I have not had time to make much progress, so that I can improve most of it with your advice. Let me know soon when Luce will have the necessary freedom since her schedule seems to be uncertain. “I have been working very sadly since I heard the news on Thursday evening, which I have not yet had the courage to tell your mother139. However, I think she will go to Paris in the course of next week, and I ask you to help her to bear our anxiety about these poor children. I suffer most of all, for my part, from my powerlessness to help them and from my ignorance of what exactly is happening. Do not be afraid to give me all the news you can on this subject. “See you soon, my darling children. “I give Michel and Aline and their parents a very tender kiss. Your: P. Langevin” 139 This concerns the arrest of my sister Hélène and her husband, the physicist Jacques Solomon, on 2 and 3 March 1942.

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The arrest of Hélène and Jacques had hit Paul Langevin hard, as it had hit us all. This new blow came shortly after an unpleasant surprise: at the beginning of January 1942, the Prefect of the Seine ordered my father to vacate his service flat in the rue Vauquelin and to replace him with another director, the ‘collaborator’ Jean Thibaud, previously director of the Institute of Atomic Physics in Lyon and professor at the Faculty of Science in that city.

Jacques Solomon murdered Paul Langevin did not know that Jacques Solomon and his wife Hélène had been in hiding for many months, but he had felt for some time that their silence was unusual. If they had become illegal, it was because they were part of the leadership of the group in charge of anti-Nazi propaganda in the University, with Politzer and Jacques Decour. Luce and I had also collaborated with them in writing and distributing the clandestine newspaper L’Université libre, which they directed and composed. I will not repeat here the terrible emotion that shook us all at the news of Jacques’ heroic death in May 1942. I had to overcome my immense grief to defend my thesis at the Sorbonne on 4 June. My father learned of the tragedy one evening at the end of May. He immediately went to the girls’ school. He was very sad. He told Miss Dibon and Miss Feiss about the assassination of someone that he loved like a son and the imminent deportation of his daughter Hélène Solomon. “Can you,” he said, “put a record on for me? – Which one? – Beethoven’s 9th symphony”. He listened to the record, running his hand over his forehead and temples several times, a gesture that he used often. When the record was finished, he stood up. His eyes were full of tears. “I can’t tell you anything more for today”. And he left. Lapierre’s arrest was likely to get my father into trouble. This is how Robert Vassart tells the story: “One day, Paul Langevin came to see me at the public prosecutor’s office and brought me a clandestine booklet Cahier de la résistance, which we were circulating. “Do you know,” he said, “that my friend Lapierre has just been arrested by the Germans in Périgny-les-Roses, a small village in l’Aube where he had retired? During the search, the Gestapo found the same booklet and a letter from me. They immediately came to search rue Raymond-Poincaré. One of them sat down in front of my wireless set and turned the on-off button. The panel did not light up. I told him straight away: “It doesn’t work”, and he turned off the power. It was only the light on the panel that didn’t work and of course the needle was on London. I was lucky.

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“Before searching, a German officer approached an open book on my desk. It was Tolstoy’s “War and Peace”, borrowed from the municipal library in Troyes. The German officer, who knew French well, read a few lines. Tolstoy was talking about the disastrous retreat of the French army to Russia in 1812. “This time, with the German army, it will not be the same at all,” said the officer. Langevin did not reply...” A few days earlier, on 31  May 1942, Paul Langevin had sent Frédéric Joliot-Curie a long letter responding to a problem posed by Joliot, the measurement of radiation using a torsion pendulum. In this letter140, Paul Langevin presented to Joliot the original solution he had just worked out, justifying the validity of the apparatus by a complete calculation of its sensitivity.

A curious German approach In the middle of 1942 my father was the object of a strange approach by the leaders of the German air force, which amounted to offering him a deal in disguise: his liberation and his return to Paris in exchange for his collaboration with the occupying forces in the scientific field. Here is the translation of the letter addressed to him: “Dr. Paul Attaché to the Grand Master of Aviation Paris Liaison Centre Quai d’Orsay 51 to 53 Paris, 4 June 1942 “To Professor Langevin 21, rue R.-Poincaré – Troyes “Very honoured Professor, “A university of the German Reich intends to establish a scientific working relationship with you or your institute. “If you are interested in this proposal, we would ask you to kindly indicate a date when a meeting of the German gentlemen with you could be arranged for an exchange of views. It seems that the most favourable time would be between 1 and 15 July. As for the place of the exchange of views, you will decide for yourself: however, it would probably be most convenient to consult each other where your Institute is located. “I would also ask you to tell me whether you know the addresses of Professor Biquard and Dr. Baumgard. It would be desirable that these gentlemen also participate in the exchange of views.

140 The full text of this letter was published in the journal “La Pensée”, special issue on Frédéric Joliot-Curie, No. 87 (Sept. Oct. 1959), pages 105 to 109.

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“If you have a personal connection with them, I would ask you to set a date when all three of you would be willing to talk to visitors from Germany. “With my most distinguished consideration, Dr. Paul”

FIGURE 18. Photocopy of the letter on 4 June 1942 addressed to Paul Langevin by Doctor Paul. All rights reserved.

Of course, this letter was never answered. Despite his terrible grief, P. Langevin retained his optimism and natural benevolence. He remembered to congratulate my daughter Aline, then aged eleven, on her success in the grade six entrance exam, and one year later, my son Michel on his success in the baccalaureate.

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My sister Hélène in Auschwitz On 16 October 1942, I received a letter from my father giving me news of my sister Hélène, then detained in the Romainville camp. Sadly, she was soon to be deported. My father was informed immediately. Jacques Nicolle tells what happened141 : “On 23  January 1942, the very day of his birthday, we had organised a small party in a school near his home. His joy was clouded, because several of his children could not be with him. After saying goodbye to our hosts, we walked back to his home. Just as we were about to enter the door of the building, a railway employee quickly jumped off his bicycle and, visibly moved, came up to us and asked my master if he was Monsieur Langevin. When he answered ‘yes’, the man put a pad of paper in his hand, and then, running away, exclaimed, “Don’t say that you have seen me”. “When we unfolded this piece of paper we found that that it was a message that Hélène Solomon-Langevin had thrown onto Troyes station a few moments earlier to warn her father that she was being deported.” As Robert Vassart points out in his article in La Pensée, my sister was able to write these few lines in her cattle car, specifying in particular that she thought she would be transported to Oranienburg142 and she had succeeded in throwing the paper through the air vents onto the platform. The emotion this news caused to my parents and to us can be appreciated. The letters my sister sent from Auschwitz had to be written in German. Despite the difficulties of translation, it was a great joy for my parents to hear from Hélène and to know that she was still alive. In correspondence dated 8  November 1943, my father says of a letter from Hélène: “We believe we have clarified the identity of the people Hélène mentions in her last letter. Jeanne Letonneau is Jeanne Tonnelat, daughter of my colleague at the Collège de France and a teacher herself here at the school on Rue des Terrasses. Claudette is Miss Claude Bloch, currently with Hélène and Marie-Élisa. I have just received a card from this Claudette, who confirms the news from Hélène herself. Little Maurice is Claudette’s son, Yvonne must be René Blech’s wife and Simone must be Mrs or Miss Dudach, whose sister you know, I believe. Lucienne is certainly Lucienne Fajon. I will send Hélène the reassuring news you gave me about little Michel143. I can imagine how painful it must have been meeting his grand141 Jacques Nicolle: Souvenirs sur P. Langevin, Cahiers rationalistes, 161, February 1957, p. 74. 142 My sister was wrong, she was deported to Auschwitz. 143 This is Michel Politzer, son of Georges Politzer, shot by the Nazis in May 1942, and a fellow combatant with Jacques Solomon and Hélène.

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parents. What sadness there is all around us, next to our own! I have just learned of the death of several old friends whom I loved very much: Quillard, Dupont, Boucherot. We must take courage to see the end of this long ordeal. Seeing you will give me some...” My father makes a mistake in interpreting the name: Lucienne is not Lucienne Fajon, for the good reason that Étienne Fajon’s companion was called Juliette, not Lucienne. But mistakes were commonplace, because, as Hélène explained to me, the Jewish deportees in Auschwitz were not allowed to write or receive letters, and so they instructed their friends to express themselves in muted tones. In the two letters I was able to find from my sister Hélène to my parents and translated by my father, the first one, a letter of April 2, 1944, which arrived in Troyes on April 17, 1944, is relatively easy to understand: “Auschwitz, 2 April 1944 “Dear parents, “I received on 10-3 your short letter of 15-2 and only on 18-3 your other letter of the same day and on 25-3 that of 25-2. Be careful to fix in each letter the stamps you enclose. “I am very happy with the news from everyone and especially the news about my little Michel144. I am happy to know that he is learning well. I really want him to learn music and swimming and especially Latin so that next year he can enter the fifth grade without any difficulties. It is a pleasure for me to know that he is cared for so well! “You cannot imagine how much I think of you all. How close I feel to you and how warmly, my dear dad, my dear little mum, I would like to be able to express my affection and my gratitude. Tell also my great Maurice145 that I have never stopped thinking about him. “How is Alice146? Please give her a kiss for me. Does she still live in the same flat? Everything in that flat147 is very dear to me and I wish she could keep it. If this is not possible, I have every confidence that you will make the best of it. “Tell my dear Juliette148 that I thought a lot about her on 23 March and about Louis149, that I am very grateful to him and that I trust him to

144 Once again, it is Michel Politzer. 145 This is obviously Maurice Thorez. 146 Alice is Alice Habib, wife of Dr. Iser Solomon (Jacques Solomon’s parents), mother-in-law of my sister Hélène, who died in deportation. 147 This is my sister’s own flat at 3, rue Vauquelin, Paris 5. 148 It is indeed, as my father understood, the mother of Mrs Claude Bloch. 149 Louis is the father of Mrs Claude Bloch.

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make Maurice150 a man worthy of his father. My dearest wish is that Gaston can take care of Juliette and Adeline151 until I return. “I am still receiving your wonderful parcels as well as those from the Red Cross. Thank you all for this. Have you heard from Gricha? I am still in very good health. Many kisses Hélène”

Rapporteur at the Academy for the election of Joliot It has been said that Paul Langevin and Joliot not only wrote to each other but also saw each other often during the period of his forced residence in Troyes. At the beginning of June 1943 and with my father’s agreement, Frédéric Joliot challenged the collaborating academics by writing that he was a candidate for Édouard Branly’s seat. The following note can be read in the minutes of the Academy: “In the meeting of 16 June 1943, the President of the Academy of Sciences announced that M. Frédéric Joliot had asked the Academy to include him among the candidates for the vacant place in the Section of General Physics following the death of M. Édouard Branly.” But the challenge was further underlined by the fact that, in agreement with Joliot, Paul Langevin asked to be rapporteur to the Committee, bringing together the members of the General Physics Section, on the candidature of F. Joliot. Here is what Langevin wrote to Joliot on 7 May 1943: “Troyes, 7 May 1943 “My dear friend, “From what Cotton has written to me, I think that the Physics Section will agree to give me the report on your work. If it is convenient for you, I will start as soon as possible and will be grateful to you for providing me with the necessary documentation. It will probably be easy for you to do so on the occasion of your application, if you prepare one, at least in the restricted form imposed by the circumstances. “Have you seen L. de Broglie? “I apologise for disturbing you in this matter, but your offprints have been packed with my library and I would have some difficulty in finding them at the moment. Make arrangements with Éliane152, who will be happy to send me what you give her. 150 Maurice, as P. Langevin said, is the son of Mrs Claude Bloch. 151 Gaston, Juliette and Adeline are members of the family of Mrs Claude Bloch. 152 Éliane Montel, Paul Langevin’s secretary.

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“Good luck with your campaign. I assume that your college teaching is about finished and will this leave you free during this period of unrest, which is not without interest. “When I have reviewed your work and prepared my report, I will ask you to come back and chat with me for a day or two. “Love to you. P. Langevin” The General Physics Section did indeed agree, since (thanks to the kindness of the honorary secretary Louis de Broglie and the archivist, to whom I am most grateful) I was able to find, in Frédéric Joliot’s file in the archives of the Académie des sciences, Paul Langevin’s handwritten report on the titles and works of his former student and friend. Following this report, the General Physics Section, through Cotton, presented Joliot, who was elected on 28 June by 23 votes out of 43. That same year, 1943, the Germans confiscated all the furniture in the Troyes flat. Only two stools remained in the kitchen: my father on one, Robert Vassart on the other, and both chatted in this unexpected setting. Finally, in the attics of the Troyes town hall, we discovered some Henri II furniture, which was used in the past when the town was responsible for furnishing the director of the École normale. My father received, among other things, three magnificent ultra-modern leather armchairs that German generals had used and then abandoned. And it was in this unexpected and heterogeneous furnishing that he lived until his departure for Switzerland on 2 May 1944.

Worries for the grandsons But in June, my father had to suffer a much more serious upset: he learned of the arrest of two of his grandchildren, my son Michel and his cousin Bernard, who were caught distributing leaflets against the Service du Travail Obligatoire153 in a student demonstration on the Boulevard St-Michel, just the day before their examination in elementary mathematics. They were arrested with two of their comrades, the son of André Pagès, and Serge Durand, son of the consulting engineer Tony Durand. The leaflets had been printed by Michel on a children’s printing shop, pompously called Imprimerie Langevin by the police. These children, the eldest of whom was no more than 17  years old, were treated as dangerous ‘terrorists’. They were “cooked” for several days at the

153 * The employment and deportation of French workers as forced labour, instituted on 4 September 1942.

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Renseignements généraux154, which was headed at the time by the ferocious Rothé. But the kids stood their ground and there was no backsliding in their group. In view of the military developments on the Eastern Front, the judges were not very zealous, the case dragged on and finally the teenagers were sent back to the juvenile court, which released them as having acted without judgement. At the beginning of 1944, to distract himself, Langevin continued to solve problems of pure mathematics. He talked to his grandson Michel, who had gone to see him, about hyperbolic functions, whose role in physics, he said, was as important as that of circular functions, the chain equation, and also about the pursuit curve described by the dog that strayed from its master’s (straight) path to reach him, assuming that it always headed in the direction in which it saw its master.

Visits and messages of friendship Throughout my father’s stay in Troyes, courageous colleagues and faithful friends wrote to him to express their sympathy. Among them were Mr. Edmond Faral, Mrs Ramard, Mr. Henri Berr, Maurice Viollette and the mineralogist Lacroix. Jacques Nicolle’s dedication never wavered for a moment. René Lucas scrupulously kept my father informed of everything that was happening at the ESPCI. On 31  December 1941 he told the exile of the tragic end of Holweck, who had been assassinated ten days earlier. And on 21 April 1942, he let him know that Jean Perrin had died. Many made the trip to Troyes: the cardiologist Laubry, André Mayer from the Collège de France, Paul Montel, Professor Tiffeneau from the Faculty of Medicine, Ernest Kahane, Biquard, Professor Lagrange from the Faculty of Sciences in Dijon, Aimé Cotton, Fabry, Francis Perrin and Marie-Elisa Nordmann. M.  Gustave Monod, remembering his collaboration with Roger Gal on the projects for the renewal of education during the war, wrote: “... This was underground work. Roger Gal, Alfred Weiler, Emile Weil and I decided to meet every Thursday morning at the home of one of us. Roger Gal played a particularly important role at that time, because he had a personal relationship with Paul Langevin and could reflect our thoughts to him. I remember Roger Gal’s mysterious, unexpected absences. He would go to Troyes to see the man we considered to be the true great master of the University and he would bring back to us the elements of what would later become the Langevin project155.” During this period, my father still received the most touching and comforting testimonies of friendship from Messrs Debierne, Labeyrie, Maublanc, Maurain, Paul Le Rolland, Henri Wallon and Wyart, from Aline Perrin and 154 * The intelligence service of the French National Police. 155 Gustave Monod: Testimony, in «  Hommage à la mémoire de Roger Gal  », Publication of the Institut Pédagogique National.

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Maria Rabaté. I am afraid that I have forgotten many of those who helped to soften for him the rigours of his separation and the pain that suppression caused him by affecting his family. May they find my apologies here, and may they all believe in the deep and unalterable gratitude of Paul Langevin’s children.

Escape from France At the end of 1943 and the beginning of 1944, when it was clear that the situation in Germany was becoming increasingly critical under the pressure of the Soviet army and that the Anglo-Saxon invasion was approaching, we all insisted that my father should evade German surveillance by agreeing to the escape plans that Joliot-Curie and his other friends had formed. He was, in fact, a hostage of choice for Germany, which was on the verge of collapse. Paul Langevin hesitated for a long while, fearing that his family would be the object of reprisals after his departure. In the end he decided. But my mother was ill and bedridden with a temperature of 40 °C. And Paul Langevin only agreed to leave when he was sure that a safe haven had been found for her. The departure took place at the very beginning of May (since, as we shall see later, he was already in Switzerland by 5 May).

FIGURE 19. The false identity card with which Paul Langevin managed to reach the Swiss border on 2 May 1944. © Bibliothèque de l’ESPCI.

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Joliot-Curie brought him a false identity card, which he filled in himself in the name of his maternal grandfather, Pinel Léon. Thus, the initials P. L. corresponded with his own. Several attempts were made to put his fingerprints on the card, first with shoe polish, then with Indian ink. Finally, shoe polish was adopted. Before his departure, my father took all the compromising documents and those he held dear to him to the Collège de jeunes filles de Troyes. Despite all objections, he refused to have his moustache shaved off and even more energetically he defended the mouche, which nevertheless made him very recognisable. He left rue Raymond-Poincaré at about 3  p.m. on 2  May and went to give the Commandant a final signature (about the 364th). From the Commandant’s office, he went towards the station, where he had agreed to meet Mr and Mrs Stains, Troyes industrialists, who had a car. He then took the train to Paris from a station near Troyes156, and then left for Montbéliard.

FIGURE 20. Paul Langevin, helped by two F.T.P., crossed the Swiss border clandestinely on 2 May 1944 towards Porrentruy, at Damount. All rights reserved. 156 Barberey: following an air raid, trains could no longer leave Troyes.

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Some from the resistance movement were waiting for him. They drove him as close as possible to Switzerland. He had to cross the border on foot; a resistance worker named Philippe helped him to walk, even carrying him in difficult places. He arrived in Neuchâtel where he was fondly received by his friend Perret, a chemistry teacher who we are happy to thank here. The Swiss government immediately put an end to his difficulties by finding him somewhere to stay...

