Becoquerel, [Antoine-] Henri

(b. Paris, France, 15 December 1852; d. Le Croisic, Brittany, France, 25 August 1908)

physics.

Becquerel is known for his discovery of radioactivity, for which he received the Nobel Prize for physics jointly with the Curies in 1903, and for other contributions to that field which he made during the halfdozen years when he was most active in it. He was a member of the Academy of Sciences, became its president, and was elected to the far more influential post of permanent secretary. He held three chairs of physics in Paris—at the Museum of Natural History, at the École Polytechnique, and at the Conservatoire National des Arts et Métiers—and attained, high rank as an engineer in the National Administration of Bridges and Highways (Ponts et Chaussées).

Henri’s father, Alexandre-Edmond Becquerel, and his grandfather, Antoine-César Becquerel, were renowned and prolific physicists, both members of the Academy of Sciences and each in his turn professor of physics at the Museum of Natural History. When Henri was born in the professor’s house at the Museum, he was born almost literally into the inner circles of French science. He was educated at the Lycée Louis-le-Grand, from which he went to the École Polytecdhnique (1872–1874) and then to the École des Ponts et Chaussées (1874–1877), where he received his engineering training and from which he entered the Administration of Bridges and Highways with the rank of ingénieur. On leaving the Polytechnique, he married Lucie-Zoé-Marie Jamin, daughter of J.-C. Jamin, academician and professor of physics in the Faculty of Sciences of Paris. Before the end of his schooling he had begun both his private research (1875) and his teaching career (1876) as répétiteur at the Polytechnique. In January 1878 his grandfather died; his wife died the following March, a few weeks after the birth of their son Jean. At this time Becquerel succeeded to the post of aide-natural-iste, which his father had hitherto held at the Museum, and from then on, his professional life was shared among the Museum, the Polytechnique, and the Ponts et Chaussées.

Becquerel’s early research was almost exclusively optical. His first extensive investigations (1875–1882) dealt with the rotation of plane-polarized light by magnetic fields. He turned next to infrared spectra (1883), making visual observations by means of the light released from certain phosphorescent crystals under infrared illumination. He then studied the absorption of light in crystals (1886–1888), particularly its dependence on the plane of polarization of the incident light and the direction of its propagation through the crystal. With these researches Becquerel obtained his doctorate from the Faculty of Sciences of Paris (1888) and election to the Academy of Sciences (1889), after two preparatory nominations (1884, 1886), in the second of which he polled twenty of the fifty-one votes. He had in the meantime been promoted to ingénieur de première classe in the Ponts et Chaussées.

With his doctorate achieved, Becquerel became substantially inactive in research. In 1890 he married his second wife, the daughter of E. Lorieux, an inspector general of mines. Following the death of Edmond Becquerel in 1891, he succeeded in the following year to his father’s two chairs of physics, at the Conservatoire National des Arts et Métiers and at the Museum. In the same year Alfred Potier withdrew from active teaching because of illness, and Becquerel took over his lectures in physics at the école Polytechnique. Two years later (1894) he became ingénieur en chef with the Ponts et Chaussées and the next year (1895) was named to succeed Potier at the Polytechnique.

Thus the beginning of 1896 found Becquerel, at the age of forty-three, established in rank and responsibility, his years of active research behind him and everything for which he is now remembered still undone. In the very opening days of the year, Roentgen had announced his discovery of X rays by a mailing of preprints and photographs, but Becquerel’s personal knowledge of the discovery dates to 20 January, when two physicians, Paul Oudin and Tousaint Barthélemy, submitted an X-ray photograph of the bones of a living hand to the Academy. From Henri Poincaré, who had received a preprint, Becquerel learned that in Roentgen’s tubes the X rays arose from the fluorescent spot where a beam of cathode rays played on the glass wall. Thus a natural, if perhaps not plausible, inference arose that the visible light and invisible X rays might be produced by the same mechanism, and that X rays might accompany all luminescence.

Becquerel’s researches had been essentially descriptive, with a primary commitment to observation and a careful avoidance of theorizing. Nevertheless, this X-ray hypothesis caught his fancy. He had some personal acquaintance with luminescent crystals, he was familiar with his father’s researches on them, and he began to hunt for a crystalline emitter of penetrating radiation. On 24 February 1896 he reported to the Academy that fluorescent crystals of potassium uranyl sulfate had exposed a photographic plate wrapped in black paper while they both lay for several hours in direct sunlight. On 2 March he reported comparable exposures when both crystals and plate lay in total darkness. By his working hypothesis, that would have been impossible because the luminescence of potassium uranyl sulfate ceases immediately when the ultraviolet radiation that excites it is withdrawn. One might speculate, nevertheless, that the penetrating rays persisted longer than the visible fluorescence when their common excitation was cut off. Becquerel did so, conscientiously condemned the speculation as unjustified, and then proceeded to act upon it.

