(b. Paris, France, 23 January 1872; d. Paris, 19 December 1946)
Langevin, the second son of Victor Langevin, an appraiser-verifier in the Montmartre section of Paris, very early displayed his liking for study. His mother, great-grandniece of the alienist Philippe Pinel, encouraged this inclination; and Langevin was always first in his class from the time he entered the École Lavoisier until he left the École Municipale de Physique et Chimie Industrielles de la Ville de Paris in 1891. (The latter school was established in 1881 by Paul Schützenberger to train engineers.) Langevin’s enthusiasm was aroused by his contact with the school’s director and by his laboratory work, which was supervised by Pierre Curie.
To further his knowledge Langevin attended the Sorbonne (1891-1893) while teaching a private course and learning Latin on his own. In 1893 he placed first in the competitive entrance examination for the École Normale Superérieure, but he did a year of military service before attending the school. At the École Normale Supéerieure he heard the lectures of Marcel Brillouin and undertook research with Jean Perrin (then an agrégé-préparateur). Langevin placed first in the competition for the agrégation in physical sciences in 1897 and left for Cambridge to spend a year at the Cavendish Laboratory with J. J. Thomson. Under Thomson’s direction, he worked on ionization by X rays, in the process discovering, independently of Sagnac, that X rays liberate secondary electrons from metals. Also while at the Cavendish he met J. Townsend, E. Rutherford, and C. T. R. Wilson: all of them soon became friends.
Upon returning to Paris, Langevin established a home (1898). He had four children: Jean (b. 1899), André(b. 1901)—both of whom became physicists—Madeleine (b. 1903), and Hélè (b. 1909). Still on scholarship, he was obliged to continue to give private lessons.
During this period the atmosphere in the Paris laboratories was one of intense excitement. At Jean Perrins’s laboratory Langevin continued his investigations of the secondary effects of X rays and, on close terms with the Curies, he was present at the birth of the study of radioactivity. Langevin completed his doctoral dissertation in 1902 at the Sorbonne. It dealth with ionized gases and was based on investigations he had begun at Cambridge. After being named Préparateur to Edmond Bouty at the Sorbonne, Langevin entered the Collége de France in 1902 to substitute for E. E. N. Mascart, whom he replaced in 1909. Meanwhile, he thought also at the École Municipale de Physique et Chimie, succeeding Pierre Curie (1904), and then at the école Nationale Supérieure de Jeunes filles (Sévres), replacing Marie Curie, who had been widowed (1906). Langevin loved teaching, and he excelled at it.
In his laboratory at the Collège de France, Langevin continued to study ions in gases, liquids, and dielectrics (1902-1913). In this work he was assisted by his students, including Edmond Bauer, Eugene Bloch, and Marcel Moulin. In his dissertation he had already given a method of calculating the mobility of both positive and negative ions during their passage through a condenser by considering their diffusion and recombination. Moreover, for the first time he communicated his results concerning secondary X rays (1898). Langevin was never in a hurry to publish; his written work is scanty in relation to the extent of his work. Whether it was a question of theory, of experimental results, or even of techniques or apparatus, he spent a long time seeking a simple and clear statement; often his publisher would snatch from him a manuscript filled with changes written in his clear, firm hand.
