In order to obtain a thin layer, the compound to be tested is ground to a fine powder and then sifted through a fine gauge uniformly over the area, so that the plate is only partially covered. Footnote 160: Rutherford, _Phil. Mag._ Jan. 1899. Footnote 161: Owens, _Phil. Mag._ Oct. 1899. Footnote 162: Rutherford and Miss Brooks, _Phil. Mag._ July, 1900. Footnote 163: Since the ionization at any point above the plate is the resultant effect of the α particles coming from all points of the large radio-active layer, λ is not the same as the coefficient of absorption of the rays from a point source. It will however be proportional to it. For this reason λ is called the “absorption constant.” Footnote 164: Townsend, _Phil. Mag._ Feb. 1901. Footnote 165: Durack, _Phil. Mag._ July 1902, May 1903. Footnote 166: Bragg and Bragg and Kleeman, _Phil. Mag._ Dec. 1904. Footnote 167: Villard, _C. R._ 130, pp. 1010, 1178, 1900. Footnote 168: Becquerel, _C. R._ 130, p. 1154, 1900. Footnote 169: Rutherford, _Phys. Zeit._ 3, p. 517, 1902. Footnote 170: McClelland, _Phil. Mag._ July 1904. Footnote 171: Paschen, _Phys. Zeit._ 5, No. 18, p. 563, 1904. Footnote 172: A. S. Eve, _Phil. Mag._ Nov. 1904. Footnote 173: Paschen, _Annal. d. Physik_, 14, p. 114, 1904; 14, 2, p. 389, 1904. _Phys. Zeit._ 5, No. 18, p. 563, 1904. Footnote 174: Paschen, _Phys. Zeit._ 5, No. 18, p. 563, 1904. Footnote 175: Rutherford and Barnes, _Phil. Mag._ May 1905. _Nature_, p. 151, Dec. 15, 1904. Footnote 176: Barkla, _Nature_, March 17, 1904. Footnote 177: Becquerel, _C.R._ 132, pp. 371, 734, 1286. 1901. Footnote 178: Mme Curie, _Thèse présentée à la Faculté des Sciences_, Paris 1903, p. 85. Footnote 179: A. S. Eve, _Phil. Mag._ Dec. 1904. Footnote 180: In a recent paper (_Phil. Mag._ Feb. 1905), McClelland has, in the main, confirmed the experimental results obtained by Eve. An electrometer was used instead of an electroscope. He finds, in addition, that the amount of secondary radiation depends on the angle of incidence of the primary rays, and is greatest for an angle of 45°. In a letter to _Nature_ (Feb. 23, p. 390, 1905), he states that more recent experiments have shown that the amount of secondary radiation from different substances is a function of their atomic weights rather than of their densities. In every case examined, the amount of secondary radiation increases with the atomic weight, but is not proportional to it. Footnote 181: Rutherford and McClung, _Phil. Trans._ A. p. 25, 1901. Footnote 182: Meyer and Schweidler, _Wien Ber._ 113, July, 1904. Footnote 183: Rutherford and Grier, _Phil. Mag._ Sept. 1902. CHAPTER V. PROPERTIES OF THE RADIATIONS.