In Switzerland The date of the journey is set by a letter from my father dated 6 May 1944, which reached us thanks to the dedication of the F.T.P. and the courage of a colleague and friend of Luce’s, Miss Foullié (pseudonym Claire SainteSoline157), who was kind enough to pass on letters to us despite the risk. Here is the text of this letter, in a deliberately allusive style: “6 May 1944 “My beloved children, “You must have known that the operation was carried out and was admirably successful yesterday 5 May. Today I have the opportunity to send you a more explicit note to reassure you completely. I do not know, depending on what you decide, whether it will come directly to you or through your friend, to whom I am very grateful. Please inform your mother, Jean, Madeleine and all those who are interested in the patient that all is well and that he is being cared for here with the best of intentions until he makes a full recovery, which we all hope will not be long in coming. I hope to be able to take advantage of similar occasions to this one to send news from time to time. “I think of you all very fondly and send you my best kisses. Your old dad and granddad” My father wrote to us from Switzerland many times. The last letter was sent to us by him after the Liberation of Paris: “Neuchâtel, 30 August 1944 “My beloved children, “You can imagine how much I have thought of you during the glorious days that have just passed since the great news of the liberation of Paris arrived here just a week ago, and also my concern to know how you have survived

157 Claire Sainte-Soline was a eminent writer, and was president of the panel of the Prix Fémina. She died recently. We are very pleased to pay this fitting tribute to her memory.

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since there seems to have been serious fighting around your home158. This note will reach you through Mr Heilbronner, whose family has done much to welcome me here and to soften for me the momentary exile, which will soon end. In a few days, I will go to Geneva to find a way to join you at last! “Why can’t I be with you! “I send you a very tender kiss. See you soon. Your dad and grandpa” Irène Joliot-Curie and her children visited Paul Langevin in Neuchâtel. They had come to take refuge in Mont-la-Ville to avoid any reprisals from the Germans. One day in September 1944, they went for a walk together in La Chaux-de-Fonds. After seeing his friends again, Paul Langevin waited impatiently for the time to return to Paris.

With the French Interior Forces (FFI) of Haute-Savoie As far as my father’s return is concerned, the best thing to do is to let one of the F.T.P. who witnessed his return to France speak159, P. L. (ex-Laffont): “Paul Langevin returned to France after more than four months of exile, to Haute-Savoie, gloriously liberated by the F.F.I.  He was welcomed at the border near Annemasse by several officers of the Francs-Tireurs et Partisans (resistance). The emotion of the great scientist was so great that, after brief greetings, everyone respected his silence. And, in splendid weather, we made our way to Annecy, where a first reception was prepared by the regional staff of the FTP at the Hôtel d’Angleterre. As soon as the car entered the courtyard, all the officers of the various offices as well as many unit commanders gathered around their chief, C.O.R.  André Bonfils, to greet Paul Langevin. “Delayed by some duty, I was one of the last to arrive in the small, already smoky room where the great physicist was chatting, relaxed, with our comrades. I was surprised to see this eminent and elderly man rise at my approach and address me in a tone of the most attentive politeness, when I was introduced to him. This simplicity had already won the hearts of the valiant soldiers around him. Paul Langevin never tired of listening to the stories of the legendary struggle of the patriots of Haute-Savoie

158 What my father did not know was that, fearing arrest, we had not stayed at our home, 38 rue de Vaugirard, but had moved in to 10 rue Vauquelin, where the F.F.I.(French Forces of the Interior) Paris-South headquarters was located. 159 P. L. (Ex-Laffont): Le retour de Paul Langevin en France, La Pensée, 12, 77-78.

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against the Nazi invaders. At any moment, he would interrupt one of us, ask for details, generously sharing our enthusiasm. “Paul Langevin then received, in the name of the Regional Military Committee and from the hands of our young comrade Nancy, heroic staff auxiliary, a card of honorary membership of the FTP of HauteSavoie. Everyone stood to attention, and our friend’s hands trembled as he received this modest item of his distinction. After this moving moment, we parted. “Military honours were paid by two F.T.P. sections to the scientist in the courtyard of the hotel. (And Paul Langevin never knew how much trouble these guerrilla fighters had gone through to hastily relearn, in their garages, the forgotten art of parades!) “Later, at an intimate dinner, in a beautiful villa overlooking Lake Annecy, the main civil and military leaders of the Resistance gathered around the eminent scientist. At table, Paul Langevin was very animated, and held the attention of our comrades by his brilliant conversation, but was always measured and respectful of the opinion of his companions. When the name of the sinister Georges Claude was mentioned in connection with the “V 2 ”, Langevin explained to us at length how this pseudo-savant had usurped the title of inventor of jet engines. He pointed out that true men of science are also men of progress, and that there is a rigorous link between the obscure scientism of Georges Claude and their adhesion to Nazism. Finally, in the name of our country’s science, he paid a vibrant tribute to all the combatants and particularly to the working class, which had given the French spirit the conditions for its free development. For his part, he pledged to put all his knowledge and all his life at the service of the workers. We know how, for 20 years, Paul Langevin had been keeping this promise daily: there was therefore hardly any need to renew it. But Langevin’s words, on the evening of his return to liberated France, were charged with a particular gravity. Many of my comrades and I still hold that minute as the highest moment of that period of fervent struggle. “The reception went on late into the night. Langevin went from one group to another, members of the Departmental Liberation Committee, secretaries of the National Front or of the glorious illegal C.G.T.160, members of the federal bureau of the French Communist Party, officers of the F.T.P. He worried about our difficulties with infinite concern, about our efforts and our hopes. He was passionate about everything. And this was the most striking feature of his character: one could not imagine an indifferent Langevin entrenched in an ivory tower. He brought to everything 160 * The Confédération générale du travail was a trade union organisation that was made illegal during the war by the Vichy Government.

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the curiosity and rigour of a scholar, the dedication and honesty of a politician, and above all the inexhaustible goodness of a generous heart. “Leaning against a high Savoy fireplace, puffing on a magnificent cigar, very alert despite the fatigue of the day, Langevin was now talking to us about his most cherished work: La Pensée. We were invited to discuss at length on the following subject: how to find all those who had first supported the journal? This question was of great concern to the scholar. Time and again, he asked us for advice. “Then the Sorbonne was mentioned and several famous academics who had once led their students to deviate from the Vigilance Committee of Anti-Fascist Intellectuals. Langevin vehemently opposed them, as did Politzer, Decour and Solomon, all authentic defenders of anti-fascist thought. He protested against certain professors – whose names I do not wish to mention – who had abused their authority to publish, under the Nazi boot, works that conformed to the ideology of the occupiers and whose teaching, as he said, had poisoned several generations of students. “As the head of the School of Physics, any educator who gave his students an adulterated education despised their dignity. A teacher worthy of the name must, first of all, respect the science that he dedicates himself to serve. The night was drawing to a close. Already, our comrades of the Departmental Committee of the Liberation were getting into their cars to go on to the congress in Valence. As I took leave of the scientist, overloading him with expressions of respect that he considered excessive, Langevin retorted gently: “Why don’t you simply call me ‘Comrade’? It is my best title... and have I not earned it?” The next day, the F.T.P. took Paul Langevin to Lyon. He had been preceded by an official telegram on 20 September 1944 from the Ministry of National Education to M. Farge, Commissioner of the Republic, which read as follows “M. Joliot, Director of the Centre national de la recherche scientifique, requests you to organise urgently a direct return to Paris for Langevin, in order to avoid fatigue. Contact Denivelle161, 31, rue des Grands Jardins in Montbéliard (Doubs). Joliot”

161 Denivelle was a Resistance industrialist and friend of Joliot, who had already collaborated in the escape of Paul Langevin to Switzerland.

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On the radio in Lyon Not only were there warm receptions in Lyon, but my father was also asked to make a statement on the Lyon radio. In Lyon he found many friends, in particular his former pupil and co-worker Pierre Biquard162 who, being Jewish, had had to leave Paris and settle in Lyon. He was also happy to meet Aragon, Elsa Triolet and Madeleine Braun. The latter, who had worked for a long time with Langevin and Victor Basch during the Spanish war, and who took over the direction of the National Front newspaper Le Patriote after the liberation of Lyon, wrote the editorial of this daily newspaper on 26 September 1944, under the signature of “Nicole”, her pseudonym in the underground. I extract this passage: I FOUND LANGEVIN by NICOLE ... it is difficult to say how one feels about Langevin... one could say of him that he is everything: great, good, human, spiritual, endearing... He has such a radiance of intelligence that whoever talks to him feels grown up and better. There are few men who can leave this impression. When I saw him again I thought of my first meeting with him. It was a long time ago. I was sixteen. It was at a meeting at the Collège de France, given by a young professor. Langevin had come to listen. The small room was full. I was standing. Langevin, who did not know me, got up and gave me his chair. Confused, I did not know where to put myself; and I later analysed this feeling I had experienced when I saw this great scientist give up his seat to a little girl, during the time, more than 3 years, that I worked almost daily with him, during an already difficult period at the time of the Spanish war. Langevin, overwhelmed by multiple tasks, did everything to facilitate mine, crossing Paris to save me a journey. He kept giving me his chair. And his life was full of these small gestures. Another image of Langevin is that of a cheerful, witty, bon-viveur. I remember a word from him at a Peace Congress in Brussels. There was a committee meeting where General Pouderoux, a former fire brigade general, was explaining his point of view on the need, in his opinion, not to alarm people about the possibility of war, but on the contrary to reassure them! At this point, Langevin leaned over to me and asked innocently: “What do you think of this fireman who doesn’t want us to shout ‘Fire’?”

162 * Biquard had worked with Langevin on problems in ultrasonics, and, had published a transcription of Langevin’s 1923 course on ultrasound, given at the Collège de France.

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Paul Langevin’s speech on the radio in Lyon on 24 September is especially interesting, as he develops his ideas in broad terms on how to set about the rehabilitation of the country. Here is the text: “The two days that have passed since my return to battered but liberated France will leave me with moving memories and at the same time bring me great hope. As I passed through the countryside, and into the great city of Lyon, active life has resumed everywhere under the protection of an armed and disciplined youth, in spite of the difficulties caused by the distressing destruction. “Drawing new strength from the will to prove itself worthy of its heroes and martyrs and to maintain the union sealed by suffering, a strong and united people feels itself on the march towards a new and great future. It knows that its duty to those who have sacrificed themselves and to future generations is not to let anything be lost of the present opportunities, so dearly bought, to achieve more social justice and more freedom through profound structural reform. “While continuing with our allies the struggle against the barbarous external enemy, which allowed itself to be led from humanity by demented guides, we must, as far as the past is concerned, do justice to the enemy within by pursuing without weakness those who, by their actions, placed themselves outside the nation. “Then there will be the great task of reconstruction. Without giving the hostile forces time to recover their former dominance, now that the country has been liberated we must achieve the liberation of each of its children; material liberation through a profound transformation of the economic system and working conditions, and spiritual liberation through an organisation of education which allows each person to develop his or her aptitudes to the full for the greater benefit of the community, and this in a form which maintains and ensures the union of all for the benefit of each. “I am leaving Lyon today to go to Paris, where I will carry with me the very deep impression I received yesterday of will of the citizens of Lyon for the union of all in a vast National Front163.”

163 A big meeting was organised by the Front national to welcome Langevin’s arrival in Lyon.

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FIGURE 21. Lyon, 24  September 1944. Aragon, Elsa Triolet and Paul Langevin. All rights reserved.

Return to Paris My father left Lyon the same day, in a car that Yves Farge had put at his disposal with a driver and an FTP bodyguard. He set off for Paris in the company of Aragon and Elsa Triolet, who also wanted to return quickly to the capital. When he arrived in Dijon, although the most direct route by no means passed through Troyes, he insisted strongly that a detour be made, which would allow him to see the old capital of Champagne again, and especially the dear friends who had softened his exile in this city. The next day, 25 September, the travellers left Troyes late, which meant that they did not arrive until about 5 o’clock in the evening at 10 rue Vauquelin, where Luce and I welcomed them on their arrival. We had not yet moved back to rue de Vaugirard and were still living in a flat normally used by the laboratory lads of the ESPCI in the attic above the chemistry laboratories, with our windows facing rue Rataud. Needless to say, we were delighted to see our father again after such a long separation, but we were, like his friend Vassart at the same time, struck by the change in his features. The terrible stress that he had suffered for so long had considerably aggravated the terrible illness that was to strike him down 2 years later.

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The collaborator Jean Thibaud not only didn’t make the necessary gesture of returning the directorial flat of the School to Paul Langevin on his return to Paris, but also refused to vacate it even when Langevin was officially reinstated in his functions. So, we witnessed the scandal that my father had to live with us in a laboratory boys’ flat in a school of which he had been and remained the director. Very important people, for example Rector Roussy and Ambassador Bogomolov, came to visit Paul Langevin in this small attic flat. In any case, we had the great pleasure of having my father with us for several weeks, something that almost never happened in ordinary times, given his many duties and obligations. Lack of communication meant that my mother, my brother and my sister Madeleine could not return to Paris until much later.

Joining the Communist Party Paul Labérenne accurately recalls this period164: “Although his health suffered greatly during these years of hardship, Paul Langevin did not think of resting, which he well deserved. On the contrary, he felt, perhaps more strongly than he had ever felt in his life, that the struggle for a better world required ever greater sacrifices. If he may have believed in his youth that the protest of a few isolated intellectuals alone could make the forces of evil recede, he now knew, and from what painful experiences, that the thinker is very little without the support of people organised for action.” My father thought that we should unite even more closely with those who were fighting for a better future. But because of his age he hesitated to join the French Communist Party. To describe the exact circumstances of his joining the day after his arrival in Paris, it would be best to give the pen to Luce, who accompanied him that day and who lived through those moving moments: “Paul Langevin’s membership of the Party remains one of the most striking memories of my life as an activist and also of my family life. From the very first moments of our life together after such a hard separation, he confided to us that the time had come for him to take the decision that was so in keeping with all his actions and thoughts during the war. On the evening of his arrival, in that small flat under the roof of the School of Physics and Chemistry, we discussed it at length: “I think like you” he said to us, “I am with you, but I am too old; it would perhaps be ridiculous to enter at seventy-two years of age into a party which needs young forces. A few hours were enough on the following day, September 26, 1944, to sweep away all 164 Paul Labérenne: La pensée et l’action, p. 305.

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these hesitations and to precipitate the decision, aware of its seriousness and feeling its necessity deeply. “We had invited Georges Cogniot to lunch to celebrate his return, his dear friend, and “adopted son” as he liked to call him, who was living at that time at 3 rue Vauquelin, just opposite the School of Physics and Chemistry, in the flat that Jacques and Hélène had set up so nicely and where they had lived for only a few weeks before starting their undercover life. “Since the liberation of Paris, Georges Cogniot had resumed his position as editor-in-chief at L’Humanité (then at 18 rue d’Enghien) with Marcel Cachin, the newspaper’s director. Paul Langevin had expressed a strong desire to see this old friend again; it was decided that a car would come to pick him and me up that very afternoon. At about three o’clock the driver knocked at the door, and when we opened it he cried out in a joyful voice: “I have come to fetch Comrade Paul!” This little phrase, pronounced so spontaneously, was the source of a chain reaction that was to overcome the last reluctance. This word camarade, which was addressed to him, filled Paul Langevin with joy and confidence. At l’Humanité, in an atmosphere of warm comradeship, he found not only Marcel Cachin and Georges Cogniot, but also the editors and the staff, who did not hide their joy at seeing him among them, toasting with him as one of their great comrades. Why should he not permanently become the comrade that all these brave militants were celebrating in him, who were still vibrant from the victorious struggles of the liberation? The obstacles of age disappeared before the youth of heart and enthusiasm. “Paul Langevin asked Georges Cogniot to take him to Jacques Duclos’ house, and as soon as he got into the car which was taking him to 44 rue Le Peletier, he said to me, all moved, “My dear, I’m going to tell you something that will please you, I’m going to join the Party. I was moved and happy, just as Jacques Duclos was when he heard the first words that Paul Langevin addressed to him with deep emotion: “I ask you,” he said, “to welcome me into your party to take Jacques Solomon’s place, without having the pretension of being able to fill the great void that he has left. “This sense of modesty could only surprise those who did not really know Paul Langevin. For my part, I had often had occasion to observe how naturally this scientist with worldwide authority was inclined to seek advice and to express his doubts. Jacques Solomon, in whom he had great confidence, could have said with what simplicity he came to discuss with him not only physics, but also everything related to dialectical materialism and politics. The long conversations they had together in their two offices in the rue Vauquelin, facing each other, are certainly not unrelated to the fact that the great scientist, who considered himself to be the disciple,

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hesitated, out of modesty, to take the place of the man who, despite his youth, possessed an uncommon strength of thought and persuasion. “How many times did he confide in me to ask me if I thought the lecture he had just given was good, if his presence at such and such a meeting was useful! He knew the critical spirit, the somewhat brutal frankness of the woman he sometimes called “the explosive woman”, but he knew that it was a real affection that allowed me to tell him all my thoughts without reticence and that is why he always listened to me with benevolence. All those who, like me, were able to speak to him with an open heart, knew this benevolence, this generous and radiant goodness that comforted you and lifted you above the petty concerns of daily life.”

The seventy-third anniversary Paul Langevin’s relations with his new comrades, the leaders of the Communist Party, and especially with Maurice Thorez, Jacques Duclos and Marcel Cachin, were immediately based on a warm and trusting friendship. The Party became for him an extended family, whose members were united by the deepest comradeship. This was evident on my father’s seventy-third birthday. He celebrated his birthday, on 25 January 1945, together with his comrades in the Party leadership, who organised a fraternal meal in his honour. The meal was presided over by Maurice Thorez, who gave a moving speech saying how proud the Party of the working class and the people was to have such a valuable intellectual in its ranks. My father, deeply touched, replied with simplicity and modesty, almost apologising for having been so late in joining a Party whose ideas he had long shared. I will always remember the cordiality and joy that marked this brotherly assembly, in the enthusiastic atmosphere of the liberation. A few weeks later, on 3 March 1945, a public tribute to Paul Langevin was organised by the Union française universitaire (UFU) which also wanted to mark my father’s birthday; the solemn session took place in the large amphitheatre of the Sorbonne, under the chairmanship of the rector, Gustave Roussy. Gustave Roussy, Edmond Lablénie, President of the U.F.U., Professors Aimé Cotton, Joliot-Curie, René Lucas, Jacques Nicolle, Doctor Henri Mondor on behalf of the Resistance doctors, Georges Cogniot who was sent by the Communist Party, and Jacques Debû-Bridel, spokesman for the National Council of the Resistance, all brought greetings and homage from science and work, from the University and from the nation. Paul Langevin replied with an admirable speech, which was like an account of his life, all divided, he said, between the service of knowledge and that of justice.

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Eighty-two delegations were present that evening, including those from the French Academy, the Academy of Medicine and the Consultative Assembly. The Minister of Education was in the room. Dozens of messages and telegrams arrived, including those from the Royal Society and the USSR Academy of Sciences. But, for my father, the days of celebration were barely hesitations in an active and creative life, which had resumed its course, as intense as ever, upon his return to Paris.