Langevin’s position at the Collège de France was of particular importance to his development, for it freed him to lecture on subjects for which the standard French curriculum had little place. Although he continued after his arrival there to involve himself deeply with the experimental work of students, his own research and teaching turned increasingly to contemporary problems in theoretical physics. For most of the thirty years after he assumed the chair, he was the leading, and at times virtually the only, practitioner and expositor of modern mathematical physics in France. Einstein caught his role and status precisely when he wrote:
Langevin’s scientific thought displayed an extraordinary clarity and vivacity combined with a quick and sure intuition for the essential point. Because of those qualities, his courses exerted a decisive influence on more than a generation of French theoretical physicists. … It seems to me certain that he would have developed the special theory of relativity if that had not been done elsewhere, for he had clearly recognized its essential points.1
The last portion of that evaluation is in part a response to Langevin’s first published theoretical papers, presented during 1904 and 1905. They dealt perceptively and authoritatively with a coherent set of current problems developed in the work of Lorentz, Larmor, and Abraham: the concept of electromagnetic mass, its rate of increase with velocity, and the related contraction hypotheses which suggested the impossibility in principle of determining the earth’s motion through the ether. Both reports from his students and the speed with he assimilated the special theory of relativity after 1905 suggest in addition that Langevin’s own thoughts, at least on the relation between mass and energy, were developing along lines close to Einstein’s before the latter’s work appeared in 1905.2
That same year is the one in which Langevin published what was perhaps his most original and enduring contribution to physical theory, a quantitative account of paramagnetism and diamagnetism which demonstrated, he said, that it was “possible, using the electron hypothesis, to give precise meaning to the ideas [molecular models] of Ampère and Weber.”3
To account for paramagnetism Langevin assumed that each molecule had a permanent magnetic moment m due to the circulation of one or more electrons. In the absence of an external field, thermal motion would orient the moments of individual molecules at random, so that there would be no net field. An external field, however, would tend to align molecular moments, the extent of the alignment depending both on the field strength and on the intensity of the thermal motion, the latter determined by temperature. Applying Boltzmann’s techniques to the problem, Langevin showed that for low fields the magnetic permeability of a gas should be given by μ = m2N/3kT, where N is the number of molecules per unit volume, K is Boltzmann’s constant, and T is the absolute temperature.
The proportionality of susceptibility to the reciprocal of temperature was, Langevin emphasized, a result which Pierre Curie had found experimentally in 1895. Using the latter’s measurement of the proportionality constant, he noted further that the observed susceptibility of oxygen could be accounted for by the orbital motion of even a single electron with velocity 2 • 108 cm/sec. At this time atoms were usually thought to consist of many hundreds of electrons. That so few were needed to explain magnetic properties suggested to Langevin that the electrons with this function might well be the superficial ones, i.e, the valence electrons, which were responsible also for chemical properties. Niels Bohr, whose route to the quantized version of Rutherford’s atomic model was deeply influenced by Langevin, was to suggest precisely the same correlation.4
The even more puzzling phenomenon of diamagnetism Langevin explained in terms of molecules within which the orbital electronic motions canceled each other, so that no net molecular moment remained. An increasing magnetic field would, however, accelerate the electrons moving in one direction and retard those moving in the other, thus producing a small net moment in a direction opposed to the field. Again Langevin’s treatment was quantitative. It predicted that, as Curie had found, diamagnetic susceptibility should be independent of temperature, and it permitted computation of plausible values for the radii of electronic orbits. As a tool for investigating both magnetism and molecular and atomic structure, Langevin’s impressive theory was vigorously developed by a number of physicists, especially Pierre Weiss, and Langevin was himself invited to discuss its current state at the famous first Solvay Congress in 1911.
Three years before that, in 1908, Langevin, whose skill in kinetic theory had first been developed when he worked on ionic transport, turned briefly to the theory of Brownian motion developed by Einstein in 1905 and, via a more direct route, by Smoluchowski in 1906. The result was simplified, still-standard treatment which, unlike Smoluchowski’s, produced precisely Einstein’s formula for the mean-square displacement. Subsequently Langevin took up the subject of thermodynamics and reconsidered its basic notions, starting from theories of Boltzmann and of Planck (“the physics of the discontinuous”) in 1913. At the same time he presented “the notions of time, space, and causality” with their relativistic significance. It required much difficult work to arrive at these elucidations; but he presented them simply, sometimes humorously (for example, Jules Verne’s cannonball  and Langevin’s rocket or cannonball ).
Although Langevin concerned himself with philosophical questions, he did not neglect the technical applications of his work. In 1914 he wa called upon to work on ballistic problems and was later requested by Maurice de Broglie, his reiend and former student, to find a way of detecting submerged enemy submarines. Lord Rayleigh and O. W. Richardson (1912) had thought of employing ultrasonic waves. In France a Russian engineer, Chilowski, proposed to the navy a device based on this principle; but its intensity was much too weak. In less than three years Langevin succeeded in providing adequate amplification by means of piezoelectricity. His team called the steel-quartz-steel triplet he developed a “Langevin sandwich.” Functioning by resonance, it “finally played for ultrasonic waves the same roles as the antenna in radio engineering.” Langevin continued to do important work in acoustics and ultrasonics after the war.
