[111]. In an examination of uranium the writer (Phil. Mag. p. 116, Jan. 1899) found that the rays from uranium consist of two kinds, differing greatly in penetrating power, which were called the α and β rays. Later, it was found that similar types of rays were emitted by thorium and radium. On the discovery that very penetrating rays were given out by uranium and thorium as well as by radium, the term γ was applied to them by the writer. The word “ray” has been retained in this work, although it is now settled that the α and β rays consist of particles projected with great velocity. The term is thus used in the same sense as by Newton, who applied it in the Principia to the stream of corpuscles which he believed to be responsible for the phenomenon of light. In some recent papers, the α and β rays have been called the α and β “emanations.” This nomenclature cannot fail to lead to confusion, since the term “radio-active emanation” has already been generally adopted in radio-activity as applying to the material substance which gradually diffuses from thorium and radium compounds, and itself emits rays.
[112]. This method of illustration is due to Mme Curie, Thèse présentée à la Faculté des Sciences de Paris, 1903.
[113]. Giesel, Annal. d. Phys. 69, p. 834, 1899.
[114]. Meyer and Schweidler, Phys. Zeit. 1, pp. 90, 113, 1899.
[115]. Becquerel, C. R. 129, pp. 997, 1205, 1899.
[116]. Curie, C. R. 130, p. 73, 1900.
[117]. Rutherford, Phil. Mag. January, 1899.
[118]. Rutherford and Grier, Phil. Mag. September, 1902.
[119]. Becquerel, C. R. 130, pp. 206, 372, 810, 979. 1900.
[120]. M. and Mme Curie, C. R. 130, p. 647, 1900.