After the liberation Multiple activities As soon as he returned to Paris, Paul Langevin resumed his responsibilities as director of ESPCI. He reorganised the teaching, which had been severely disrupted during the war. Thanks to his driving force the school was raised to the rank of a higher education establishment, without losing its specific character, shortly after his death. During a friendly lunch for the sixty-second graduating class, on 29 June 1946, Paul Langevin still found the strength, despite his fatigue, to give one of the last addresses of his life. In a voice already shaded by illness, he retraced the main stages of his life as he bade farewell to the École. Here is what the Bulletin des anciens élèves of October-November-December 1946 says about this speech: “Professor Langevin, in a very moving improvisation, hinted that this might be the last meeting he would attend as Director of the School, the doctors having advised him to give up some of his work. He briefly spoke about the gradual improvements in teaching and expressed his satisfaction that more and more former students were occupying positions of primary importance in research and industry. He expressed his regret to the students that his state of health had not allowed him more recently to maintain more intimate and frequent contact with them, as he had been able to do in previous years.” He was still a professor at the Collège de France. According to the beautiful expression of the administrator, Paul Langevin continued to be “part of the living soul of this establishment” 165. Edmond Faral expressed himself in these terms before the assembly of professors on 16 February 1947, and he added: “Langevin so dominated everything that was being done in international physics at the highest level that he seemed to preside as sovereign over 165 My father’s retirement due to the age limit preceded his death by only a few weeks.

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the work of all other scientists. His prominent position arose from his superbly penetrating and lucid mind, whose authority was universally recognised and respectfully accepted. Written traces that still remain of his exceptional work are either too rare or too scattered or too little known. But all of us here know about it, and we can give as examples the presentations that he made in this assembly when it was a question of creating a new teaching programme or an election to a vacant chair. It was clear then that he was a kind of arbiter mundi. Some of the papers that he read on these occasions are preserved in our archives: they would suffice, on their own, to establish the reputation of any man.” My father also continued his activity as a member of the Academy of Sciences, an activity that had continued thanks to the dignified attitude of his colleagues. They had appointed him rapporteur of the Clément Félix Foundation prize on 23  December 1940, shortly after his arrest by the Gestapo. This gesture was all the more courageous as the commission accepted Paul Langevin’s proposal to award the arrears of the foundation to Pierre Biquard, chef de traveaux at ESPCI who, being of Jewish origin, had been obliged to flee to Lyon, in the “free” zone. Paul Langevin was then repeatedly elected by his colleagues in the Academy of Sciences to the physics prize commissions during the years 1941 to 1944. After the liberation, Paul Langevin and Frédéric Joliot continued their fight for peace within the Institute, with the agreement of Maurice de Broglie and Louis de Broglie. Here is what can be read on this subject in the minutes of the session of 13 December 1945: “... The Academy of Sciences of the Institute of France considers: 1. that, as a result of scientific research, atomic energy has been made available to the human community as a new source of energy; “2 that, as a result, there are many possibilities for a better world economy; “3 that, as a result, these many possibilities were developed first of all in the form of the atomic bomb with its horrible destructive power; “4 that the world of science, which has created these possibilities, is fully aware of its responsibilities in this respect; “5 that it is inadmissible, from the point of view of the development of science and the resulting consequences for social welfare and public health, for the results of scientific research to be kept secret. “Consequently, the Academy appeals to the sense of responsibility of governments and scientific institutions to direct their action towards the beneficial applications of scientific research for civilisation, to the exclusion of those which may threaten it. “The Academy also expresses the desire that the scientific community be enabled to participate effectively and permanently in discussions on these issues.”

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Not only did Paul Langevin return to his old pre-war occupations, but he even accepted new responsibilities. In December 1944 he was elected president of the Ligue des droits de l’homme to replace Victor Basch who had been assassinated by the militia and for whom he delivered a moving eulogy at the Sorbonne on 7 January 1945.

The Union française universitaire and the Union rationaliste My father’s activity was also evident in the l’Union française universitaire, which sought to define a “French science policy” in the spirit of the formula “French scientists at the service of the nation”. Jean Orcel asked Paul Langevin to give the inaugural lecture on Friday 10 May 1946, for which he chose the theme: La pensée et l’action (Thought and action). My father said: “First of all, I would like to emphasise the ever-closer solidarity, the reciprocal fertilisation between thought and action, between science and technology, between theory and experiment or, as our Soviet friends say, between theory and practice, the expanded and daily form of experiment. Our science is largely derived from the need to act. This is well known in mathematics, from arithmetic and geometry to differential and integral calculus. The progress of astronomy is linked either to the problem of measuring time, or to the desire to predict the relative positions of the stars or the future of men, or, in antiquity as in the Renaissance period, to the growing needs of navigation. Optics, as developed above all by Galileo, Kepler, Descartes and Newton, followed a parallel path to that of astronomy, as it tried to satisfy ever-growing needs for precision in observing the sky. “At about the same time, the development of mechanics, which also began with Galileo, Descartes, Huyghens and flourished with Newton, was closely linked to the problems posed by ballistics and astronomy. Differential and integral calculus was created in the eighteenth century to answer questions posed by mechanics, ballistics and architects. In the nineteenth century, thermodynamics and the precise knowledge of the laws governing gases and vapours developed as the applications of the steam engine began to assume central importance. And since our great Sadi Carnot first stated the essential principles of thermodynamics under this influence, this science has dominated much of pure and applied physics and chemistry. Thus, in all these examples, the need to act determined the activity of thinking. “On the other hand, the needs of thought, once manifested, the concern to understand, what I have called “holy curiosity”, does not let the mind rest until it has constructed an interpretation of natural phenomena, either

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to calm historical anxiety, or, more and more clearly, for the purpose of intellectual satisfaction. The results of this research, the resulting pure sciences, have been surprisingly fruitful from the point of view of action by the unexpected applications to which they have given rise. By following the path that leads to science through the needs of action, one knows in advance what one wants to achieve and seeks increasingly rational and precise means to reach it... “... Not only does thought solve the problems posed by the needs of action, but also, once set in motion and building pure science, it proves extraordinarily fruitful in creating new possibilities for action. “It seems that we can even go further and affirm that no truly scientific research, however abstract and disinterested it may seem, remains without finding an application sooner or later, and that no outcome is achieved without the effort of thought.” The Rationalist Union also needed my father’s help. In 1945, he chaired, among others, the meetings of Edmond Vermeil on “Hitlerism and Rationalism”, of Henri Laugier on “Some Conditions of French Greatness”, of Georges Cogniot on “Rationalism and Secularism”, of Paul Couderc on “Astronomy and the Evolution of Reason”, and of Frédéric Joliot-Curie on “Atomic Disintegration”. All these meetings took place at the Sorbonne. After my father’s death, René Maublanc remembered the meetings of the Board of Directors of the Rationalist Union where “Paul Langevin generously distributed the treasures of his broad and deep reflection, his kindness, his bonhomie and his good humour”.

Collaboration with La Pensée René Maublanc, who met my father at the management meetings of La Pensée, said of him: “He had a very rare gift: he knew how to be at ease in any company and how to make everyone in his company feel at ease. Social relations would be easier and more pleasant if there were many similar men. No one was a better listener than Langevin, not only with more patience, but also with a greater sense of becoming more intelligent. Anatole France had a similar ability. “It has been said of Langevin, to his credit, that he was a lay saint. If by this is meant that he was capable of total dedication to causes he believed to be just, that he could sacrifice his strength, his freedom and even his life without hesitation, then this is undoubtedly right. “But there is in the word sanctity the idea of an asceticism, of a renunciation of the joys of the Earth which does not fit well with the man we loved. Paul Langevin carried in him, on the contrary, a love of life and all

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its joys, a healthy and joyful optimism which complements and explains his unrepentant rationalism. “But his supreme virtue was goodness…” Paul Langevin devoted his second article in La Pensée after his return to Paris to nuclear energy166: “... We are actually witnessing, in a particularly dramatic form, the beginning of a new era, that of induced transmutations. It opens up perspectives that go far beyond the old dream of the alchemists. It is no longer a question of synthesising gold, which would add nothing to the happiness of mankind, but instead of making available to mankind the inexhaustible reserves of energy hidden by nature at the very heart of atoms, concentrated in their nuclei and whose existence was revealed to us barely fifty years ago by the discovery of radioactivity, with which the names of Henri Becquerel and Pierre and Marie Curie are associated. This discovery will perhaps be as important for the future of civilisation as that which enabled man to master the power of fire. Its applications, thus far limited to the medical field, will far exceed those of the steam engine and the internal combustion or jet engine... “... This material liberation would make spiritual liberation, the development of culture, not only possible thanks to the leisure it would provide, but also necessary because of the need for man to create and operate increasingly delicate and complex machines. “Just as it became necessary to give the working man, at the beginning of the present capitalist period, a basic primary education – to know how to read, write and count – so that his professional value and the surplus value of his work increased, so the use of ever more sophisticated techniques and the operation of ever more sophisticated machines in an ever more cohesive and united human community will require of each individual, in the interest of all, an ever higher degree of education, an ever more complete understanding of the structure of the world and of the laws which govern nature and man.” So ends this remarkable glimpse of the future.

Education reform As a logical consequence of his conception of the development of civilisation, one of the problems that most fascinated Paul Langevin was the realisation of what he called ‘justice in the school’. As we have seen, he had considered this question since well before the war but after 1945 it was central to his concerns. He was therefore very pleased when the decree of 8 November 1944 166 La Pensée, No. 4 (July-August-September 1945), pp. 3-16.

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created a ministerial study commission for educational reform, of which he was appointed chairman. Although already seriously ill, for almost two years Paul Langevin devoted a great deal of his effort and time to the work of this commission, which he convened at the ESPCI. One can even say that this consumed his remaining strength. On the radio in June 1945, Paul Langevin set out the principles on which he intended to base ‘French educational reform’167: “The efforts which were made either before or during the occupation168, are being continued by this study commission for the reform of education which I have the honour of chairing and which was set up exactly eight months ago to prepare the solution of the very many complex problems posed by the reconstruction of our national education... “... The aim is to provide every child with the means to develop his or her abilities to the full, so as to enable him or her to occupy the place in society, in the nation and in humanity in the broadest sense, which corresponds to his or her value, a value which results both from his or her abilities and from the effort he or she has made to develop them. “A reform of this kind is, of course, directed towards the particular interests of each individual, but it is also in the general interest, because we must realise that the human resources at our disposal, and at the disposal of our country in particular, are now and have been for centuries used as poorly as they could be. It is therefore a question of putting each person in his or her place and making the best possible use of each one in the interests of the community. The principle thus set out implies consequences and we have tried to set out these consequences in order to prepare our project and the means of putting it into effect. “A first consequence is obviously that of free education at all levels. But free education alone is not sufficient and must be accompanied by family allowances for children, pre-salaries for young workers and subsidies for students. The future must not be compromised by differences in material conditions, since it is well established that difficulties in feeding children have a profound effect on their school careers. Children must be adequately nourished if they are to be expected to make the intellectual effort of which they are capable. “There is another consequence that, above all, results from the parallel aspect of the development of the personality. This twofold aspect concerns 167 This text was printed in the special issue of Pour l’ère nouvelle, pp. 189-190, already cited. 168 Allusion to the paragraph on education in the Programme of the National Council of the Resistance (March 1944), to the Communist project for educational reform drawn up by Georges Cogniot (September 1943) and to the work of Algiers.

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vocational training, preparation for a trade, and also general culture, in other words the training of the worker in the most general sense of the word and the training of the man. “To briefly recall what we mean by general culture, I would say that we mean everything that places the human being in relation to his fellow human beings, connects him to them and inserts him into the species, in both place and time... “With regard to place, each of us must have the possibility of understanding and appreciating other forms of activity and other states of mind than our own; and, in time, each of us must have as clear a sense as possible of the development of the human situation and problems in order to feel solidarity with both our ancestors and our descendants, to be aware of being a link in the long history of the development of the human species, and to feel responsible for passing on to our descendants the civilisation inherited from our ancestors, enriching the treasure it represents to the best of our ability. “We want the school to develop in each child a taste for culture, to be an initiation into culture, and not shut the door on culture. As we say, from the moment the child leaves school, culture is for each of us the work of our whole life. Since we want to emphasise professional aptitudes and as far as possible place each person in the situation which enables him to be of most service to the community, first educational and then vocational guidance must play a considerable role in the structure of our national curriculum. The problem of guidance is one of the most important that we have to solve. “Finally culture, what we call general education, cannot continue if it is not parallel to vocational training for all. There is still too small a proportion of children who receive training in our education for a trade that they will practice. It is absolutely necessary that we provide vocational training in schools, both for manual and intellectual occupations. This is done for intellectual jobs. We need to do it for all manual occupations as well.” Miss Seclet-Riou, his faithful collaborator and dear friend, summed up the work he accomplished within the Commission of which she was the secretary, in a posthumous tribute to my father on 19 December 1956: “… I remain astonished by the wealth and scope of the work accomplished under his impetus and with his constant participation. All the problems posed by national education, for the present and the future, were defined, analysed and resolved 10 years ago by Paul Langevin, ... “To remember Paul Langevin in the Commission for the Reform of Education is first of all to recreate an atmosphere. The opening session took place on 29 November 1944. The war was not over. Many families

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were still living in distress. Many heroes were still to die. But Paris was vibrating with the enthusiasm of its liberation, with the popular joy of a dearly-paid, hard-won deliverance, which carried with it, we thought, more than promises: the assurance of a beautiful and clear future. Despite our material and moral ruins, Paul Langevin was already working to rebuild France. “It was a time when the French loved each other, at least those who, in the face of Hitler’s odious victory, had not bowed their heads, had not abandoned the slightest bit of their love of country, their national pride, their democratic convictions. The spirit of resistance animated the Commission, creating between its members a family bond, a cement of comradeship. The President’s handsome face, scarred by fatigue, anguish and pain, and yet luminous and serene, appeared as the figurehead, marked by past storms, and the bearer of all hopes. “It was a time when the French loved each other enough to admit and understand their differences, since on the essentials they had not diverged. Within the Commission, there were great intellectuals, scholars, teachers, professors of all disciplines and French people of all origins. In them there lived, clearly and powerfully, the consciousness of their national value, of the unique value of the working class, “the only one who that remained faithful to the desecrated France”. It seemed simple then, and just and logical, to provide the whole nation with all the possibilities of culture. The most complete democratisation of our University, demanded by Langevin, with all the arguments of his lucid intelligence, all his passion for justice and his love of the people, unanimously accepted, became the main idea which guided even the most technical discussions. “As the months passed, however, the clear atmosphere of the aftermath of the Liberation became somewhat clouded. But at no time did it deteriorate to the point of calling into question the great principles so passionately and effectively defended by Paul Langevin. The unanimity of thoughts and hearts, which he had brought to life so well, remained intact at the end of the work. The plan for the reform of education which bears his name, associated with that of his great and dear friend, Professor Wallon, received the unanimous approval of the Commission, as a tribute to the man whose generous intelligence had inspired its work and for two years had followed and guided it step by step. “Paul Langevin was indeed an assiduous and scrupulous chairman, a chairman of the highest order, directing the debates not by authority, but from within, one might say, participating in all simplicity in the exchange of views, more concerned to provoke the expression of the thoughts of others than to formulate his own. He listened more than he spoke, but each of his interventions was like a light, clearing the views of the future from old prejudices, agreeing on the possible and the desirable,

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confronting all the proposals with the guiding principles accepted by all. The final report, by its clarity, its logic, its balance, its harmony, bears the mark of the rigorous vigilance he brought to bear in keeping the great principles, the inspiring thoughts, in their proper place in relation to future achievements and actions. “All those who took part have vivid memories of the first working session. Everyone brought their own experiences, desires, wishes, and preferences, perhaps even prejudices. But all were drawn in by the same feeling. We sensed that we were witnessing the birth of something great through this meeting, actually taking place before our eyes, of the most significant representatives of the ‘resistance university’ from here and there. The foundations of the joint work, accepted by common agreement, were Durry’s report on the Algerian project and the project of the ‘résistance métropolitaine’, presented by Georges Cogniot under the significant title: “Outline of a French policy of education” 169. Our common feelings and thoughts were synthesised and dominated by Paul Langevin’s introductory talk, defining the ultimate aims of education as the dissemination and triumph of culture. In his deep and measured voice, Paul Langevin began this fundamental presentation. “At a time when the question of educational reform is being raised, the cornerstone of the reconstruction of our country, it is important to see clearly what should be the aims and the main features of an organisation which will enable each child to develop his personality to the full, giving each one the means of access, for the greater good of all, to the form of activity in which he can render the most service by virtue of his aptitudes and his personal effort. At a decisive moment in our history, it is a question of posing, and as far as possible resolving in the broadest sense, this problem of the humanities which has long preoccupied educators, of defining, in their unity and diversity, the aspects of culture and professional training which university education should provide... “I will distinguish three closely related aspects of this problem: the individual aspect, the collective or human aspect and the national aspect, as it stands before you today...” Miss Seclet-Riou then gave details of the Commission’s meetings and referred to the last plenary session: “... The 52nd session, on 11 July 1946, was the last plenary session to be chaired by Paul Langevin. For many of us, this session had the melancholic nuance of the end of any year, of any separation, even temporary, of those who had worked together for many months. The President made 169 Outline of a French education policy, 30  September 1943. Text presented on behalf of the Communist Party to the Resistance groups.

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a point of closing this meeting with optimistic words, but which in retrospect have taken on a testamentary value: “Let each of us imagine,” he said, “what the conditions can and must be for the development of our human resources, that are currently unexplored and unexploited, and what we must do to bring them to fruition. When we think of culture and its creation, we are left confused by what has come out of such poor management, and full of hope for what could arise from better management. Let us act as a good father who seeks to get the best out of his children, out of each one of them... “... Back in the rue Vauquelin at the end of September170, Paul Langevin contacted his closest collaborators to hurry with the resumption of the Commission’s work and at the same time to outline the final report. On 25  November, a meeting took place in his office. Despite his tiredness, he thought intensely about the near and distant future. In the immediate future, he entrusted several of us with the responsibility of preparing the main chapters of the overall report. As if he felt that time was running out, he gave us only a few days to submit a first draft of our work. “Paul Langevin was unable join Pierre George, Roger Gal and me at the meeting on 3 December. “During this fortnight, which was to be the last of his life, he remained preoccupied with the work of the Commission. He was thinking about this great work, to which he gave his intelligence, his time, his strength and the best of himself, and it was in his mind until his last breath. It was like the crowning achievement of his life…”

From the Paris city council to the Encyclopaedia of the French renaissance Throughout this period, Paul Langevin was active as a communist. Upon his return to Paris, he was appointed honorary member of the Parisian Liberation Committee, which sat in the City Hall before the regular municipal elections in April 1945. Paul Langevin was nominated by the Communist Party in these elections and was elected municipal councillor of the 5th arrondissement.