He did not neglect his general studies. He showed that, like X rays, the penetrating rays from his crystals could discharge electrified bodies (in modern terms, could ionize the air they passed through). He found evidence to suggest that the rays were refracted and reflected like visible light, although later he attributed these effects to secondary electrons ejected from his glass plates and mirrors. Nevertheless, he devoted a substantial effort to searching out the radiation that had first excited his penetrating rays. He kept some of his crystals in darkness, hoping that their pent-up energy might dissipate itself and make them ready for reexcitation. He tried other luminescent crystals and found that only those containing uranium emitted the penetrating radiation. He tried ingeniously but unsuccessfully to release the energy of uranyl nitrate by warming its crystals in darkness until they dissolved in their own water of crystallization. He tested nonluminescent compounds of uranium and found that they emitted his penetrating rays. Finally, he tried a disk of pure uranium metal and found that it produced penetrating radiation three to four times as intense as that he had first seen with potassium uranyl sulfate.

With this last announcement, on 18 May, Becquerel’s discovery of radioactivity was complete, although he continued with ionization studies of his penetrating radiation until the following spring. What he had accomplished at the most general level was to establish the occurrence and the properties of that radiation, so that it could be identified unambiguously. Of more importance, he had shown that the power of emitting penetrating rays was a particular property of uranium. However, the implications of this second conclusion were by no means clear at the time. Becquerel characterized his own achievement as the first observation of phosphorescence in a metal. His immediate successors, G. C. Schmidt and Marie Curie, started with quite conventional views about the rays and came only gradually to realize that such radiation might also be emitted by other elements. Both then searched among the known elements, finding that only thorium was also a ray-emitter. Marie Curie and her husband, Pierre, pushed on to search for unknown elements with the same property, however, and so discovered polonium and radium. With these discoveries, the field of radioactivity (a term that the Curies coined) was fully established.

Nothing that Becquerel subsequently accomplished was as important as this discovery, by which he opened the way to nuclear physics. Nevertheless, there were two other occasions on which he stood directly on the path of history: when he identified electrons in the radiations of radium (1899–1900) and when he published the first evidence of a radioactive transformation (1901).

Marie Curie’s work, which attracted Becquerel’s attention, brought the Curies within the circle of his acquaintance and turned him back to radioactive studies. He became the intermediary through whom their papers reached the Academy, and they lent him radium preparations from time to time. Toward the end of 1899 (his first report is dated 11 December), he began to investigate the effects on the radiation from radium of magnetic fields in various orientations to the direction of its propagation (in modern terms, the magnetic deflection of the beta rays from shortterm decay products in equilibrium with the radium). In this work he united two descriptive traditions, the magneto optics of his own experience and a line of qualitative studies of the discharge of electricity through gases. He soon moved from these to J. J. Thomson’s more radical program of quantitative observations on collimated beams, in which Thomson had shown (1897) that the cathode rays were corpuscular and consisted of streams of swiftly moving, negatively charged particles whose masses were probably subatomic. By 26 March 1900, Becquerel had duplicated those experiments for the radium radiation and had shown that it too consisted of negatively charged ions, moving at 1.6 × 10¹⁰ cm./sec. with a ratio of m/e = 10^-7 gm./abcoul. Thus Thomson’s “corpuscles” (electrons) constituted a part of the radiations of radioactivity.

At this period an idea was current, although seldom formally expressed, that radioactivity should be a property only of rare substances like radium, and not of ordinary chemical elements. Perhaps under the impulse of such a notion, Becquerel undertook to remove from uranium a magnetically deviable (or beta) radiation he had recently identified. His method was borrowed from André Debierne, who had found it effective with actinium. To a solution of uranium chloride, he added barium chloride and precipitated the barium as the sulfate. The precipitate entrained something, for the deviable radiation of the uranium was diminished; by a long repetition of such operations he succeeded in July 1900 in reducing that radiation, in one specimen, to one-sixth of its original value. In confirmation of this result, he found that earlier that spring, Crookes had succeeded, by more effective chemical procedures, in separating from uranium the photographically active radiation, which he now attributed to a substance provisionally named uranium X. Something over a year later, Becquerel realized the logical incongruity of these two successes. It had been relatively easy to remove the apparent radioactivity from uranium by chemical purification, yet no one who had investigated uranium over the last five years had ever observed a nonradioactive specimen. It followed, then, that whatever radioactivity was lost in purification must always regenerate itself; and he verified this logical conclusion on his own earlier specimens. The uranium had regained its lost radioactivity, and the barium sulfate precipitates had lost all that they had carried down. The explanations he attempted were thoroughly confusing, but the facts remained.

