(b. Lyons, France, 28 September 1860; d. Bayonne, France, 13 January 1934), physics.
Villard’s main work involved the experimental study of cathode rays. X rays, and radioactivity from 1897 to 1907 or thereabouts.
Villard entered the École Normal Supérieure in 1881 and received the agrégé in 1884. After teaching in provincial secondary schools and in Paris, he received permission to work in the chemistry laboratory of the École Normale, where he conducted all his research. Villard received the Wilde and the La Caze prizes of the Paris Academy of Sciences and was elected a member of its general physics section in 1908. During his later years he received a pension d’honneur from the Caisse Nationale des Sciences.
Villard’s background was in physical chemistry. During his first ten years at the chemistry laboratory of the École Normale, he worked on the hydrates of argon, methane, methylene, and acetylene, and on topics associated with change of phase. Villard retained a chemical point of view after having become involved in radiation physics, considering, for example, the “chemical action’ of X rays and the “reducing action” of cathode rays.
Since Villard made all his own apparatus he was quite familiar with all of its technical details and, in addition to his research, he designed instruments and techniques that were useful to practical radiologists.
In his early work on cathode-ray tubes, Villard was interested in the nature of the material that moves in the tube. In this context he also considered the nature of canal rays, a puzzle since Goldstein had observed them in 1886. In 1898 Villard Observed the positively charged material moves toward the cathode and that when there are holes in the cathode, a visible stream, causing heating, passes through it. He suggested that the positive material forms the canal rays after it passes through the cathode. He was not, however, able to deflect the stream moving through the cathode by means of electric or magnetic fields; and he assumed that it had lost its charge at the cathode. On the basis of the reducing action of various rays in the cathode tube, Villard concluded that the moving material is hydrogen. (In the same year Wien managed to deflect canal rays with electric and magnetic fields, and he found that their e/m was on the order of that of the hydrogen ion.)1
Villard was the first to observe a penetrating radiation, which he named γ radiation, following the pattern of Rutherford’s names for α and β rays. In 1900, while studying the secondary emission produced by both cathode rays and radium radiation on passing through a metal sheet, he observed that a component of the radium radiation was sufficiently energetic to pass directly through; and he associated it with the “nondeviable” (uncharged) component. As a further test Villard used a more active radium source given by the Curies and sent the radiation consecutively through two photographic plates. He concluded that the “X rays” emitted by the radium are much more penetrating than the charged rays. (At this time Villard appears already to have associated the penetrating radiation with X rays; the electromagnetic nature of the γ rays was not proved until 1914.)
In 1906 Villard began studying the aurora experimentally by simulating the phenomenon with cathode rays in a magnetic field. (Olaf Birkeland, a Norwegian, had produced the first laboratory model of the aurora in 1896 by sending cathode rays toward a magnetized sphere.) Villard, who had been studying the properties of cathode rays produced when the cathode is in a strong magnetic field, “made some very fine experiments,” according to C. Störmer, and “succeeded in producing threadlike currents of cathode rays which made it possible to follow the trajectories in detail.”2 Villard was interested in the theoretical implications of his work and, differing from others, believed the source of the aurora to be terrestrial.
1. W. Wien, “Untersuchungen über die electrische Entladung in verdünnten Gasen,” in Wiedemann’s Annalen der Physik und Chemie, n.s. 65 1898, 440–452; “Die electrostatische und magnetische Ablenkung der Canalstrahlen,” in Verhandlungen der Physikalischen Gesellschaft zu Berlin (1898), 10–12.
2. C. Störmer, The Polar Aurora (Oxford, 1955), 290.
I. Original Works. Villard’s papers include “Sur les rayons cathodiques.” in Comptes rendus de l’Académie des sciences, 126 (1898), 1339-1341, 1564-1566; 127 (1898), 173–175; 130 (1900), 1614-1616; “Sur la réflexion et la réfraction des rayons cathodiques et des rayons déviables du radium,” ibid., 130 (1900), 1010-1012; “Sur le rayonnement du radium,” ibid., 1178-1179; and “Sur l’aurore boréale,’ ibid., 142 (1906), 1330-1333; 143 (1906), 143–145.
Villard’s papers published before 1900 are listed in the Royal Society Catalogue of Scientific Papers, XIX , 352. Those published after 1900 are listed in Science Abstracts.
II. Secondary Literature. The discussion of Villard’s work presented in connection with the award of the La Caze Prize is in Comptes rendus… de l’Académie des sciences, 145 (1907), 1002-1005. E. Borel’s obituary address on Villard is ibid., 198 (1934), 213–215.