170 In July-August, at the Sorbonne, Paul Langevin had presided over the 7th International Congress of New Education, which brought together 1200 educators from twenty different countries and where he was a lively participant. As soon as the Congress was over, on 21 August, my father left for Geneva, in order to obtain information from the International Bureau of Education on the teaching methods in use abroad. He stayed in Switzerland until 27 September.

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In spite of his poor health and as conscientious as ever, my father held his own surgery at the town hall in 5th every Saturday from 09h30 to 11h30. He mainly dealt with educational issues in the municipal council One episode clearly demonstrates Paul Langevin’s popularity after the Liberation. On 28 March 1945, when the General Council, of which he was also a member, proceeded to elect its officers, one by one the councillors went to the gallery to place their ballot papers in the ballot box. The letter L was called “Mr Langevin” by the secretary. My father stood up; immediately, on all the benches, from the right to the left, applause crackled. Paul Langevin also chaired several meetings in 1945 organised by the Communist Party, in particular the meeting for women attended by Maurice Thorez, Mathilde Péri and Jeannette Thorez-Vermeersch. The inaugural session took place at the Palais de Chaillot on 10 June 1945 where Paul Langevin, assisted by Georges Tessier, Louis Aragon and Henri Wallon, defined the meaning of the Encyclopédie de la renaissance française, which, in the minds of its promoters, was to continue the work undertaken by Diderot. My father gave a very distinguished speech at the meeting, where he said: “On the level of action, where doctrine must be translated into method, dialectical materialism seems to prove as fruitful as on the level of explanation or understanding. It seems to permit an extension of the experimental method itself...” Unfortunately, through the negligence of the main person responsible, this Encyclopaedia did not succeed.

Once again fighting for peace... As a communist activist, my father devoted all his energy to the fight against warfare. At the beginning of July 1946, he took part in the congress of the National Union of Intellectuals and launched a poignant call for action for peace at the opening ceremony: “By taking the theme “French thought in the service of peace”, for the Congress which opens today, the l’Union nationale des intellectuelles wanted to emphasise the paradoxical fact, so contrary to our hopes for the Liberation, that at the very moment when the cessation of hostilities has just been declared and the peace conference is proceeding with great difficulty, the hoped-for détente has not occurred... We are gathered here to reflect together on this threatening situation, to try to understand its causes, and to convince ourselves of the duty of each of us to use all our powers of thought, expression and action to avert the danger that hangs over us all. Our humanity has never had more powerful means to forge its future: today it is at a crossroads; may we help it to choose the road that leads to a peace that is all the more beautiful because war can be even more atrocious...

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“The results of the conquests of science, and especially the most recent one, have made us doubt the human value of our science. Nearly 20 years ago, the crises resulting from the development of technical applications, the scourge of unemployment, and the absurd spectacle of the voluntary destruction of human wealth, had already caused us to propose that the new Prometheus, science, should be put in chains. The same paralysis of our great efforts to understand the world and to dominate it would also result from the secrecy imposed on certain research by nations jealous of preserving their lead in any field, a decision that would generalise the stupid method of trade secrets used by too many industrialists at an international level. As there is no secret that cannot be penetrated or even surpassed with sufficient effort, that outcome would be an atmosphere of distrust and a race to the bottom, where all science and all humanity would perish. Moreover, who can foresee the consequences of any new discovery? Those that we are so proud about or so worried about have begun in a very modest way. It would therefore be necessary to prohibit all publication, to stop all scientific life and to deprive ourselves of the benefits that knowledge of the world makes possible, alongside the dangers. “In reality, these dangers result from the fact that these new and powerful means of action are developed in a world where individual and collective egoism has reigned supreme and gave rise to conflicts that could only be intensified and broadened by harnessing new forces... “... The great new fact about the atomic bomb is that, apart from its incomparable power, there seems to be no way of protecting ourselves from it, unless we return to the savage state and the stone age. “To help avoid this regression and to prepare for the happy peace that science makes possible, there is no other way for the scientist than to put his thought at the service of social or international justice, to develop the companionable inner man, to leave his profession and the ivory tower in which he practises it, to take a greater interest than he has done up to now in the use of his discoveries for good or for evil, and to sound the alarm to alert the entire species, which is being put in danger by certain leaders. “Remarkably, and to their credit, the American scientists who worked on the development of the atomic bomb were the first to understand the task that I have just outlined171.”

171 As Paul Langevin spoke these words, news came through of the resignation of Dr.Oppenheimer, who had presided over the making of the atomic bomb in the U.S.

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... and for justice My father joined the Communist Party as a man of science. As Professor Bernal said, Paul Langevin “set an example of a socially conscious intellectual”. He was convinced that a scientist does not fully discharge his responsibilities if he does not actively advocate justice in social relations, so as to create conditions in which science can fully flourish. On 5 March 1945, at the ceremony organised by the French University Union for his seventy-third birthday, my father said: “... Science has evolved so rapidly over the last two centuries that Justice, always a little lame, has not been able to keep up, and our social or international organisations no longer correspond to our ability to act. “In order to establish harmony, it is necessary for Science to reach out to Justice, by applying scientific methods to the study of human problems, and by developing a civic conscience among those who contribute to the development of science. Like the intellectuals who, at the time of the Dreyfus affair, put their strength of mind at the service of individual justice, it is today a duty for those who create science to watch over the use that men make of it. “For more than 20 years this conviction has led me, along with good militant comrades, to devote part of my strength to the defence of social or international justice... “I have followed the immense Soviet experiment from the beginning with passionate interest, because I felt that it was moving towards justice based on science. As I became better acquainted with it, I gave its guiding ideas more and more complete support, confirmed by my recent enrolment in the French Communist Party. “These ideas derive from eighteenth century thoughts about human progress, adapting them to new conditions. I am grateful to them for having helped me to understand better the evolution of my own science and for having confirmed my confidence in the future of human endeavour.”

The death of Paul Langevin My father’s self-imposed overwork, what he called the ‘whirlwind’ of visits, meetings, telephone calls, and his scrupulous determination to answer letters from what he called his “persecutors” and to clear up the backlog172, all this hastened his end. As has been noted he had long been afflicted with an 172 The stock of unfulfilled requests, articles to be written, letters to be drafted, constituted what my father called his “remorse”. Every holiday period, the ‘remorse’ filled a large wicker trunk, which Paul Langevin hoped to dispose of, always in vain.

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illness that was then incurable and probably due to prolonged x-ray irradiation, not to heart disease as was generally believed. As he felt that he had not finished the work for the reform of education that he considered most important and urgent, he resisted his fatigue and weakness until the end. He struggled to recover. He maintained a great lucidity until his last moments. He wanted to have all his family and friends around him. He died surrounded by the affection of his family and many faithful friends, who could hardly overcome their distress to speak to him, as he urged them to do, in order to fix an increasingly elusive thought with the help of dialogue. Dr. Ghisa Ghinsberg gave my father attentive and vigilant care, which inspired him with confidence and hope almost to his last breath, in spite of the physical sufferings that he stoically endured. As has been rightly said, his last words reflected his overriding concerns. He muttered several times: “Justice, goodness!” Paul Langevin was such an affectionate and exceptional father that none of his children ever fully recovered from the terrible blow of 19 December 1946. As soon as Paul Langevin’s death became known, condolences poured in from all over the world and from all walks of life. While the National Assembly was hearing my father’s eulogy, the Political Bureau of the French Communist Party addressed a tribute to our family that appeared on 20 December in L’Humanité: “Paul Langevin, an honour to French science, a noble heir to the great tradition of thought which has always been represented by the best minds of our people, brilliantly continuing the work of the Encyclopaedists of the eighteenth century into the conditions of modern science and society, set an example of the most clear-sighted and courageous patriotism in the struggle against the fascist aggressor which he engaged in without hesitation and supported without rest and with total self-sacrifice. “Paul Langevin, an active communist and deeply attached to his party, was able to contribute effectively to the development of the theory of modern rationalism, dialectical materialism. He leaves a high example to all French intellectuals, the most advanced of whom will be keen to continue his work by joining the ranks of the French Communist Party in even greater numbers.” The family received hundreds of telegrams from my father’s friends and former students, from communist organisations in France, Algeria, Brazil, etc.

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The funeral of Paul Langevin As soon as the government learned of my father’s death, it decided that a national funeral would be held for him. The date was set for the afternoon of Saturday 21 December, at the Collège de France.

FIGURE 22. Paul Langevin’s funeral (21 December 1946). All rights reserved.

My father’s body was laid to rest in a chapel set up in the entrance porch of the College that he had so often hurried across to reach his laboratory and the pupils waiting for his unforgettable courses. I have a poignant memory of the ceremony where a large Parisian crowd had gathered alongside the authorities and intellectual personalities. As Jacques Nicolle said, “we saw all the people of Paris, united in brotherhood before his coffin, workers from the suburbs and intellectuals, representatives of the provinces, towns and countryside, and Breton sailors followed by miners from the North in working clothes. What could be more moving than to read among so many testimonies on the register of signatures deposited at the Collège de France the names of ambassadors of the most diverse foreign powers next to those of the modest craftsmen of the 5th arrondissement! The workers of the Renault factories signed next to the members of the Academy of Sciences...” A huge tricolour flag floated over the pediment of the Collège de France. Wreaths and bouquets were piled up on the pavements.

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Military honours were paid to Paul Langevin, the Grand Cross of the Légion d’honneur. My father’s life and work were remembered in moving terms by Georges Cogniot, on behalf of the Central Committee of the French Communist Party, by Frédéric Joliot-Curie, on behalf of Paul Langevin’s pupils and his colleagues at the Academy of Sciences, and by the Minister of National Education, Naegelen. The President of the Republic and the President of the Council also attended the ceremony. Then the long procession moved off in rows of ten to cross Paris to Père-Lachaise. Despite the bitter cold and the strong north wind, tens of thousands of workers, employees and intellectuals accompanied my father to his final resting place to the sound of Chopin’s Funeral March. An endless crowd formed a border along the route. United as brothers in mourning, all these men who loved goodness and justice mourned both the great scientist and the courageous comrade in battle. His widow leant on the arm of Marcel Cachin as she walked. A large number of ambassadors followed behind the family and the Political Bureau of the Communist Party, including the representative of the USSR, Bogomolov, and the representative of Vietnam, Than Van Druy. Academics included the rector of the Academy of Paris, Gustave Roussy, the director of the National Centre for Scientific Research, Georges Teissier, colleagues from the Collège de France with Henri Wallon, the Academy of Sciences with Elie Cartan, Louis and Maurice de Broglie, members of the Council of State, jurists, writers and artists and a whole crowd of eminent personalities. Not least was the moving tribute of the pupils of the École primaire supérieure Turgot, who had massed on the pavement of the rue Turbigo to greet the remains of Paul Langevin. The same was true a little further on for the students of the École des métiers de la chaussure and those of the École de commerce on Avenue de la République. For the family, plunged into grief and turmoil, all these expressions of sympathy, from the greatest personalities to the humblest workers, proved once again the importance of the person that we were mourning and the loyalty of Paris to his memory and to his example. This was a precious comfort in our grief. Paul Langevin remained alive in the memory of his peers, his comrades in the struggle, the faithful and the people who loved him. The first posthumous tribute paid to him was a radio address by René Maublanc on 22 December 1946, in the name of the Rationalist Union: “Paul Langevin was not content with being one of the world’s foremost physicists; he was truly an example of humanity of a very rare kind. Not content with understanding certain ideas and problems that only a dozen or so scientists in the world were able to discuss with him, he wanted none of the theoretical or practical questions that might worry

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the humblest of his fellow men to be foreign to him; he covered all the miseries, he was indignant at all the injustices, he risked his health, his freedom and his life to fight against all oppression. And always, whether it was a question of Einsteinian relativity or the defence of human rights, of the determinism of nature or of political action in the party of which he had wanted to be a devoted militant, he showed the virtues that command respect and affection: modesty, disinterestedness, simplicity, good-humoured cordiality and the supreme virtue, goodness.” During a meeting arranged by René Maublanc for the Rationalist Union at the Richelieu Amphitheatre at the Sorbonne on 21 January 1947, Frédéric Joliot-Curie, the new president of the society, remembered with feeling the person who, he said, embodied the very spirit of scientific rationalism, and recalled the magnificent lectures on “science and determinism” that my father had given in the spring of 1930. The National Union of Intellectuals organised a solemn ceremony in memory of my father in the great amphitheatre of the Sorbonne on 25  February 1947. One after another, M.  J.  Cabannes, Frédéric JoliotCurie, Eugénie Cotton, René Lucas, the rector Gustave Roussy, our friend J. D. Bernal, then René Maublanc, Albert Bayet and Marcel Cachin each praised the scientist, the philosopher and the man of action. The audience listened with emotion to the performance of Gabriel Fauré’s Elegy and Beethoven’s Moonlight Sonata, which my father had loved so much. By playing his favourite pieces of music, by playing a recording of one of his speeches, the organisers brought the man himself to life. The day after, it was the turn of the Academy of Sciences of the USSR and the University of Moscow to solemnly mark their admiration for the work of Paul Langevin. Serge Vavilov, President of the Academy, and after him, Grigori Landsberg, Abram Ioffé, Vladimir Kemenov and other scientists spoke at an event organised in the large physics amphitheatre of that university. The Groupe français d’Éducation nouvelle, for its part, held a commemorative meeting, under the presidency of Henri Piéron and Henri Wallon, in the large amphitheatre of the Faculty of Medicine on 31 March. At about the same time, beautiful and touching ceremonies were held in Troyes. One of my father’s good friends, M. Casati, eloquently recalled the years when Paul Langevin was in forced residence in that city. It is unfortunately impossible to quote here all the articles written in the press by faithful friends in memory of my father during this period. Let us recall, almost at random, the moving tributes of my father’s friend Pierre Vaillant, of Georges Fournier, of André Wurmser, of Senèze, then Secretary General of the National Union of Teachers, of Jacques Hadamard, of Paul Le Rolland, of Madame Cotton, of Miss Seclet-Riou, of Francis Jourdain and of Albert Bayet. A large number of these texts appeared in No. 12 of La Pensée. Finally, on 15 December 1947, on the occasion of the solemn prize-giving ceremony under the Dome, Louis de Broglie paid Paul Langevin a very beautiful tribute.

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The transfer of the ashes of Paul Langevin and Jean Perrin to the Pantheon On 15 June 1948, Bill 4562 on the transfer of the ashes of Jean Perrin and Paul Langevin to the Pantheon was presented to the National Assembly. The explanatory memorandum concluded with these words: “To stay aligned with the life and work of these two scientists, the tribute paid to Jean Perrin and Paul Langevin during a difficult period must not only serve science by enhancing its prestige, but also be the occasion for events that help scientific progress directly. This is why the support requested is partly intended for the organisation of a working meeting of scientists from all countries, who are invited to the ceremonies for the transfer to the Pantheon.”

FIGURE 23. Procession to the place de la République. The transfer to the Panthéon (November 1948). All rights reserved.

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It was Georges Cogniot who, on behalf of the Finance Committee, presented the favourable opinion on the project to the National Assembly on 16 September: “The government asks the National Assembly to grant the Minister of Education a credit of 5 million francs for the transfer of the ashes of Paul Langevin and Jean Perrin to the Pantheon. “Your committee proposes to grant the credit subject to the following observations: “1. The bill indicates that the tribute paid to the two illustrious scientists should be the occasion for events contributing directly to scientific progress, and it mentions a working meeting of scientists from all countries invited to the transfer ceremonies. One can only applaud the convening of this meeting, but it seems that at the same time the best, most lasting and most useful way to honour Paul Langevin and Jean Perrin must be the publication of their works. Under the present conditions of publishing, especially publishing of a high and theoretical character, make it almost impossible without State support to publish these works. They are all the more necessary to print because they have been produced largely in the form of teaching courses and thus essential and original elements remain absolutely unpublished. The study of this problem could be entrusted to the National Centre for Scientific Research. “2. It goes without saying that the public, in particular young students, school children and their teachers and popular organisations, should be associated with the transfer ceremonies, from which all official dryness and vain formalism would thus be removed. The ideas of the two illustrious dead were well known in this respect. It is therefore important that the transfer should take place either on a Sunday or on a Thursday173. “With these observations, your committee gives a favourable opinion on Bill No. 4562.” In fact, the government organised four scientific colloquia, which were held from 15 to 19 November at the Collège de France, and in which many French and foreign scientists, collaborators and friends of my father, took part. Among them were Aimé Cotton, J.  D.  Bernal, Joliot-Curie, L.  Rosenfeld and Georges Teissier. The scientific works of Jean Perrin and those of Paul Langevin (which my brother collected) were also published. They were published in 1950. But if the first part of the Finance Committee’s wishes was granted, the same could not be said for the second part. Instead of setting the date for the transfer of the ashes on a Sunday or Thursday, as had been requested, the government decided on Wednesday 17 November, at 9 a.m. 173 * Schools were closed on Thursdays in France at this time.

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The choice of a day of the week other than Thursday was clearly a deliberate attempt to restrict the popular tribute paid to the two great departed. This is why my mother felt it necessary to send a letter of protest to the President of the Republic. This letter had no effect. We came up against the absolute will of the government to maintain the planned day and not to accept that the people of Paris, and especially the militants of the Communist Party, could participate in the parade. The claimed pretext was that the “dignity” of the ceremony would be compromised by their presence. We had to give in under the threat of the total suppression of the ceremony. The petty manoeuvre of the government was defeated by the people of Paris. The people succeeded in giving the transfer of the ashes an extremely imposing character and the meaning of a mass demonstration in honour of the two scientists who had linked their fate to that of the workers. The bodies had been exposed at the Palais de la découverte, symbolically chosen for the last and solemn wake of 16 to 17 November. I still remember our surprise when, on the morning of the 16th, the mortal remains of Paul Langevin were taken from Père Lachaise and travelled with us across the whole of Paris, where the traffic stopped as if the life of the capital was suspended for a moment at the passage of one of its greatest sons. In the Palais de la découverte, draped with immense flags, around the two coffins placed side by side and covered with a tricolour, former students mounted the guard of honour while colleagues and friends of the deceased scientists gathered. In the night and the silence, the strains of Beethoven’s Funeral March made the sadness even more poignant. At nine o’clock in the morning, under a little freezing rain falling from a uniformly grey sky, in harmony with the funereal pomp of the day, the procession set off. Troops rendered the honours. Two groups of sixteen students, flanked by torches, took turns to carry the two coffins. Students also held the velvet cushions on which their masters’ decorations were placed. The families of the scholars and their close friends followed in two parallel columns. Among the officials were the Minister of Education and Rector Sarrailh, followed by teachers in gowns. Finally came the great crowd of delegates, among whom it was easy to recognise the representatives of the French Communist Party, of the C.G.T., of the Municipal Council of which Paul Langevin had been a member, of the Société des gens de lettres de France-U.R.S.S. which had chosen my father as its president, of the Ligue de l’enseignement and of the Union rationaliste. Tens of thousands of people accompanied Paul Langevin and Jean Perrin to the Pantheon while, along the route, despite the early hour and the bad weather, large numbers of Parisians stood at the side and paid their last respects to the deceased.