Langevin received many honors. In 1915 he was honored by the Royal Society of London, and in 1928 he became a member of that body (he had still not been elected to the Académie des Sciences). He was elected to many other foreign academies and to the Académie de Marine in Paris. His relativistic views still appeared revolutionary: he had invited Michelson and Einstein to speak at the Collège de France as early as 1922. Then came the theories of Louis de Brogile. Langevin, at first surprised, soon became their strongest advocate (1924). The Académie des Sciences elected Louis de Broglie in 1933 and Langevin in 1934. In writing his “Notice,” Langevin relived forty years during which he had constantly contributed to deepening our understanding of the universe.
Internationally Langevin’s influene became paramount in 1928, when he succeeded H. A.Lorentz as president of the Solvay International Physics Institute, of which he had been a member since 1921. On his initiative a message of sympathy was sent in 1933 to Einstein, who was already being persecuted by the Nazis.
As passionate in his concern for justice as in his quest for truth, in this period Langevin joined various movements supporting victims of fascism and, denouncing the horrors of war, actively participated in campaigns aimed at securing peace. To his mind the same enemy—reaction—opposed the new seientific theories, the modernization of teaching, individual liberty, and the spirit of brotherhood.
When war broke out, Langevin testified in favor of the forty-four Communist deputies excluded from their seats following the signing of the German-Soviet pact. In March-April 1940 he was invited by the navy to direct research on ultrasonic depth finders.
After the departure of the French government from Paris, Langevin again became director of the École Municipal de Physique et Chimie Industrielles (the duties of that office having already been delegated to him); but on 30 October 1940 he was arrested by the Wehrmacht. Dismissed by the Vichy government and imprisoned in Fresnes, he ws finally placed under house arrest in Troyes. Messages of sympathy came to him from all over the world. Peter Kapitza invited Langevin to join him in the Soviet Union, but Langevin refused to leave France. Resigned, and surrounded by devoted friends, he resumed his calculations. He then learned of the execution of his son-in-law, the physicist Jacques Solomon (1942), and of the arrest and deportation of his daughter, Hélène Solomon-Langevin (1943). Fearing for his safety, his young friends Frédéric Joliot, H. Moureu, Denivelle, and P. Biquard persuaded Langevin to flee in May 1944. Warmly welcomed in Switzerland, he worked on educational reform for postwar France.
As early as 1904 Langevin had denounced the obstacles to progress found in scientific instruction in the form of “ossifying dogmas” that hinder the recognition of “fruitful principles”—such as rational mechanics vis-à-vis he atomic theory. Faithful to his own thought, the reprinted this article (“L’esprit de L’enseignement scientifique”) in 1923. He took up the same theme again in the hope that liberated France would direct its youth along progressive paths in thought and action, and inculcate in them the notions indispensable to philosophers and technicians alike.
After returning to the École Municipale de Physique et Chimie Industrielles in October 1944, Langevin devoted his greatest efforts to educational reforms and to the support of his political friends. His daughter Hélène, returned from Auschwitz, sat in the Assemblée Consultative. He joined her as a member of the Communist Party—several members of which were also members of the government—in the hope of encouraging a brotherhood that capitalism had not succeeded in establishing.
Langevin died following a brief illness. The govenment, which had made a grand officer of the Legion of Honor, accorded him a state funeral. his remains were transferred to the Pantheon in 1948 at the same time as those of Jean Perrin.
1. A. Einsterin, “Paul Langevin,”s in La Pensée,12 (May-June 1947), pp. 13-14.
2. E. Bauer, L’électromagnétisme hier et aujourd’hui (Paris, 1949), p. 156 n.
3. P. Langevin, “Magnétisme et théeorie des électrons” in Annales de chimie et de physique,5 (1905), 70-127; quotion from end of introduction.
4. Cf. John L. Heilbron and Thomas S. Kuhn, “The Genesis of the Bohr Atom,” in Historical Studies in the Physical Scienes,1 (1969), 211-290.