These facts were brought squarely to the attention of Ernest Rutherford and Frederick Soddy, who had just succeeded in separating a thorium X analogous to Crookes’s uranium X. Their subsequent tests showed a similar regeneration of the lost radioactivity of the thorium. From this they inferred, and immediately verified, a regeneration of the thorium X in those specimens and then came to realize that this chemically distinct thorium X could have been formed there only by a transformation of the thorium. On the basis of these and similar experiments, in a few months they formulated the transformation theory, which became the basic theory of radioactivity.

In 1903 the Nobel Prize for physics was divided between Henri Becquerel and Pierre and Marie Curie. It was an appropriate division. Becquerel’s pioneer investigations had opened the way to the Curies’ discoveries, and their discoveries had validated and shown the importance of his. On 31 December 1906, Becquerel was elected vice-president of the Academy of Sciences, serving in that capacity during 1907 and succeeding to the presidency in 1908. On 29 June 1908 he was elected as one of the two permanent secretaries of the Academy, following the death of Lapparent. On confirmation by the president of the republic, he was installed in that office on 6 July, taking his seat beside Darboux, who had taught him mathematics nearly four decades before at the Lycée Louis-le-Grand. He died soon after at Le Croisic in Brittany, the ancestral home of his wife’s family, the Lorieux.

In an assessment of Becquerel’s scientific powers, it should be noted that he had little taste for physical theories, either his own or those of others, and much of his research effort was dissipated on observations of no great significance. Against this, he displayed an admirable versatility in experiment in unfamiliar as well as familiar fields. His greatest asset, however, was a strong, persistent power of critical afterthought. On those rare occasions when Becquerel did pursue a hypothesis, this critical power continually corrected his enthusiasms and redirected his line of investigation; so that, for example, while he persistently searched for X rays in phosphorescence, he managed to discover the inherent radioactivity of uranium.

BIBLIOGRAPHY

I. Original Works. Becquerel wrote an account of his radioactive investigations in an extended mémoire, “Recherches sur une propriété de la matieère. Activité radiante spontanée ou radioactivité de la matière,” in Mémoires de l’Académie des sciences, Paris, 46 (1903). Aside from this, his scientific work is to be found in some 150 papers and notes in various journals. No single list of them exists, but those published up to 1900 may be found in the Royal Society’s Catalogue of Scientific Papers, IX, 166–167, and XIII, 395–396. His papers on radioactivity are listed in the bibliography of 214 items included with his mémoire. Among these papers on radioactivity, all of them in the Comptes rendus de l’Académie des sciences, Paris, are “Émission de radiations nouvelles par l’uranium métallique,” 122 (1896), 1086–1088; “Sur quelques propriétés nouvelles des radiations invisibles émises par divers corps phosphorescents,” ibid., 559–564; “Sur les radiations émises par phosphorescence,” ibid., 420–421; “Sur les radiations invisibles émises par les corps phosphorescents,” ibid., 501–503; “Sur les radiations invisibles émises par les sels d’uranium,” ibid., 689–694; “Sur diverses propriété des rayons uraniques,”123 (1896), 855–858; “Sur la loi de décharge dans l’air de l’uranium éectrisé,” 124 (1897), 800–803; “Recherches sur les rayons uraniques,” ibid., 444; “Influence d’un champ magnétique sur le rayonnemen; des corps radio-actifs,” 129 (1899), 996–1001; “Sur le rayonnement des corps radio-actifs,” ibid., 1205–1207; “Contribution à l’étude du rayonnement du radium,” 130 (1900), 206–211; “Déviation du rayonnement du radium dans un champ électrique,” ibid., 809–815; “Sur la dispersion du rayonnement du radium dans un champ magnétique,” ibid., 372–376; “Note sur le rayonnement de l’uranium,” ibid., 1583–1585, and 131 (1900), 137–138; and “Sur la radioactivité de l’uranium.” 133 (1901), 977–980.

II. Secondary Literature. There is a somewhat padded biography by Albert Ranc, Henri Becquerel et la découverte de la radioactivité, Sciences et Savants, no. 3 (Paris, 1946). Important material is in G. Darboux, E. Perrier, M. Vieille, and L. Passy, “Discours prononcés aux funerailles de M. Henri Becquerel,” in Comptes rendus de l’Académie des sciences, Paris, 147 (1908). 443–451: and in the sketch included in Les Prix Nobel en 1903 (Stockholm, 1906), pp. 62–63.

Critical studies include Sir O. Lodge, “Becquerel Memorial Lecture,” in Journal of the Chemical Society, 101 (1912), 2005–2042, for a contemporary assessment; and L. Badash, “‘Chance Favors the Prepared Mind’: Henri Becquerel and the Discovery of Radioactivity,” in Archives internationales d’histoire des sciences, 18 (1965), 55–66, for a modern one.

Alfred Romer