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The coffins were laid down under the dome of the Pantheon, and the Minister of Education recalled the careers of Jean Perrin and Paul Langevin. The day before, a solemn ceremony had already taken place in the great amphitheatre of the Sorbonne in the presence of the President of the Republic, where eulogies for the two scientists had been given by the Rector Sarrailh, Edmond Bauer, Madame Cotton, Jean Cassou, Henri Laugier, Henri Wallon, John D. Bernal and Louis de Broglie. Since 17 November, my father and his friend Jean Perrin have been laid to rest, alongside Berthelot and Painlevé, in the crypt of the Pantheon. Every year until recently, the organisations of which Paul Langevin was a member called their members to a commemoration at the Pantheon to mark the anniversary of my father’s death. The procession formed at the Collège de France, where those present would gather, united in their respect for the memory of Paul Langevin and that of Jacques Solomon. Then we went up rue Saint-Jacques and rue Soufflot, our arms laden with flowers. Each year, our pace became slower, because my mother and our faithful friend, Marcel Cachin, led the march. Those who spoke under the dome included Hadamard and Sicard de Plauzoles from the Ligue des droits de l’homme, representatives of the Union rationaliste or La Pensée such as René Maublanc and Henri Wallon, and political activists. * *  * The ceremonies at the Pantheon are just one of the examples of the continuing loyalty to my father’s memory. The great light he shed on the world has not been extinguished. In many cities, schools, streets, large apartment buildings and rest homes bear his name. Countless people still feel guided by him with clarity, propriety and goodness. The Academy of Sciences, the ESPCI, the journal La Pensée, the pedagogical societies, the Rationalist Union and other organisations are preparing to celebrate the centenary of his birth with dignity. If, as John D. Bernal said, “the measure of a great man is the power and duration of the mark he leaves on the world”, my father deserves to be called that. The moving pages of memories that were sent to me, on the occasion of the preparation of this book, by René Lucas, Léon Brillouin, Henri Le Boiteux, Jean Saphores, the Bulgarian academician G.  Nadjakov and the Academician A.  B.  Datzeff confirm that Paul Langevin’s influence is still alive and kicking, that his example is a precious source of inspiration for all men concerned with the difficult problem of the union between thought and action. This certainty is a deep comfort for all of us who lived with him, who loved him and who cannot forget him. I am writing these notes in my country house in Samoreau, on the edge of the forest of Fontainebleau. This house is full of memories of my father. When, in 1938, we were looking to buy an old farmhouse in this region

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(since the daughter of Georges Urbain, who had kindly given us her house in Recloses, was taking it over for her own use), my father, with his usual generosity, took out a mortgage to transform the granary into bedrooms and to install central heating. This was the creation of what he jokingly called “the first five-year plan” of our development. We hoped to have him with us often during the holidays; unfortunately, his multiple occupations meant that he came to Samoreau only rarely. In this house, which he loved, we keep his manuscripts, his books and his letters, as well as the first desk he used and his first library. When I finish this book, I will consider my duty fulfilled if I have only succeeded in showing how much the man, who was a great scholar and a profound thinker, also knew how to be a tender father, a generous friend, a sensitive and passionate heart and a man in the full sense of the word.

Testimonials Some memories of Paul Langevin By Jean Saphores Honorary Professor at the École supérieure de physique et chimie In 1904, I entered the École de physique et chimie, which was then hardly more than 20 years old. Langevin was then standing in for Pierre Curie. The harsh discipline that reigned at the Collège Chaptal had become intolerable for me. My fate was decided by the desire to get away from it as quickly as possible. That long ago it was possible be admitted without difficulty to the school in the same year as the elementary mathematics baccalaureate. And I entered quite naively, ‘resigned’ to becoming a chemist, to escape the constraints that I no longer wanted to accept and without which I would probably have ended up as an agrégé in history! My first semester at the school was a disappointment. Most of the teachers did not have the ‘profession’ of secondary school teachers. However, I am very grateful to one of them, Albert Lévy. Formerly from a polytechnic he had, in his sixties, moved into chemistry. I certainly owe it to him to have started to understand the role and spirit of mathematics and I learned to love it. I saw Paul Langevin for the first time in the second semester of that first year. He was in charge of the general electricity course. Having naturally assessed the inevitable ignorance of young students, he opened his teaching with a few generalities of mechanics and energetics, which were indispensable for a clear understanding of the theories he was about to expound. During a career of half a century I have never known a teacher who was so totally devoted to his task and so unsparing in his efforts. His hour-and-ahalf lessons lasted two hours, which caused those in charge of the practical work, who were waiting for us, to despair. His 60 lessons became 90. If any clarification seemed necessary we went to the Collège de France, where he was then deputising for Mascart (who he was soon to replace), and even to his home.

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Langevin’s course was a dazzling revelation to me. I often emerged in a trance-like state from the old half-darkened room where he taught, rather like Moses coming down from Sinai. This admirable teaching definitely decided my vocation. I entered the school resigned to becoming a chemist, but when I had to choose between the two specialities, I opted for physics, filled with enthusiasm. Langevin had totally converted me! In 1908 a few months after my graduation, he asked me to direct the practical work in his teaching laboratory. I did so until 1925, with a fouryear interruption imposed by the war. Between these two dates, I attended his lectures at the Collège de France and participated in the ‘seminars’ he had set up there to organise lectures and discussions among young physicists on the problems of the day. These meetings were such a success that we were often joined by experienced scientists, mathematicians and physicists. I remember, for example, meeting Hadamard and Borel around the table. I believe that Langevin was, at that time in France, the most effective man teaching physicists. The School of Physics and Chemistry, created in 1882 by the city of Paris, was attached to the Faculty of Science in 1926. Albin Haller, member of the Institute and professor of organic chemistry at the Sorbonne, who had directed the school for 20 years, had passed away the previous year. Paul Langevin replaced him as Director of Studies at the School, having collaborated with him from 1906, the date of his appointment, until he died in 1925. At his request and without applying, I was proposed by the Board of Directors of the School to succeed him in his chair of general electricity, a post I then held for 33 years. I then realised the full weight of such an onerous legacy, which his advice alone often helped me to carry. From then on, I watched as countless visitors waited at the door of his director’s office every day. He always welcomed them all with untiring kindness and did not discourage any of them. I tried to be discreet and to consult him only when it was essential. As a result, my contact with my ‘master thinker’ became less frequent. Nevertheless, he remained the man that I most deeply admired and loved, apart from my family. So far I have only mentioned professional memories; I have hundreds of others concerning the “man” rather than the scientist. I would have to write a book to tell them. I am going to mention a few of them at random. The declaration of war in August 1914 took me away from him. I went to the front as a second lieutenant in the infantry and by September 1915 I had become a captain: there was a shortage of them, as the losses had been heavy. Towards the end of that year, the Minister of Armaments, Albert Thomas, not satisfied with the dynamism of certain services, created three ten-member commissions to examine technical inventions and suggestions that might be of interest to the army at war. Each of these commissions

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was to include a combatant officer. At Langevin’s request, I was appointed to one of them. The same job, for the other two, was given to Jolibois, who taught chemistry at the École des mines, and to Canac, a physicist, from Madame Curie’s laboratory. It was not a ‘planque’174, as we used to say in the army. We would only come to Paris for two or three days for very infrequent meetings, while keeping our commitments at the front. I remember, at the beginning of this time, a pleasant lunch to receive the three of us, organised by Madame Curie, Langevin and Blaise, who was then teaching organic chemistry at the Faculty of Science and at the school. Many ideas, inevitably concerning the ongoing war, were exchanged across the table. At coffee time, the three guests were asked “All three of you are captains and are fighting in the front line. How long do you think it would take to be fit to command a division?” Clearly, this was just our civilian “bosses” getting a better understanding of command problems. After a few minutes of reflection, I answered: “Three months” with the feeling of being bold. Jolibois was more daring; one month seemed to him to be enough. Canac, more cautious and perhaps wiser, took refuge in sitting on the fence. Langevin smiled. Blaise laughed, turned to him and said, “Well, remember what I told you! Obviously they had already discussed this problem between them. The advanced age of most of our generals worried them, as it did many other Frenchmen. During this meal, Madame Curie told us about the atrocities committed by the Tsar’s armies, who, entering into the Austro-Hungarian Empire, had undertaken to convert the Catholics to Orthodoxy with the sword. She had certainly been informed of this by her outside connections. She also told me about Rasputin’s sinister operations at the court of St. Petersburg. The “Commissions” of Albert Thomas disappeared discreetly, after a few useful and often tumultuous meetings, because the professional soldiers could not stand them. This is the common fate of commissions. After a while, towards the end of October 1917, I was living in Alsace on an exceptionally calm front. The Germans and the French spared the Alsatians and avoided violence, which would have led to destruction. Gunfire was rare and cannon fire exceptional. Receiving the daily “Regimental Orders” one morning, I read this miraculous sentence: Captain Saphores, commander of the 18th company, by decision of the G.H.Q. and at the request of the Minister of Armaments, is seconded, for a renewable period of three months, and placed at the disposal of Professor Langevin to assist him in experimental ballistics work of interest to National Defence.

174 * Sinecure.

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In this sector where nothing was happening, the colonel commanding the regiment, under which I had served for more than three years, had not even had the courtesy to pick up his telephone the day before to tell me the news. After greeting him very coldly in passing I rode to Belfort and left for Paris. Langevin had obtained this very exceptional decision through Painlevé, then President of the Council. At the time, the G.H.Q. had made it an absolute rule never to “let go” of an infantry officer, as recruitment would be too difficult. I nearly lost this posting. I had been in Paris for a fortnight and I still received, by post, the orders for my regiment. One day I found in it this paragraph that still concerned me: “By decision of the G.H.Q., Captain Saphores, of the 360th R.I., is posted to the French headquarters in Iassy (Romania). He will be transported by sea to Arkhangelsk (Russia), from where he will reach Iassy by his own means”. It was unbelievably stupid. I never understood how a French officer, who didn’t speak a word of Russian, would have any chance of travelling thousands of kilometres individually through a country in the throes of revolution! At once I guessed the origin of the affair. When Romania entered the war, there was a demand at the front for volunteer infantry captains to be deputies to the Romanian commanders, with the temporary rank of lieutenant-colonel. I was bored to death in the trenches and volunteered for anything that would get me out of there. I had wanted to go to the Army in the East, for the Indochinese riflemen and for all the rest. After my departure from Alsace, my brave and furious colonel had fished out of a drawer one of these requests that had never been sent. In ten days, it had been magnificently successful! In Paris, I was under the orders of General Moschot, commander of the Artillery Studies and Experiments. With the “decision” in hand, I visited him and told him about the affair, apologising for the probable departure. He said to me very kindly: “Forget it, I’ll go straight to the telephone to settle it with Chantilly. Even if your colonel hears of it, he won’t have stolen you!” And so I did my best to help Langevin in his experimental ballistics work, until my demobilisation in April 1919. This return to Paris had put me back in touch with a School that had become skeletal. Most of the students who had passed the successive competitive examinations had been mobilised. However, the management had managed to maintain, as best it could, a first-year teaching programme. I have forgotten the date when I came to see Langevin one morning in his office in the rue Vauquelin, when he said to me: Here I am in great embarrassment. I had found one of our former students who had been mobilised in Paris to replace Debierne in his general physics class. Bur he is being sent to the provinces. Since you are here, can I ask you to take charge of this course? I did it myself for two years, here are all my files, I’m giving them to you.

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It was not the time to argue, and my only question was: “When should I start?” And with the smile I loved so much, his answer came: “Tomorrow morning, at 08h30. Needless to say, I changed my plans for the rest of the day and the night that followed. The next morning, freshly shaved and in uniform, I entered the lecture theatre to long and loud applause from my young audience. Shortly after the armistice towards the end of 1918 military regulations granted me a week’s leave. Langevin, who was directing research in Toulon on the use of ultrasound to hunt German submarines, was about to depart and he invited me to go with him. There I met my dear friends Marcel Tournier and Fernand Holweck, who were working with him. I also met his old friend Jean Perrin, whose teaching I had once followed and admired at the Faculty. I had never been to this beautiful coast before, and if I am ending my days here today, perhaps I owe my choice to this distant journey. Very often, we all had our meals together, on the sunny terrace of a small restaurant, set up on the edge of a large square, at the foot of old plane trees, chatting happily to each other. As we were all talking at the same time, it was difficult to hear one other and I remember Perrin, sitting next to Langevin, pinching his arm until he shouted: “Paul,” he said, “you are not listening to anything I say!” With the war finally over, all our thoughts were obviously on the conditions for peace. With the help of the sun and no doubt the rosé wine from the Var, I made a stupid suggestion: First of all, we should force the Germans to change their language! Together, Langevin and Perrin, bursting into laughter, cried out, “What a savage!” I was demobbed in April 1919  and resumed my duties at Physique et Chimie. In 1920, not yet married, I was spending my family holidays in the Royan region. One morning I received a letter from Langevin who was in Lamaloules-Bains in the Hérault, which read: “We are almost neighbours, so please come and spend a week with my family”. The railway from Bordeaux to Cette (now Sète) was only single track, the rails of the second having been dismantled and sent to the front. The broken windows of the wagons had been replaced by wooden panels. So either you closed them and travelled in the dark, or in the smoke with the windows open. In eighteen or 20 hours of improbable travel, I finally reached my “neighbour”. Langevin was living with all his family in a large and delightful old house in the middle of the vineyards. A famous Virgin stood near the house, the object of a pilgrimage on August 15. Around coffee time, I learned from the “master”, who hated inactivity, that very early the next day we were going to leave for a long hike through the Causses du Tarn with his two sons Jean and André.

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We thoroughly explored for a week. Langevin’s stamina was extraordinary. As an old infantryman who had almost become professional, I could never get him to accept the principle of the hourly stop. He said that this stop cut the legs of the walker! And from six or seven o’clock until midday and beyond, we roamed the wilderness, amusing ourselves by watching a few sheep sucking stones. In three years of military life, in which I have so often travelled all the back roads, I have never suffered so much as in the first days of this expedition. Fortunately, in Sainte-Enimie, a good old moustachioed nun took care of my feet and encouraged me to continue to please the Lord. Along the rough, sunny road, we exchanged rare remarks and I remember that one day while walking, the discussion opened up on the problem of vocation. Jean Langevin said that he was worried about not being rigorously certain of his career and his father, laughing, replied: “How could you be? I still don’t know!” He was then forty-eight years old and universally recognised as one of the great physicists of his time. Although Langevin devoted all his intelligence and dynamism to the problems of resistance and response to German aggression during the First World War, he had not lost his ideas. The day before we left together for Toulon in December 1919, he had taken part in a meeting in Paris in defence of the French sailors who had mutinied in Odessa. And if in the inter-war period he devoted the greater part of his activity to the defence of his political and social views, he always did so with the utmost respect for those of his colleagues. I remember the joy with which we shook hands on the day after the 1924 elections, which finally swept away the Chambre “bleu horizon” 175 and, we thought, the residues of an absurd nationalism. The following year, however, he chose Copeau, a chemist, who was a practising Catholic and held conservative views, to take over as Director of Studies at the School. When, around 1934, with Alain and Rivet, he created the Comité de Vigilance des Intellectuels antifascistes (Vigilance Committee of AntiFascist Intellectuals), he never promoted it among the teaching staff that he directed. When I joined a few months later, he told me very simply and with a smile: “I knew you would come to us”. And then came the abominable Second World War. Separated from him during the exodus, I did not see him again until Paris in early October 1940. I was extremely surprised to find this man, who was by then sixty-eight years old, still full of hope and animated by a dynamism that already led him to totally refuse to accept defeat. A few weeks later, the Germans incarcerated him in the Santé prison, and then, a month later, sent him to Troyes under house arrest.

175 * The extreme right-wing government that gained power in 1919.

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It was only in 1943 that I was allowed to visit him. In spite of the very deep family sorrows that had hit him then, I found him there still as full of confidence for the future of our country. He was under the surveillance of the German police and was visited every week by a Gestapo officer. During the breakfast we had together, he told me this story. Langevin liked to smoke a cigar in the morning, when he could get them. Ringing his doorbell, a not entirely uneducated lout entered into a smoky atmosphere. And the conversation began: –  “Mr. Langevin, I don’t understand why you called us barbarians so often”. And, troubled by the smoke, the man went to the window and opened it. This gesture provided the reply: – “Sir, in a civilised world, a visitor does not allow himself your gesture. He needs the permission of the person receiving him to do so!” I was not to see my master again until after the Liberation, when he returned to his post in Paris, alas for too short a time. Although his magnificent intelligence had remained intact, the illness that would soon take him away was already eroding him. Almost 25 years have passed since he went and there are few days when I do not think of him. Not only do I owe him everything, in terms of scientific knowledge, but until my last day I will be grateful to him, above all, for having taught me to think and perhaps even more for having taught me, by example, to love people. Nice 1970

The man I knew by René Lucas Director of the School of Physics and Chemistry The first image I remember was in 1915-1916, when I was a young student at the School of Physics and Chemistry. At that time, Paul Langevin was carrying out research into the conversion of electrical oscillations into mechanical oscillations in order to obtain ultrasonic waves. This research was done with the engineer Chilowski and two of his former students, Fernand Holweck and Marcel Tournier. I often saw the admired and respected scientist pass by in the school. I was struck by his tall figure, his face lit up by a look that was full of radiance where gentleness and penetration of thought were combined. His broad forehead was dominated by black hair cut into a brush, where a few rare white hairs softened the assurance of his appearance. The sensation of the power of thought was so strong that I keep this first image intact in my memory.