1. Original Works. Langevom’s writings on X rays and ionization of gas include “Recherches sure les gaz ionisés,” his doctoral dissertation (1902), in annales de chimie et de physique,28 (1903), 289, 433; “Sur les rayons secondaries des rayons de Röntgen,” his doctoral dissertation (1902), ibid., p. 500; “Recharches récentes sur le mécanisme du courant électrique. Ions et électrons,”s in Bulletin de la Société internationale des électriciens, 2nd ser., 5 (1905), 615; “Recherches récentes sure le mécanisme de la décharge disruptive,”s ibid.,6 (1906), 69; “sur la recombinaison des ions dans les diélectriques,” in Comptes rendus … de l“Académie des scienes,146 (1908), 1011; “Mesure de la valence des ions dans les gaz,” in Radium,10 (1913), 113; and “Sur la recombinaison des ions,” in Journal de physique, 8th ser., 6 (1945), 1.
In the following citaitons Physuqye refers to La physique depuis vingt ans (Paris, 1923). On ions in the atmosphere and particles in suspension, see “Interprération de divers phénomènes par la présence de gros ions dans I’atmosphére,” in Bulletin des séances de la Société franccedilaise de physique, fasc. 4 (19 May 1905), 79; and “Electrométre enrregistreur des ions de I’atmosphére,” in Radium4 (1907), 218, written with M. Moulin.
Kinetic theory and thermodynamics are treated in “Sur une formule fondamentale de la théorie cinétique,” in Comptes rendus.. de L’ Académie des sciences,140 (1905), 35, also in Annales de chimie et de physique, 8th ser., 5 (1905), 245; “Sur la téorie du mouvement Brownian,” in Comptes rendus … de l’Académie des sciences,146 (1908), 530; and “La physique due discontinu,” in Les progrè de la physique moléculaire (Paris, 1914), p. 1, also in Physique, p. 189.
On electromagnetic theory and electrons, see “La physique des électrons,” in Rapport du Congrés international des sciences et arts à Sains-Louis, (1904), also in Physique, p.1; “Les grains d’électricité et la dunamique électromagnétique,” in les Idées modernes sur la constitution de la matière (Paris, 1913), p. 54, also in Physique, p. 70; and “L’électron positif,” in Bulletin de la société des électriciensm 5th ser., 4 (1934), 335.
Writings on magnetic theory and molecular orientation include “Magnétisme et théorie des électrons,” in Annales de chimie et de physique, 8th ser., 5 (1905), 70; “Sur les biréfringences électrique et magnétique,” in Radium, 7 (1910), 249 ; “La théorie cinétique du magnétisme et les magnétons,” presented at the Solvay Conference in 1911, in La théeorie du rayonnement et les quanta (Paris, 1913), also in Physique, p. 171 ; “Sur l’orientation moléculaire,” a letter to M. W. Voigt, in Göttingen Nachrichten, no. 5 (1912), 589 ; and “Le magnétisme,” in Sixième Congrès de physique Solvay (Paris, 1923), p. 352.
The principle of relativity and the inertia of energy are discussed in “Sur l’impossibilité de mettre en évidence le mouvement de translation de la terre,” in Comptes rendus … de l’Académie des sciences,140 (1905), 1171; “L’évolution de 1’espace et du temps,” in Scientia,10 (1911), 31, also in Physique, p. 265; “Le temps, 1’espace et la causalité dans la physique moderne,” in Bulletin de la Société francaise de philosophic,12, no. 1 (Jan. 1912), 1, also in Physique, p. 301; “L’inertie de 1’énergie et ses conséquences,” in Journal de physique, 5th ser., 3 (1913), 553, also in Physique, p. 345; “Sur la théorie de la relativité et 1’expérience de M. Sagnac,” in Comptes rendus … de l’Académie des sciences, 173 (1921), 831; “La structure des atomes et l’origine de la chaleur solaire,” in Bulletin de l’Université de Tiflis,10 (1929); “La relativité,” in Exposés et discussions du Centre de synthése(Paris, 1932); “L’oeuvre d’Einstein et l’astronomie,” in L’astronomie, 45 (1931), 277; “Déduction simplifiée du facteur de Thomas,” in Convegno di fisica nucleare (Rome, 1931), p. 137; “Espace et temps dans un univers euclidien,” in Livre jubilaire de Marcel Brillouin (Paris, 1935); and “Résonance et forces de gravitation,” in Annales de physique,17 (1942), 261.
