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indefinite time after the substance had been removed from the sunlight, and after the luminosity which properly constitutes phosphorescence had died away; and he was thus led to conclude that the activity was spontaneous and permanent. It was soon found that those salts of uranium which do not phosphoresce—e.g., the uranous salts,—and the metal itself, all emit the rays; and it became evident that what Becquerel had discovered was a radically new physical property, possessed by the element uranium in all its chemical compounds.

Attempts were now made to trace this activity in other substances. In 1898 it was recognized in thorium and its compounds;^[1] and in the same year P. Curie and Madame Sklodowska Curie announced to the French Academy the separation from the mineral pitchblende of two new highly active elements, to which they gave the names of polonium^[2] and radium.^[3] A host of workers was soon engaged in studying the properties of the Becquerel rays. The discoverer himself had shown^[4] in 1896 that these rays, like the X- and cathode rays, impart conductivity to gases. It was found in 1899 by Rutherford^[5] that the rays from uranium are not all of the same kind, but that at least two distinct types are present; one of these, to which he gave the name α-rays, is readily absorbed; while another, which he named β-radiation, has a greater penetrating power. It was then shown by Giesel, Becquerel, and others, that part of the radiation is deflected by a magnetic field,^[6] and part is not.^[7] After this Monsieur and Madame Curie^[8] found that the deviable rays carry negative electric charges,

1. By Schmidt, Ann. d. Phys., lxv (1898), p. 141; and by Madame Curie, Comptes Rendus, cxxvi (1898), p. 1101.
2. Comptes Rendus, cxxvii (1898), p. 175.
3. Ibid., cxxvii (1898), p. 1275.
4. Ibid., cxxii (1896), p. 659.
5. Phil. Mag. (5), xlvii (1899), p. 109.
6. Giesel, Ann. d. Phys. lxix (1899), p. 834 (working with polonium); Bcequerel, Comptes Rendus, cxxix (1899), p. 996 (working with radium); Meyer and v. Schweidler, Phys. Zeitschr. i (1899), p. 113 (working with polonium and radium).
7. Becquerel, Comptes Rendus, cxxix (1889), p. 1205); cxxx (1900), pp. 206, 372. Curie, ibid. cxxx (1900), p. 73.
8. Comptes Rendus, cxxx (1900), p. 647.