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Later, after three years apart, I had the pleasure of knowing Paul Langevin as a teacher, first at the School of Physics and Chemistry, then at the Collège de France. The vigour of his thinking and the admirable clarity of his lectures earned him almost religious devotion from his listeners. Many young physicists, such as Louis de Broglie, Léon Brillouin and Francis Perrin, flocked to the Collège de France where he dealt masterfully with the most fundamental problems of physics. Scientists who had reached the height of their scientific fame, such as Elie Cartan, Charles Mauguin and Edmond Bauer, did not fail to come and follow with scrupulous attention the developments on the theory of relativity. Unfortunately, these admirable lectures, in which the master rethought the foundations of physics in front of his audience, were never published. I also have a very vivid memory of the beautiful meeting when Albert Einstein, passing through Paris, came to the Collège de France to take part in a discussion on the foundations of relativity, a meeting at which both Paul Painlevé and Jacques Hadamard also contributed to the discussion. When I was appointed as Paul Langevin’s assistant at the School of Physics and Chemistry in 1922, I had the opportunity to get to know the scientist better, through our many discussions. How often I was struck by his anticipation, which made him guess the questions I planned to ask him! In 1925, I had the pleasure of spending most of my summer holidays with him and his family. This time I found out about the man, not the scientist, in the broadest and highest sense, made of goodness and simplicity. This was a delight for me. Paul Langevin was happy to go for long walks through the countryside with his children in the magnificent foothills of the Cévennes. Some walks were too long to get home in the daylight, and so were divided by a night spent in the simplest way possible, in a barn that was discovered during the evening. The years that followed were gradually overshadowed by political developments in Europe. Sensing the mortal peril that threatened humanism due to the rising tide of fascist and racial ideologies, Paul Langevin and a few scientists took a stand that would later expose them to attacks from Hitler’s agents. Many intellectuals, threatened in Germany or in Austria, left their country. Many passed through Paris and came to see Paul Langevin, whose kindness and inexhaustible generosity were extended in every way to help these first victims of Hitlerism. After the defeat of 1940, the hardest trials were to befall Paul Langevin and show his courage and fortitude. The Liberation of 1944 finally put an end to this nightmare and Paul Langevin’s return to Paris was a source of immense relief for his family and friends. Back at his beloved School of Physics and Chemistry, he devoted himself, alas for a short time, to the work of the Langevin-Wallon Commission for educational reform. Indeed, Paul Langevin was convinced

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that it was profoundly right and of the utmost importance for the country to allow every person’s intelligence to achieve its full potential. A magnificent anniversary was held in 1945 in the great amphitheatre of the Sorbonne to celebrate this man whose qualities of intelligence were equalled only by the qualities of his heart. A relentless illness took him away in 1946. In a single ceremony dedicated to Paul Langevin and Jean Perrin, the nation held a grand funeral for two of its greatest sons.

A great scholar and humanist by Léon Brillouin Professor at the Collège de France Dear friend, I promised you some memories of your father, whose great figure as a scholar and a humanist I so admired. But where to start? How do I sort out all these images that beset me, and where do I make the distinction? These memories are mixed with all my recollections as a child and as a student, at the beginning of my own career. I don’t know how to classify these ghosts; I give up trying to put them in an artificial order and I let my pen run, at random, in the reflections of a lost time. For it is a whole lost era, disappeared, that is being reshaped before me. It is not only my youth, which is already long gone, but the time before the world wars. After the European war of 1870, there was a period of peace, or a semblance of peace, when wars were only colonial and distant; life in Europe was stable, quiet and without storms. A student would set off on his bike and ride all over Europe without papers. A passport? Balzac talked about it, but what was it? Nobody cared. To get some money, or to receive your letters at the poste restante, all you had to do was show an old, stamped envelope; there were no police controls, and exchange rates were based on the gold standard. The Baedeker guide gave these rates to three decimal places, unchanging. Passports, worries, in short, barbarism, began at the Russian and Turkish borders. I travelled alone in this bygone world, visiting famous scientists, friends of my father or grandfather. Paul Langevin had gone to study in England and liked to talk about his travel impressions. I myself went to work in Munich at Sommerfeld. By the way, let me correct a false impression you had about this great scientist: he was a Prussian from Koenigsberg with the scar of a student fighter on his forehead, but he had a liberal and extremely independent spirit. In 1914, he refused to sign the all too famous declaration of the 93 German scholars in favour of the German army and, when Hitler came along, he came under suspicion by the regime by stubbornly supporting all his Jewish students and colleagues. Pauli has personal proof of this.

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My personal memories of your father go back to the high school years, at the very beginning of the present century. My parents had rented a small house, Swiss chalet style, on a hillside above Palaiseau, from which the view stretched as far as Corbeil. One summer your parents moved to the same village, which had kept its old-fashioned appearance and felt so far from Paris. The two families met often, and I remember the very cordial welcome and hearty snacks at your parents’ home. They lived in a fairly large dwelling in an old farmhouse, and there was a courtyard, where little monkeys played in a cage. We also went to George Sand’s house on the Yvette. I was in high school at the time, and I took the train with my father on Wednesdays and Saturdays, around 5 p.m. The Friday or Monday morning trains took me back to school. On these Wednesday and Saturday evening departures, my father would often meet Pierre Curie or your father, and they would have long scientific conversations with each other, in which magical words would come up: electrons, crystals, radium. I guessed at an unknown world, full of unimaginable wonders, and far exceeding the surprises of the 1900 Exhibition or the height of the Eiffel Tower (opened the year I was born). As a child, I used to read old physics books borrowed from my father, where the figures represented light rays through space and zigzagging between mirrors, lenses, prisms, telescopes and so on. And I often went to meet my father in his lab at the École normale (I even broke a precious crystal there once!). All the physics instruments were familiar friends to me. It was a great disappointment when I had my first physics lessons in high school: weights, balances, pressures... What a bore! “How can you spend your life studying?” I said to my father one day. He was moved by my distress. “Wait a moment”, and he looked in his library for a small book: Pascal’s letters to his brother-in-law on the discovery of atmospheric pressure and the famous experiment at Puy-de-Dôme. It enchanted me like a soap opera. I came across this wonderful feeling of discovery several times in my life. First of all, as a student at the École normale, I had to follow the physics courses at the Sorbonne. It was grim, uninteresting and badly presented (and the students had no idea of rebelling then!). At that time, your father was called upon to replace my grandfather Mascart, who was unwell, and began a series of lessons at the Collège de France: electricity, magnetism, Maxwell, and then (once he was appointed full professor) electromagnetism in moving bodies, relativity, radioactivity and so on. What a wonder! I found it difficult to follow at first, but I did not hesitate to come and disturb your father to ask for some explanations, and I sometimes brought some friends from school with me (Foch, Pérès, Canac, and so on). Soon there were also Jean Perrin’s classes and those of Madame Curie. All this gave me back the enthusiasm and the excitement of research. I often consulted Langevin’s old notebooks, as well as those of Sommerfeld’s courses in Munich (the latter were very strange,

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written in French for a month, then in Franco-German gibberish, then in German at the end of 1913). But let us return to Paul Langevin’s lectures. Very often, in his lessons at the Collège de France, he presented entirely new and already wonderfully clear theoretical developments... Many of these personal researches were not published, although some of them were written down by his students from their lecture notes. In 1911, for example, Langevin explained the principle of relativity and the famous relationship between mass and energy. This work remained unpublished for a long time, but was fortunately taken up again much later (in 1932) by Francis Perrin, in issue no. 41 of the Actualités scientifiques published by Hermann (pp. 11-18). Also, in 1911, the first Solvay Congress of Physics took place in Brussels. Sommerfeld presented a very important remark, insisting that Planck’s constant  h represented a ‘quantum of action’ in the sense of the term in mechanics. Seizing the opportunity, Langevin showed how to take advantage of this concept to calculate the value of the magneton. There was still an arbitrary coefficient in the formula (as in Sommerfeld’s paper), so Langevin’s value had an incorrect factor of 2, but the reasoning was already the same as that by which we now obtain the so-called Bohr magneton. As soon as he returned from Brussels, Langevin amazed us by summarising these discussions and bringing us up to date with the latest problems in physics. With Langevin’s teaching and then Sommerfeld my student years came to an end. I had started my doctoral work on quantum theories and submitted a note to the Comptes rendus in the spring of 1914, which contained the beginnings of the discovery of the now classic ‘Brillouin diffraction’ effect. And suddenly war broke out, interrupting our peaceful studies and plunging us into brutality. For my part, I became absorbed in the problems of military radio. Langevin tackled the detection of submarines, and invented ultra-sound, a discovery of profound importance delivered with astonishing ingenuity. His skill as an experimentalist amazed me, and I often met him, either in his lab in Paris or in Toulon, where I was posted by the Navy. Pure science, which we had nurtured with passion, was diverted from its course and turned into industrial science, war machines, bombs and counter-bombs. At last, peace returned. As soon as I was demobbed, I returned rapidly to my old notes... and I no longer understood a thing. It was an incredible effort to regain a foothold in this field, where everything had changed from top to bottom by the quantum theory. I was constantly supported by your father from that time on... ... He had a brilliant intelligence and an irresistible sweetness and charm. He was a great character and a sensitive man, extremely affectionate. All

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those who were close to him have a moving memory of him, and remember his touching gestures, so naturally tender. When I met him again, after the second war, fate had separated us for many years. We fell into each other’s arms, and in that simple embrace we felt the deep emotion that no words could express. He was my teacher, my scientific director and my dearest colleague. His great and noble figure remains before my eyes and his memory does not leave me.

In the tradition of d’Alembert176 by Marcel Tournier professor at ESPCI: head of the research division at ONERA177 This year our school celebrates its seventy-fifth anniversary (1957). Paul Langevin was a member of the seventh class. He died in 1946. For fifty-seven years, he lovingly placed his powerful intelligence and his keen sense of community life at the service of the continuing improvement of teaching at the school. Today, in 1957, in the course of a working day, a student who has recently entered the school, busy with his school activities, classes, laboratory, tests, can ignore Paul Langevin. Yet our present way of life is almost entirely organised as a result of his personal drive. The scientific trends and working methods are his. Those who, like me, knew him, saw him at work and entered his thoughts, think about him at every moment by finding him everywhere. In truth, his soul is present here. I’m trying to gather memories of my youth, after so many years. Pleased to have passed the competitive examination, it was with a passionate hope of discovering an unknown universe, populated by real scientists, that one entered the School in October. Alas, the reality was far from the dream. My new teachers were simply men. In the midst of them, dominating them all, this young man, Paul Langevin, already appeared as a great figure. He had made me take the physics exam for the entrance exam, and had already made a remark that had impressed me. Already I had felt that physics was much more than an examination syllabus with problems that are more or less solved, but instead a living reality, multiple, profound, always new, always incomplete, each day richer and more magnificent. In Paul Langevin’s deep black eyes, with his penetrating and kind gaze, I had already sensed the grandeur and beauty of an intense faith in the power of scientific knowledge. He had just been appointed director of studies when I was a student. One of the first people that he brought in was Paul Boucherot, who already had 176 Text published in 1958 in the journal Physique et chimie by the students of the École supérieure de Physique et Chimie. 177 * Office national d’études et de recherches aérospatiales.

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brilliant industrial achievements to his credit, precisely because of his scientific mind. He was not content with describing the machines known at the time, but he had the constant concern, as shown by the famous theorems which bear his name, to identify the general concepts that could serve as a guide to invention. Thanks to Langevin, too, Debierne had just entered as a lecturer. He was one of the scientists created by the teaching of the school. He was of great importance as director of research at the Radium Institute and contributed effectively as a teacher at the school to the scientific culture of a large number of our fellow students. Later on, mathematics teaching was completely changed. Famous mathematicians such as Élie Cartan and Lebesgue were recruited, to name only those who have passed away. Paul Langevin’s choice of all the members of the teaching staff was always guided by the desire to make our establishment the most efficient in its two specialities. Langevin gave precise and reliable instructions to each newcomer, which contributed to the homogeneity and value of this great system of work and discovery that was formed by the group of his former pupils. One of the dominant characteristics of his character was insight. When I sometimes came to him for advice on a job, I could be sure that long before the question was finished, his answer was ready. His erudition was enormous and his memory extraordinary. In my last year of study, I met him on Rue Lhomond. He asked me about my work in the laboratory. I was doing a project in organic chemistry on acetylacetic acid compounds. “Ah yes,” he said, “it’s all those strange acidic hydrogen bodies”. This answer was striking, coming from a physicist. A few years later, I came to talk to him about a rather thorny question of wave propagation in an anisotropic medium. “Have a look in Volume III of Lorentz’s works, which are in the library; I think you will find an answer you are looking for”. His knowledge was not limited to scientific knowledge; I believe he knew all the relatives of Balzac’s characters and their family names. Music, history, geography and philosophy, he had studied all these subjects in depth. Talking at a dinner party around 1922 with an economist: “You had made a mistake,” said Langevin, “at the time of the First World War. Didn’t you say that it couldn’t last more than a fortnight for financial reasons?” The economist replied: “It’s because we didn’t foresee that we could use fiat money”. “And yet,” replies Langevin, “it had already been used during the Punic Wars”. If he felt like broaching a difficult subject of physics in a private conversation, he sensed that the person he was talking to might be embarrassed by ignoring the question. So as not to put him in a state of inferiority, he always managed to give a reminder in one or two sentences of what should be known and avoid the embarrassment caused by ignorance.

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One day I was talking to him about chess: “You must certainly play very well”, I said. “No, I have hardly ever played, it is very disagreeable to me to enter into competition with a friend on such a trifling subject, and to show him that I am cleverer than he is”. On the other hand, during a trip to South America, he had been taught the game of matches. He had succeeded in establishing a very nice theory on this subject, which was based on base-two numeration and which, when applied, was sure to win. He was very amused by this work. I think it happened several times, when he was the first to have the idea of a famous invention, before its official author, but through modesty did not attach much importance to it. In 1909, I think, sometime after Fleming’s discovery of the diode as a detector of radio signals, I was building one in Debierne’s laboratory. Langevin passed by and asked me about the object I had in my hands. I described it to him with the little knowledge I had at that time: “If you put a third electrode between the other two,” said Langevin, “it would probably give you some interesting possibilities”. That was before Lee de Forest invented the grid. When, many years later, we had triodes in our hands, Langevin sometimes described their operation to visitors by comparing the operation of the grid to that of the drawer on a steam engine. During the First World War, I met a former pupil who has since become famous, Lucien Lévy. He told me a few years later that a few words spoken on that day had been at the origin of his discovery of the super-heterodyne, which was and still is of considerable importance. As soon as the theory of relativity was published, Langevin became enthusiastic about it and gave a luminous presentation at the Collège de France, which will long remain in the memory of those who had the good fortune to attend. Developing the consequences of the theory in dynamics, he came to discover the fundamental relation of the inertia of energy. He showed the calculations to one of his friends, who could not believe the extraordinary novelty of the result. This made Langevin hesitate, and he delayed the publication. It was not made until several months later by Albert Einstein. At the time of his imprisonment, a German officer who interrogated him accused him of having played a role in France comparable to that of the philosophers in the eighteenth century. “I had never thought,” said Paul Langevin, “of comparing myself to d’Alembert”. Yet it was the truth, Langevin was indeed from the lineage of Lagrange, Laplace, Carnot and Fresnel, who gave France its central place in the contribution to the progress of humanity.

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Let us be proud to belong to the school he led. Let us honour him often by remembering him in our hearts.

An elite intellect By Henri Le Boiteux Professor at l’École supérieure de physique et chimie I have always thought that if I were asked to concisely sum up the memory I have of my revered teacher, I would say, Paul Langevin was an “aristocrat” of the mind. I think that this expression alone can express the nobility of his thought and heart, the loftiness of his views, and show the extent of his genius, which was by no means limited to the field of science, where he was one of the greatest of his time. At the beginning of my career, I had the honour and pleasure of being one of his collaborators, after having been his student, one of those who called him le Patron with all that this term, too often forgotten nowadays, implies in terms of respect, admiration and the desire to work hard to come closer to what, humanly and scientifically, was for us an ideal. I have often observed, subsequently, that men belonging to different circles, who disagreed with him particularly in politics, paid him the same tribute and bowed to his nobility of thought as much as to his scientific genius. His kindness was obvious. I remember that when I was his assistant, a vigilant guard had to be organised around his office to prevent access by unscrupulous beggars who he did not have the courage to chase away. I experienced this kindness myself when, as a young engineer, I had to give up a research post that he had offered me at the Naval laboratory in Toulon, but which, alas, awaited the decision on a budget that was a year late, as was often the case in those days. The teacher was of the same quality as the scholar. The clarity of his presentation was matched by the depth of the views he presented. We had the impression that he was “thinking” his lecture in front of us and that he was leading his audience along the most hidden paths of his thought. Then everything was clear with an illumination that, alas, one does not always find easily when alone. I remember following his course at the Collège de France for a year. It was an extraordinary lecture on the equivalence between radiation and matter... I think that, if an argument were needed to defend what today is somewhat contemptuously called the lecture, the example of Paul Langevin alone would suffice.

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The influence of his personality is found to very clearly in most of his students. I remember that, when I was giving my first public lecture on a scientific subject, I was surprised, after the session, to find a stranger come up to me and say, “Sir, were you not a pupil of Paul Langevin?” And on my answering in the affirmative, he said, “I guessed it from your method of presentation”. Mimicry? No. But I had admired his method and recognised its value enough to deliberately use it. There is no detail in this respect that is not important. Thus the habit, which I have always maintained, of developing the calculations on the board, in perfect order, from the top left to the bottom right and so that, at the end of the course, the whole thing remains visible and perfectly ordered. Then, with the last relationship written up and the board completed, Langevin quietly left. I am reminded of another trait, which I think would be very useful to think about today. As students at the School of Physics and Chemistry at the time when relativity was beginning to make a big impact among physicists, with the enthusiasm of youth we had hoped that Paul Langevin, whose authority in this field was brilliantly asserting itself, would add lessons on this exciting novelty to his course. I was one of those who were sent by their comrades to ask him. To my comrade and friend Joliot and to myself, he replied that it was not desirable to introduce too early in the teaching process what was still in the process of evolving and that it would be preferable to assimilate perfectly the essential basics in order to be able to tackle the higher theories later on. This wise lesson I have retained and value. Paul Langevin was a man of high culture, what in past centuries would have been called a philosopher, and on all subjects his conversation was a delight to the mind. I remember that one day, when we were invited to dine with him at the home of his son, my old friend André Langevin, we had the joy of listening to him speak on a great variety of subjects. He continued as we left down the stairs and out onto the pavement, and we never tired of listening to him. When the time came to say goodbye, he addressed each of us. The street was poorly lit; we formed a small compact group, which was joined by a tramp who was passing by; he held out his hand in a gesture that was nothing less than disinterested. Confusing him with us, Paul Langevin held out his hand and said to him, as he does to everyone else: “Goodbye, my friend”. The other walked away, unaware that he had just shaken hands with one of the greatest minds of this century. When it was decided to transfer of the bodies of Paul Langevin and Jean Perrin to the Pantheon, a wake was organised at the Palais de la découverte. As I and his other students stood in emotional guard by his coffin, the Guard Band played one of Beethoven’s most grandiose works: the Funeral March from the Eroica Symphony. This tribute from the genius of music to

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the scientific genius and universal man that was my master has always had a symbolic value for me and I never hear this magnificent passage without reliving that moment when Paul Langevin entered into glory.