On physical chemistry and radioactivity, see “Sur la comparaison des molé cules gazeuses et dissoutes,” in Comptes rendus … de l’Académie des sciences,154 (1912), 594, also in Proces-verbaux des commissions de la Société francaise de physique (19 Apr. 1912), 54; “L’interprétation cinétique de la pression osmotique,” in Journal de chimiephysique, 10 (1912), 524, 527; and “Sur un problème d’activation par diffusion,” in Journal de physique, 7th ser., 5 (1934), 57.
Writings on magnitudes and units include “Notions géométriques fondamentales,” in Encyclopédie des sciences mathématiques, IV, pt. 5, fast. 1 (1912), 1; and “Sur les unités de champ et d’induction,” in Bulletin de la Société francaise de physique (17 Feb. 1922), 33.
Classical and modern mechanics are discussed in “Sur la dynamique de la relativité,” in Procés-verbaux des commissions de la Société francaise de physique (15 Dec. 1921), 97, also in Exposés et discussions du Centre international de synthése sur la relativité (Paris, 1932); “Les nouvelles mécaniques et la chimie,” in L’activation et la structure des molécules (Paris, 1929), p. 550; “La notion de corpuscules et d’atomes,” in Réunion internationale de chimie-physique (Paris, 1933); and “Sur les chocs entre neutrons rapides et noyaux de masse quelconque,” in Annales de physique, 17 (1942), 303, also in Comptes rendus … de 1’Académie des sciences, 214 (1942), 517, 867, 889.
On acoustics and ultrasonics, see “Procédés et appareils pour la production de signaux sous-marins dirigés et pour la localisation à distance d’obstacles sous-marins …,” in Brevet francais, no. 502.913 (29 May 1916), written with M. C. Chilowski; no. 505.703 (17 Sept. 1918); no. 575.435 (27 Dec. 1923), written with M. C. Florisson; and no. 576.281 (14 Jan. 1924), 1st supp. no. (1 Mar. 1924) and 2nd supp. no. (16 Oct. 1924).
See also “Note sur l’énergie auditive,” in Publications du Centre d’études de Toulon. (25 Sept. 1918); “Émission d’un faisceau d’ondes ultra-sonores,” in Journal de physique, 6th ser., 4 0 (1923), 537, written with M. C. Chilowsky and M. Tournier; “Utilisation des phénoménes piézo-électriques pour la mesure de l’intensité des sons en valeur absolue,” ibid., 6th ser., 4 (1923), 539, written with M. Ishimoto; “Sondage et détection sous-marine par les ultra-sons,” in Bulletin de l’Association technique maritime et aéronautique, no. 28 (1924), 407; “La production et l’utilisation des ondes ultra-sonores,” in Revue générale de l’électricité, 23 (1928), 626; “Sur le mirage ultra-sonore,” in Bulletin de l’Association technique maritime et aéronautique (1929), 727; “Les ondes ultra-sonores,” in Revue d’acoustique, 1 (1932), 93, 315; 2 (1933), 288; 3 (1934), 104, with notes by P. Biquard; and “Sur les lois du dégagement d’électricité par torsion dans les corps piézo-électriques,” in Comptes rendus … de l’Académie des sciences,200 (1935), 1257.
Various technical problems are treated in “Sur la production des étincelles musicales par courant continu,” in Annales des postes, télégraphes et téléphones, 5th year, no. 4 (1916), p. 404; “Utilisation de la détente pour la production des courants d’air de grande vitesse,” in Procés-verbaux des commissions de la Société française de physique(20 Feb. 1920), p. 21; “Note sur la loi de résistance de l’air,” in Mémorial de 1’artillerie française (1922), p. 253; “Note sur les effets balistiques de la détente des gaz de la poudre,” ibid. (1923), p. 3; “Procédé et appareils permettant la mesure de la puissance transmise par un arbre,” in Brevet français (22 Dec. 1927); “Banc piézo-électrique pour l’équilibrage des rotors,” ibid. (19 Dec. 1927); “Procédé et dispositif pour la mesure des variations de pression dans les canalisations d’eau ou autre liquide,” ibid. (6 Aug. 1927), written with R. Hocart; and “L’enregistrement des coups de bélier,” in Bulletin technique de la Chambre syndicale des entrepreneurs de couverture plomberie, no. 23 (1927), p. 81.