My eminent teacher Paul Langevin by Professor Georgi Nadjakov, member of the Sofia Academy of Sciences Not only the French people, but all of humanity bows with respect to the breadth and variety of Paul Langevin’s scientific work. First and foremost, he is admired by his many students, friends and colleagues throughout the world. I too had the good fortune to know him well, to work under his direction and to learn from his example when, more than forty years ago in 1925–26, I entered his laboratory to complete my scientific training, which today gives me the right to call myself his student. Paul Langevin’s work is extremely extensive and rich in its diversity. It is with good reason that he can be considered as a successor of the French encyclopaedists of the eighteenth century. In our time, in this twentieth century characterised by a spectacular development of all sciences and, in particular, of physics, – the latter, in a few decades, having given birth to so many new sciences, experimental, technical and theoretical, which came to be grafted onto it – in the century that is said to be the century of advanced specialisation, it is astonishing to recall that there was a scientist whose research activity covered so many different problems in all branches of physics. The importance of Paul Langevin for France and for the universe is immense. French science and world science had known a brilliant period linked to the names of a whole host of great French mathematicians, engineers and physicists of the end of the eighteenth and the first half of the nineteenth centuries, such as Laplace, Lagrange, d’Alembert, Poisson, Coulomb, Carnot, Fresnel and many others, who pushed mechanics in particular to a powerful position. After a relative pause in France during the second half of the nineteenth century, we see with Paul Langevin, at the beginning of the current century, a revival of these great French traditions that are reflected in simultaneous experimental and theoretical achievements. Paul Langevin is the founder of modern theoretical physics, of thorough experimental research and of the practical application of some of the remarkable scientific results he obtained. Louis de Broglie, one of the most eminent contemporary theoretical physicists, was his pupil who had worked with him before defending his famous thesis. Frédéric Joliot-Curie, one of the greatest experimentalists of our time, was also one of his pupils. Langevin had directed him towards atomic physics from the very beginning,

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with great foresight. Among his pupils, direct or indirect, one must rank almost all the current French physicists, as well as a considerable number of foreign physicists in the generations that followed. Albert Einstein the greatest physicist of our time, was among his closest and warmest friends, as was the academician Abram Fiodorovitch Ioffé, the father of Soviet physics. Added, as an appropriate complement, to the enormous scientific prestige that Paul Langevin enjoyed in the eyes of the world’s scientific opinion was a public activity that was no less important in the service of progress and social justice. Paul Langevin will always remain a great example in this respect. His speech at the last ceremony in his honour at the Sorbonne cannot be forgotten, when he said: “I had to divide my strength between the service of science and the service of justice, born as I was of a father who was a Republican to the depths of his soul and of a mother who was devoted to the self-sacrifice of this admirable Parisian people with whom I have always felt so deeply united. My father had inspired in me a thirst for knowledge; he and my mother, witnesses of the bloody repression of the Paris Commune, had inspired in me by their accounts an aversion to violence and a passionate inclination for social justice”. And, indeed, our eminent teacher followed simultaneously the two paths that life had laid out for him, with a marked predominance of scientific interests in his early years and a somewhat more pronounced public activity in his maturity and old age. I cannot forget the great confusion that I felt when, in September 1925, as a young physics assistant at the University of Sofia, I was sent abroad for scientific training and found myself at the door of the director’s office of the great scientist Paul Langevin, who had just taken up the post of Director of the School of Physics and Industrial Chemistry in Paris. My feelings were intense, since I was only at the beginning of my scientific activity and I was expecting to meet this great scientist. Finally, when I decided to knock on the door of his office, I was received very kindly and, to my great surprise, Paul Langevin enquired in great detail about my scientific interests, despite having little free time and innumerable preoccupations. Moreover, his desk was loaded with a pile of letters and various envelopes from all over the world. I was able to see this again a few days later when, entering one of the laboratories, I saw his personal assistant at the time, now Professor Pierre Biquard, sifting through the mass of international correspondence. At this first meeting I did not get a definite answer to my request to work in Paul Langevin’s laboratory for my scientific training. Instead I was advised to think seriously about it again, to order my scientific interests, to formulate them in writing in a short paper that I should present in three days. This I did, and when I presented myself for the second time to Langevin three days later, he took my paper, took a quick look at it, and said, “I will examine your paper and in three days you will have my

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final answer”. I don’t think I need to say how impatiently I waited for those three days to pass. Here I am again in front of his door; extremely anxiously, I raise my hand to knock. He greeted me with a smile and said: “I like what you have written and likewise the branch that you have chosen for your research. I will give you a corner of my laboratory.” This was the height of happiness for me! It was also the beginning of my real scientific activity. Immediately, Paul Langevin pressed a bell and told his secretary to call Professor Saphores, in charge of the course on electricity, as well as Dr. René Lucas, then head of the laboratory and now a professor, member of the Institute, who become director of the ESPCI after our master’s death. Both of them soon appeared. Langevin introduced me to them, mentioning that I was a young scientist who had come from far away, from little Bulgaria, to study with them. And, in my presence, he gave them directions as to the place that was to be reserved for me in his laboratory, adding that, after a conversation with me, every effort should be made to ensure that my little corner was equipped with all the necessary apparatus and materials. Indeed, all this was done in a very short time. The very next day, I already had all the materials on my bench that I needed to start my research, supplied by the Poulenc frères company. I was given access to all the apparatus in the electricity laboratory, and the necessary assistance to use certain apparatus in the neighbouring optics laboratory. In addition, a few days later, I also had at my disposal some brand new instruments and apparatus that had been ordered especially for me. André Langevin and another young scientist, R. Hocart, now professor of crystallography at the Sorbonne, both were admirable colleagues and companions, were also working in this laboratory which always remains engraved in my memory. An adjoining room served as the office of the editorial board of the Journal de physique. Paul Langevin was the scientific editor, although his place was taken by his other son, Jean. The editorial board received all the scientific journals of the time, encyclopaedias and many of the most recent physics books. I also had access to all this scientific literature, which, together with the resources of the library of the School of Physics and Chemistry, the library of the Sorbonne and the National Library, contributed greatly to my scientific education. This remarkable welcome and the extremely competent and skilful scientific guidance of my teacher – who had his own way of directing his students, including me, with a laconic question: “How’s the work going?” and, after receiving the necessary explanations: “Good, good, work, work!” – had the aim, as I thought then and as I understand better today, to train young researchers to think and work independently and freely. An example drawn from my own experience convinced me of this. During my research on the photoelectric effect of dielectrics, working with the Curie-Debierne quadrant electrometer, I had observed a very

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weak associated phenomenon, which had been noted for the first time by Röntgen and mentioned in his last work, partly carried out in collaboration with A. F. Ioffé, entitled: “On the photoconductivity of natural rock salt”. The phenomenon appeared as a steady deflection of the needle of the electrometer when it was isolated, regardless of whether it was being used for measurement or not. The explanation given by Röntgen for this phenomenon was that it was due to ionisation of the air in the electrometer itself, the origin of which is unknown. When I came to inform my teacher Paul Langevin of the observations I had made which to some degree disturbed my results, I saw that he was very interested in this phenomenon, perhaps because his first scientific work was in the field of Röntgen’s rays and the ionisation of gases. And since there was talk of possible ionisation from an unknown source, Paul Langevin wondered if it was not a question of an effect of radio-active waste left over from the time of Pierre and Marie Curie, whose hut, at the time of their first experiments, was located precisely where the buildings of the School of Physics and Chemistry are built today. He advised me to go to Madame Curie and ask her opinion, which I did. She, however, expressed doubts that this ionisation could be due to radioactive waste from the time of her first research. All this led me to look for another reason for this phenomenon. This scientific training in Paul Langevin’s laboratory, under his watchful eye and direct care, enabled me to choose my field of research well, and for this I am forever grateful to my teacher. This period of my life and a few other encounters with him later, in 1934  and in 1938, have left indelible marks on my life and my work. Lastly, I would like to recall a memory that is indirectly linked to the memory of Paul Langevin, and directly to the activity that Frédéric JoliotCurie and that I carried out within the World Peace Movement. At each of our meetings, during the congresses and sessions of the supporters of peace, we could not refrain from exchanging memories about our mutual teacher Paul Langevin. During the Congress of Peoples in Vienna in 1952, on behalf of other members of the Bulgarian delegation, I presented Frédéric JoliotCurie with the diploma of honorary member of the Bulgarian Academy of Sciences and a second diploma conferring on him the title of honorary doctor of the University of Sofia. In his reply to my speech of greeting, Joliot-Curie could not help but recall the one who had departed and say to me: “My dear Nadjakov, you remember, of course, the great intellect of our master, Paul Langevin”. Respectful homage and deep gratitude to the great scientist who was my dear master, Paul Langevin!

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A strong supporter of new ideas Memories of Assène Datzeff, Member of the Sofia Academy of Sciences From the end of 1934 to the beginning of 1939 I specialised in theoretical physics at the Institut Henri Poincaré in Paris, under the direction of Louis de Broglie. But in 1935, for the first and last time, I tried to be an experimenter. Thus, I had the opportunity to get to know Professor Paul Langevin more closely, to get closer to him and his world, and this left unforgettable memories with me. I knew Professor Langevin’s name well. I knew it was that of a famous physicist who had published important results on paramagnetism and on ultrasound, who had contributed to the development of special relativity and had publicised it and actively defended it. I had heard about his advanced ideas, his extensive progressive activity. For me he was a great figure, for whom one feels a deep respect. His characteristic and imposing face was known to me from his portrait, so it was not difficult for me to recognise him at the first opportunity without being introduced to him. I keep in my memory precise recollections like this one: Paul Langevin and Jean Perrin, already grey, especially the latter, coming from the Radium Institute, crossing the rue Saint-Jacques, arm in arm, absorbed in a lively conversation. I was struck by the friendship that bound Langevin’s collaborators and students together, among whom there were quite a few well-known names. They were known to be progressive, and some were committed anti-fascist activists. “This is the school spirit of Langevin”, my colleagues sometimes said. That’s why I was impressed when I saw Langevin surrounded by his students and other researchers, and when I sensed the high respect in which this intellectual elite held the “boss”. This very human atmosphere gave courage to many young people, especially those foreigners driven out of their own country. They felt like men here, while in Eastern Europe and in France’s neighbours the ‘new order’ was already in place. The first time I met Paul Langevin personally, it was to pass on Nadjakov’s greetings from Sofia. In the autumn of 1935 I applied to him to work in one of the laboratories of the School of Physics and Chemistry. I presented him with a report on the application of my idea to determine the speed of free electrons in metals. Here is what it was all about: you touch the periphery of a rotating metal disc with the edge of a thin bent metal blade, whose two arms are connected by wires to a differential galvanometer, then to a constant voltage source, and the whole circuit is closed by the disc itself. I expected that the mechanical speed transmitted by the rotating disc to the electrons coming out of it would create an asymmetry in the two currents. This was to be detected by the galvanometer, whose indication would depend on the speed of the electrons in the metal making up the disc. I had

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misjudged the magnitude of the expected effect, and it was only later that I appreciated this. Professor Langevin twice postponed the acceptance of my project in order to think about it further, and then he said to me: “I am not convinced that you will find the expected result, but the idea of the experiment is very simple and it is worth trying”. So, I was accepted at the school. I was provided with a room where I worked for about six months. The essential part of the apparatus was built in the school’s workshop. When the precision of the experiment was brought to its limit and several disturbing phenomena could not be eliminated, it became clear that the expected effect would not be detected, and so I stopped the experiment. In the course of my work I was helped by the staff of the laboratory, including the son of the boss, André Langevin, with whom we often had discussions. Langevin was lecturing at the Collège de France on electrodynamics and special relativity, which was apparently his favourite subject. When he worked out the curious consequences of relativity concerning the reciprocal deformations between two observers in two systems of inertia, there was inevitably a stir in the room, since it is not easy to remain without reacting to these surprising conclusions. In this regard, he once said during his lecture: “These things seem incomprehensible and strange to you, and in contradiction with common sense. But I will tell you that at the end of the last century, when we were learning Maxwell’s electromagnetic theory as students, it was not easy to understand. The notion of electromagnetic potential introduced by Maxwell and commonly used today seemed to us then just as unusual and difficult to grasp as the effects of special relativity are to you. So new concepts are difficult to get used to, but use makes them ordinary”. This idea made me think, although I was convinced (as I still am now) that the difficulties arising from special relativity are of a different nature and that it is not just a matter of habit. The audience was not large: ten or fifteen people, which was often the case for specialised courses at the Collège de France or the Sorbonne. Among them was the mathematician J.  Hadamard, whose wide-ranging interests also included theoretical physics, and who regularly took notes. In the physics seminar led by Langevin, work was presented from various branches of physics, and they were often very theoretical, sometimes I lost the thread, especially at the beginning. I admired the skill with which Langevin often intervened in the discussions and set the tone. Hadamard’s participation is also worth noting. As in his own seminar, with his own style, he followed the discussions, not shying away from asking questions when he did not understand the physics, but making decisive comments on the mathematical aspects. I remember Paul Langevin chairing meetings organised by the Rationalist Union at the Sorbonne or at the Salle de la rue du Four. He would open the meeting with a few words, intervene briefly during the discussion, but

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would maintain the direction. Once, in 1937 I think, he chaired a public lecture by Fournier at the Sorbonne on a delicate subject: an attempt to seek a physical explanation of the famous fine structure constant 1/137 by means of a certain geometrical model of the nucleus, a type of crystal structure. In his introduction, Langevin said that in this new and difficult field, bold attempts and new ideas should be encouraged. I don’t know whether the hypothesis in question was later supported anywhere, but at the time it seemed to me to be interesting and thought-provoking. In my heart I was glad that such ideas, even if only hypothetical, were allowed to be presented to a wide audience. I admired Langevin’s strong support for new ideas. We were young researchers of the Institut Henri Poincaré, and naturally knew that he had been on the panel for that famous de Broglie doctoral thesis in 1924 which marked the beginning of wave mechanics, and that he had quickly understood the new idea. He was quick to grasp the ideas that deserved attention, but he also could spot false ones. Once, in 1936, there was a meeting and discussion at the Salle de la rue du Four on some new problems of the nucleus, I think. Jacques Solomon had taken part in the very lively discussion, and his thorough analysis had made an impression. Paul Langevin had emphasised the importance of it, and he had drawn general conclusions. It was then that the floor was taken by a tall man, an engineer, who I had seen sometimes in the library of the Institut Henri Poincaré. He said that he could clear up the difficulties under discussion, and he began to present the basis of his theory, which was built on a certain complicated mechanical model of the ether from which elementary corpuscles, etc., could be built. The room became agitated. Paul Langevin then interrupted the speaker, kindly, telling him that his ideas were very interesting and that he could present them at a special meeting, but that here it was necessary to discuss them on the basis of the actual state of science. Without further ado he adjourned the meeting and the audience emptied the room, while the engineer continued to draw on the blackboard and feverishly expound his ‘theory’ to a pair of ironic onlookers. Since I was one of the ‘inside’ workers at the School of Physics and enjoyed the benevolence of the boss, I could go into his house through the back door when it was necessary to consult him. I found many visitors waiting in the corridor for their turn to be received: representatives of anti-fascist movements, defenders of the Spanish Republic and other people unknown to me. I had heard from many that they were received with understanding and cordiality, and that they found support. During my conversation with Langevin in his office, the phone rang frequently. He was being called from different places, from the editorial offices of various newspapers. His schedule at the school was very busy. But I knew that it was not only at the school that his life was under strain. Several times I went to the meetings organised by the Popular Front in the large hall of the Mutualité. Professor Paul Langevin had a place of

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honour on the podium. I listened to his speeches or interventions on different issues. They were generally not very long, but precise, meaningful and striking. I was impressed by the high regard in which he was held by the participants, most of whom were workers. Many times I saw Paul Langevin at the demonstrations organised by the Popular Front in 1937 and after. He was in the front row with the leaders. At that time, as a young physicist, I had the idea that there is perhaps no more honourable destiny for a scientist than that: to share the feelings of the masses and to be an active participant in a popular movement. One day I invited him to do his best to come to Bulgaria to give some lectures, assuring him that he would be warmly received by students, intellectuals and workers, and by a large section of our people. He smiled and told me that he had heard a lot about my country and that he would come with pleasure, but that it would not be possible for him at the moment, because he would have to pass through fascist countries and he would not have a visa. The Rome-Berlin axis was already blocking Europe from north to east. His visit to Bulgaria was postponed indefinitely. At the beginning of March 1939 I was back in Sofia. Shortly afterwards, the Second World War broke out. At its end, when news from France began to arrive again, one of the first pieces of information that provoked our feelings was about Professor Paul Langevin. We knew that during the German occupation he had actively participated in the Resistance and that he had been saved in time, to everyone’s relief. So, the 1872 republican’s son never betrayed his father’s ideals, and at the same time won the love of future generations. He left his name inscribed not only among those of eminent scholars, but also in the glorious cohort of defenders of justice and human rights for all.

Appendix Paul Langevin: Curriculum Vitae Born in Paris, 18th arrondissement, 24 January 1872. Died in Paris, 5th arrondissement, 19 December 1946. Buried in the Pantheon, Paris, 18 November 1948. 1884. Entered as a student at l’École Lavoisier 1888. Entered as a student at l’École de Physique et de Chimie. 1893. Entered as a student at l’École Normale Supérieure. 1897. Agrégé of physical sciences. 1897. City of Paris scholarship at the Cavendish Laboratory, Cambridge. 1898. Scholar of l’École Normale at the Faculty of Sciences of Paris. 1900. Laboratory assistant at the Faculty of Sciences in Paris. 1902. Replacement professor at the Collège de France. 1903. Substitute professor at the Collège de France. 1905. Professor at l’École de Physique et de Chimie. 1909. Full professor at the Collège de France. 1911 to 1927. Member of the first five Solvay Physics Congresses. 1920. Scientific director of the Journal de Physique. 1926. Director of l’École de Physique et de Chimie. 1928. Chairman of the Scientific Committee of the Solvay International Institute of Physics. 1930 to 1933. Chairman of the 6th and 7th Solvay Physics Conferences. 1934. Member of the Academy of Sciences of Paris. 1945. President of the French Education Reform Commission. Doctor Honoris causa from the Universities of Manchester, Leeds, Bristol, Cambridge, Brussels and Liège. Honorary Professor at the University of Buenos Aires. Honorary Member of the Faculty of Sciences of Santiago, Chile. Member of the Royal Society and the Royal Institution of London. Honorary Member of the Academy of Sciences of the U.S.S.R. Member of the Royal Society of Sciences of Göttingen, of the Academy of Lincei in Rome, of the Academie de Marine, of the Academies of Sciences of Prague, Bologna, Buenos-Aires, Copenhagen, of the Royal Academy of Ireland. Grand-Croix de la Légion d’Honneur, Commander of the British Empire.