On teaching and pedagogy, see “L’esprit de l’enseignement scientifique,” in L’enseignement des sciences mathématiques et des sciences physiques(Paris, 1904), also in Physique; “Le théorème de Fermat de la loi du minimum de temps en optique géométrique,” in Journal de physique, ser. 6, 1 (1920), 188; “La valeur éducative de l’histoire des sciences,” in Revue de synthése, 6 (1933), 5; “La réorganisation de 1’enseignement public en Chine,” in Rapport de la mission d’experts de la Société des Nations, written with C. H. Becker, M. Falski, and R. H. Tawney (Paris, 1932); “Le problème de la culture générale,” in Discours d’ouverture du Congrès international d’éducation nouvelle, Nice, July 1932, also in Full Report of the New Education Fellowship (London, 1933), p. 73; “L’enseignement en Chine,” in Bulletin de la Société française de pedagogie, no. 49 (Sept. 1933); and “La Réforme générale de l’enseignement (Premier rapport sur les travaux de la commission ministérille),” in Bulletin officiel de l’éducation nationale, no, 23 (15 March 1945), p. 1461.
Other publications incude “Notice sur less travaux d Monsieur p. Curie,” in bulletin des anciens éléves de l’École municipale de physique et chimie industrielles (Dec. 1904); “Henri Poincaré, le physicien” in henri Poincaré (Paris, 1914), also in Revue du Mois,8 n (1913); “Paul Schutzenberger” in Discours Prononcé à l’occasion du centenaire de P. Schutzenberger (1929); “L’orientation actuelle de la physique,” in L’orientation actuelle des sciences (paris, 1930), p. 29; “La physique au Collége de France” in Volume du centenaire (Paris, 1932), p. 61; “Ernest Solvay” (Brussels, 1932); “Paul Painlevé, le savant,” in Les Cahiers rationalistes,. no. 26 (Nov. 1933) “La valeur humaine de la science,” preface to l’Évolution humaine (Paris, 1933); and “Discours prononcés à l’occasion du cinquantenaire de l’École municipale de physique et chimie industrielles” in Cinquante années de science appliquée à l’industrie, 1882-1932.
II. Secondary Literature. See P. Biquard, Paul Langevin, scientifique, éducateur, citoyen (Paris, 1969), with preface by J.D. Bernal and a bibliography; Louis de Broglie, Notice sur la vie et l’oeuvre de Paul Langevin (Paris, 1947); S. Ghiseman, Paul Langevin (Bucharest, 1964) La pensée (Paris), no. 12 (May-June 1947), spec. no. “In memoriam”; o. A. Staroselskaya Nikitina, Paul Langevin (Moscow, 1962); and A. R. Weill, “Paul Langevin,” in Mémorial de l’artillerie française, fasc. 4 (1946).
See also André Langevin, Paul Langevin, mom père (Paris, 1972).
Adrienne R. Weill-Brunschvicg
"Langevin, Paul." Complete Dictionary of Scientific Biography. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/langevin-paul
"Langevin, Paul." Complete Dictionary of Scientific Biography. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/langevin-paul
Paul Langevin (pōl läNzhəvăN´), 1872–1946, French physicist and chemist. He was professor of experimental physics at the Collège de France from 1909 and at the École municipale de Physique et de Chimie, Paris, from 1904 (director from 1929); dismissed by the Vichy government in 1940, he resumed his posts in 1944. He is noted for his work on the electron theory of magnetism and for his research on sound devices for submarine detection.
"Langevin, Paul." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/langevin-paul
"Langevin, Paul." The Columbia Encyclopedia, 6th ed.. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/langevin-paul
"Langevin, Paul." World Encyclopedia. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/langevin-paul
"Langevin, Paul." World Encyclopedia. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/langevin-paul