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Publications I. Gas ionization 1900. Sur l’ionisation des gaz. Bull. Soc. Fr. Phys., 3, 39. 1900. Les ions dans les gaz. Bull. Soc. Int. Elect., 17, 203. 1902. Recherches sur les gaz ionises. C. R. Acad. Sci. Paris, 134, 414-417. 1902. Sur la recombinaison des ions dans les gaz. C. R. Acad. Sci. Paris, 134, 533-536. 1902. Sur la mobilité des ions dans les gaz. C. R. Acad. Sci. Paris, 134, 646-649. 1903. Recherches sur les gaz ionisés. Ann. Chim. Phys., 28, 289, 233. 1903. Note sur les rayons secondaires des rayons de Roentgen. Ann. Chim. Phys., 28, 500. 1903. Sur la loi de recombinaison des ions. C. R. Acad. Sci. Paris, 137, 177-179. 1904. Sur la conductibilité des gaz issus d’une flamme (with M. E. Bloch). C. R. Acad. Sci. Paris, 139, 792-794. 1904. Recherches récentes sur le mecanisme du courant électrique. Ions et électrons. Éclair. Élect. 45, 361, 401; Bull. Soc. Int. Élect. (Ser 2) 5, 615 (1905). 1905. Recherches récentes sur la decharge disruptive. Bull. Soc. Fr. Phys. 4, 25. 1905. Recombinaison et diffusion des ions gazeux. J. Phys. Theor. Appl. 4, 322-334. 1905. Sur les ions de l’atmosphère. C. R. Acad. Sci. Paris, 140, 232-234. 1905. Sur un enregistreur des ions de l’atmosphère (with Moulin). C. R. Acad. Sci. Paris, 140, 305-307. 1905. Remarques à propos de la communication de M. E. Bloch. Bull. Phys. 4, 84. 1905. Interprétation de divers phénomènes par la présence de gros ions dans l’atmosphère. Bull. Soc. Fr. Phys. 4, 79. 1906. Recherches recentes sur le mécanisme de la décharge disruptive. Bull. Soc. Int. Élect. (Ser 2), 6, 69; Radium, Paris, 3, 107. 1907. Electromètre enregistreur des ions de l’atmosphère (with Moulin). Radium, Paris, 4, 218-230. 1908. Sur la recombinaison des ions dans les diélectriques. C. R. Acad. Sci. Paris, 146, 1011-1017. 1913. Mesure de la valence des ions dans les gaz. Radium, Paris, 10, 113-119. 1949. Sur un analyseur de mobilité pour les ions gazeux. J. Phys. Radium, 10, 177, 225, 257.

II. Kinetic and thermodynamic theory 1905. Une formule fondamentale de théorie cinétique. Ann. Chim. Phys., 5, 245. C. R. Acad. Sci. Paris, 140, 35-38. 1908. Sur la theorie du mouvement brownien. C. R. Acad. Sci. Paris, 146, 530-534. 1913. Sur les chocs exceptionnels des molecules gazeuses. Radium, Paris, 10, 142-146.

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1913. La physique du discontinu. (Presented at the Société française de Physique), Les Progres de la Physique Paris: Gauthier-Villars (1914) and La Physique depuis vingt ans. Paris: O. Doin (1923), 189-264. 1914. Thermodynamique et statistique. Rev. du mois. 97, 29-38.

III. Electromagnetic theory and electrons 1904. La Physique des électrons. (Report of the International Congress of Sciences and Arts, St. Louis.) Rev. Gén. Sci. Pur. Appl. (1905); La Physique depuis vingt ans. Paris: O. Doin (1923) 1-69. 1905. Sur l’origine des radiations et l’inertie électromagnetique. J. Phys. appl. 4, 165. 1910. La théorie electromagnetique et le bleu du ciel. Bull. Soc. Fr. Phys. 4, 80. 1912. Les grains d’électricité et la dynamique électromagnétique. (Presented at the Société française de Physique), published in Les idées modernes sur la constitution de la matière. Paris: Gauthier-Villars (1913, p. 54), and in La Physique depuis vingt ans. Paris: O. Doin (1923) 70-170. 1913. La théorie du rayonnement et les quanta (with M. de Broglie). Rapports preséntés au premier Conseil Solvay, Bruxelles, 1911. Paris: Gauthier-Villars. 1934. L’électron positif. Bull. Soc. Int. Électriciens, 4, 335-279. 1935. Remarques au sujet de la note de M. Prunier “Sur une expérience de Sagnac qui sera faite avec un flux d’électrons”. C. R. Acad. Sci. Paris, 200, 48-51.

IV. Magnetism and molecular orientation 1905. Sur la théorie du magnétisme. C. R. Acad. Sci. Paris, 139, 1204; Bull. Soc. Fr. Phys. 4. 13; Rev. Gén. Sci. Pur. Appl. (1905). 1905. Sur la théorie du magnétisme. J. Phys. Théor. Appl. 4, 678. 1905. Magnétisme et théorie des électrons. Ann. Chim. Phys., 5, 70. 1910. Sur les biréfringences électrique et magnétique. Radium, Paris, 7, 249-261. C. R. Acad. Sci. Paris, 151, 475-478. 1911. La Theorie cinétique du magnétisme et les magnétons. (Report from the Solvay Council) in La Theorie du rayonnement et les quanta. Paris: Gauthier-Villars (1912), and La Physique depuis vingt ans. Paris: O. Doin (1923) 171-188. 1912. Sur l’orientation moléculaire. Letter to W. Voigt, Nachr. Ges. Wiss. Göttingen, 5, 589. 1919. Remarques à  propos de la communication de M.  Bauer. P.  V.  Soc. Franç. Phys. 18.

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V. Relativity 1905. Sur l’impossibilité physique de mettre en évidence le mouvement de la translation de la terre. C. R. Acad. Sci. Paris, 140, 1171-1173. 1911. L’évolution de l’espace et du temps (Presented at the Congrès de Philosophie de Bologne), Scientia Bologna, 10, 31-54; and La Physique depuis vingt ans. Paris: O. Doin (1923) 265-300. 1912. Le temps, l’espace et la causalité en Physique moderne. Bull. Soc. Fr. Philos.12, 1-46; La Physique depuis vingt ans. Paris: O. Doin (1923) 345-405. 1913. L’inertie de l’énergie et ses conséquences. J. Phys. Théor. Appl. 3, 553592. La Physique depuis vingt ans. Paris: O. Doin (1923) 301-344. 1920. Les aspects successifs du principe de relativité. Bull. Soc. Fr. Phys. 138, 5. La Physique depuis vingt ans. Paris: O. Doin (1923) 406-423. 1921. Sur la théorie de la relativité et l’experience de M. Sagnac. C. R. Acad. Sci. Paris, 173, 831-834. 1922. Le principe de la relativité. Bibliothèque de Synthèse scientifique (Ed. E Chiron). Paris. 1922. L’aspect general de la théorie de la relativité. Bull. Scient. Étudiants de Paris. 2, 2-22. 1929. La structure des atomes et la chaleur solaire. Bull. Univ. Tiflis, 10, 1931. L’oeuvre d’Einstein et l’astronomie. Bull. Soc. Astronomie Fr., 45, 277-297. 1931. Déduction simplifiée du facteur de Thomas, dans Sommerfeld, Vereinfachte Ableitung des Thomas Faktor (Convegno di fisica nucleare), Rome, p. 137. 1932. La relativité, Exposés et discussions du Centre de Synthèse. Paris: Hermann. 1936. Espace et temps dans un universe Euclidian. Livre jubilaire de Marcel Brillouin. Paris: Gauthiers-Villars. 18-26. 1937. Sur l’expérience de M. Sagnac. C. R. Acad. Sci. Paris, 205, 304-306. 1938. La relativité. Les Cahiers rationalistes, 7, 103.

VI. Chemical physics 1911. Remarques au sujet des communications de M. Fouard. P. V. Soc. Fr. Phys. (1 December). p. 84. 1912. Sur la comparaison des molécules gazeuses et dissoutes. C. R. Acad. Sci. Paris, 154, 594-596. 1912. L’interprétation cinétique de la pression osmotique. J.  Chim. Phys. 10, 524 and 527. 1913. Remarques au sujet de la communication de M. Wertenstein. P. V. Soc. Fr. Phys. (7 March). p. 34. 1913. Remarques au sujet des communications de M. Fouard. P. V. Soc. Fr. Phys. (4 April). p. 42 1934. Sur un problème d’activation par diffusion. J. Phys. Radium, 5, 57-60.

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VII. Quantities and units 1905. Ions, electrons, corpuscules; les quantités élémentaires d’électricité (with H. Abraham). Recueil de Mémoires publié par la Société française de Physique. Paris: Gauthier-Villars. 1912. Notions géométriques fondamentales. Encycl. Sci. math. 4, 5(1), p. 1. 1921. Sur les grandeurs champ et induction. Bull. Soc. Fr. Phys. 162, 3. 1922. Sur la nature des grandeurs et le choix d’un système d’unités électriques. Bull. Soc. Fr. Phys. 164, 9.

VIII. Classical and new mechanics 1921. Sur la dynamique de la relativité. P. V. Soc. Fr. Phys. and Exposés et Discussions du Centre de Synthèse sur la Relativité. Paris: Herman (1932). 1928. Les nouvelles méécaniques et la chimie (transcribed by H. Granjouan). Réunion internationale de Chimie-Physique. Paris: Presses universitaires, 550-569. 1933. La notion de corpuscules et d’atomes. Presentation at La  réunion internationale de Chimie-Physique. Paris: Herman. (1934)

IX. Acoustics, ultrasound and piezoelectricity 1916. Sur la production des étincelles musicales par courant continu. Ann. des Post. Télégr. Téléph. 4, 404. 1916. Procédés et appareils pour la production de signaux sous-marins dirigés et pour la localisation d’obstacles sous-marins (with Constantin Chilowsky). French patent 502,913. 29 May 1916. 1918. Procédés et appareils d’emission et de réception des ondes élastiques sous-marins à l’aide des propriétés piézo-électriques du quartz. French patent 505,703. 17 Sept 1918. 1918. Note sur l’enérgie auditive. Publication du Centre d’Études de Toulon. 25 Sept 1918. 1923. Emission d’un faisceau d’ondes ultra-sonores (with C. Chilowsky and M. Tournier). J. Phys. Appl. 4, 537. 1923. Procédés et appareils pour le sondage et la localisation en distance d’obstacles sous-marins, au moyen d’échos ultra-sonores (with Charles Louis Florisson). French patent 575,435. 27 Dec 1923. 1923. Utilisation des phénomènes piézo-électriques pour la mesure de l’intensité des sons en valeur absolue (with M. Ishimoto). J. Phys. Radium 4, 539. 1924. Procédé et appareils permettant la mesure direct ou l’enregistrement des profondeurs ou des distance en mer par la méthode de l’écho ultra-sonore. French patent 576,281: 14 January: additions 1 March 1924 and 16 October 1924.

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1924. Procédé et appareils pour la mesure par lecture directe de la distance d’un obstacle dans l’air. French patent 577,055. 11 Feb. 1924. 1924. Sondage et détection sous-marine par les ultra-sons. Bull. Ass. Tech. Maritime. 28, 407; Rev. hydrographique du Bureau Int. de Monaco, 1(2), 9; 1925 Rech. et Invent. 113, 441. 1926. Procédé et disposition améliorant l’efficacité des projecteurs ultra-sonores piézoèlectriques. French patent 662,035, 27 January 1926. 1926. Sondeur ultra-sonore. Rech. et Invent. 132, 119. 1927. Le phare ultra-sonore de Calais. Revue maritime, 88, 481-492. 1927. A  propos des bruits parasites ultra-sonores. Rev.  hydrographique du Bureau Int. de Monaco, 4, 161. 1927. Banc piézo-électrique pour l’equilibrage des rotors. French patent 659,871. 22 December 1927. 1928. La production et l’utilisation des ondes ultra-sonores. Presentation at la Société des Ingenieurs civils (9 March). P. V. Soc. Ing. Civ. 5, 119; Rev. Gen. Électr. 23, 626-634. 1929. Sur le mirage ultra-sonore. Bull. Ass. Tech. Maritime. 727-733; On the ultra-sonic mirage Rev. hydrographique du Bureau Int. Monaco (1931), 8(1), 140-143. 1929. Les vibrations ultra-sonores et leurs applications. Bull. Univ. Tiflis, 22, 10. 1931. La directivité en acoustique sous-marine. Bull. Ass. Tech. Marit. 37-47. 1932. Les ondes ultra-sonores (notes of P. Biquard). Rev. Acoust. 1, 93-109, 315-335. 1933. Les ondes ultra-sonores (notes of P. Biquard). Rev. Acoust. 2, 288-299. 1934. Les ondes ultra-sonores (notes of P. Biquard). Rev. Acoust. 3, 104-132. 1935. Sur les lois du dégagement d’électricité par torsion dans les corps piézo-électriques (with J. Solomon). C. R. Acad. Sci. Paris, 200, 1257-1259.

X. Other technical publications 1920. Utilisation de la détente pour la production de courants d’air de grande vitesse. Bull. Soc. Fr. Phys. 139, 7. 1922. Note sur la loi de résistance de l’air et sur la correction d’élasticité proposée par M. le capitaine Darrieus. Mémor. Artill. Fr. 2, 253. 1923. Note sur les effets balistiques de la détente des gaz de la poudre dans une tuyère convergente-divergente. Memor. Artill. Fr. 3, 1. 1927. Procédé et dispositif pour la mesure des variations de pression dans les canalizations d’eau ou autre liquide. French patent 639,151 6 Aug 1927 (with R. Hocart). 1927. Procédés et appareils permettant la mesure de la puissance transmise par un arbre. French patent 659,658. 19 December 1927. 1927. L’enregistrement des coups de bélier. Bull. Tech. Chamb. Synd. des Entrepreneurs de Couverture-Plomberie, 23, 81.

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XI. Nuclear physics, gravitation 1942. Sur les chocs entre neutrons rapides et noyaux de masse quelconque. C. R. Acad. Sci. Paris, 214, 517-522; Ann. Phys. Paris, 17, 303. 1942. Sur les chocs entre neutrons et noyaux. C. R. Acad. Sci. Paris, 214, 867-869. 1942. Sur le ralentissement des neutrons. C. R. Acad. Sci. Paris, 214, 889-891. 1942. Resonance et forces de gravitation. Ann. Phys. Paris, 17, 265.

XII. Teaching and education 1904. L’esprit de l’enseignement scientifique, Talk given at the Musée pedagogique. L’enseignement des sciences mathématiques et des sciences physiques. Paris: Imprimerie Nationale. La Physique depuis vingt ans. Paris: O. Doin (1923) 424-453. 1911. Exposé expérimental des phénomènes fondamentaux de l’électrostatique au moyen de l’électrometre à quadrants. Talk given at the Société française de Physique. Notes by M. J. Villey, J. Phys. Théor. Appl. (Ser 5) 1, 460. 1920. Le théorème de Fermat et la loi du minimum de temps en optique géométrique. J. Phys. Théor. Appl. (Ser. 6) 1, 188. 1926. La valeur éducative de l’histoire des Sciences. Bull. Soc. Fr. Pédagogie, 22; 692-700. Rev. Synthèse, 6, 5 (1933). 1931. La contribution des sciences physiques a la culture generate. Bull. Soc. Fr. Pédagogie, 41. 1931. Science et laïcite. Groupe fraternel de l’esseignement. Paris: Deshayes. 7-28. 1932. The reorganisation of education in China. (with C.  H.  Becker, M. Falski and R. H. Tawney). League of Nations’ Institute of Intellectual Co-operation. Paris. 1932. Le problème de la culture générale. Opening discussion of Congrès international d’Education nouvelle. Nice. Full report of the new Education Fellowship, 73. London (1933). 1932. Discours d’ouverture du 6th  Congrès mondial d’éducation nouvelle. Pour l’ère nouvelle, 80. 1932. Le problème de la culture générale. Pour l’ère nouvelle, 81, 239-245. 1933. L’Enseignement en Chine. Bull. Soc. Fr. Pédagogie, 49. 1945. Culture et Humanité: Discours d’ouverture des travaux de la Commission de réforme de l’enseignement. Bull. Officiel du Ministère de l’Education nationale. 1946. Discours prononcé au Congrès. Bull de l’Ass. pour la paix par l’éducation. 1946. La reforme de l’enseignement et ses rapports avec l’éducation nouvelle. Pour l’ère nouvelle.

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XIII. Sundry other publications 1906. Pierre Curie. Revue du Mois. 2, 5-36. 1910. E. Mascart. Annuaire du Collège de France. 1911. Victor Regnault. Revue du Mois. 11, 129. 1914. Henri Poincaré, le physicien. Henri Poincaré. Paris: F. Alcan. Revue de métaphysique et de morale, 1913, 21, 665-718. 1923. La Physique depuis vingt ans. Paris: O. Doin. 1927. Les étapes de la pensée scientifique. Opening discussion at the Congrés de l’Association française pour l’avancement des sciences. Constantine. 51st session, p. 23. Paris: Masson. 1929. Paul Schutzenberger. Discours prononcé à l’occasion du centenaire de P. Schutzenberger. 1930. L’orientation actuelle de la Physique. Talk given at l’École normale supérieure. L’orientation actuelle des sciences. Paris: Alcan. 29-62. 1932. La Physique au Collège de France. Talk given at the 40th anniversary of the Collège de France. Volume du Centenaire, 61-79. 1932. Ernest Solvay. Homage national á Ernest Solvay, 16 October, Bruxelles. l’Institut International Solvay. 1932. La science et la paix. Cahiers des droits de l’homme, 28, 651-655. 1933. Paul Painlevé, le savant. Les Cahiers rationalistes, 2, 230-243. 1933. La valeur humaine de la Science. Preface to L’Evolution humaine. Paris: Quillet. xi-xv. Cahiers Rationalistes, 8, 35-50 (1940). 1933. L’évolution de la science électrique depuis cinquante ans. Presentation at the 50th anniversary of the Société Française des Électriciens à Paris. 1934. La jeunesse devant le fascism. Paris: Comité de vigilance des intellectuels antifascists. 1937. Fascism et civilisation, Clarté, 7, 51-56. 1938. Defense de la paix et de la liberte. Clarté, 23, 827-30. 1938. Fidélité au serment. Paix et Liberté Jun 1938, 11-15. 1939. La Science comme facteur d’evolution morale et sociale. Cahiers Rationalistes, 75, 114-137. 1939 Hommage à G. Urbain. La Pensée, 1, 89-91. 1943. Les Actions mutuelles dans le monde physique. Comptes rendus 10th semaine Internationale de Synthèse 7- 10 June 1938. Paris: Presses Universitaires. 1944. Statistique et Déterminisme. Compte rendu 7th semaine International de Synthèse 3-8 juin 1935. Paris: Presses Universitaires. 1946. La Pensée et l’Action. Paris: l’Union Française Universitaire. 1950. Extracts from Paul Langevin’s philosophical and educational writings, assembled by P. Laberenne, in La Pensée et l’Action. Paris: Editeurs français reunis.