Footnote 25: Runge and Precht, _Phil. Mag._ April, 1903. Footnote 26: Watts, _Phil. Mag._ July, 1903; August, 1904. Footnote 27: Runge, _Phil. Mag._ December, 1903. Footnote 28: Debierne, _C. R._ 129, p. 593, 1899; 130, p. 206, 1900. Footnote 29: Giesel, _Ber. d. D. Chem. Ges._ p. 3608, 1902; p. 342, 1903. Footnote 30: Debierne, _C. R._ 139, p. 538, 1904. Miss Brooks, _Phil. Mag._ Sept. 1904. Giesel, _Phys. Zeit._ 5, p. 822, 1904. _Jahrbuch. d. Radioaktivität_, no. 4, p. 345, 1904. Footnote 31: Giesel, _Ber. d. D. Chem. Ges._ 37, p. 1696, 1904; Hartmann, _Phys. Zeit._ 5, No. 18, p. 570, 1904. Footnote 32: Mme Curie, _C. R._ 127, p. 175, 1898. Footnote 33: Mme Curie, _Thèse_, Paris, 1903. Footnote 34: Crookes, _Proc. Roy. Soc._ May, 1900. Footnote 35: Berndt, _Phys. Zeit._ 2, p. 180, 1900. Footnote 36: Marckwald, _Phys. Zeit._ 4, No. 1 b, p. 51. Footnote 37: Marckwald, _Ber. d. D. Chem. Ges._ p. 2662, No. 12, 1903. Footnote 38: Elster and Geitel, _Annal. d. Phys._ 69, p. 83, 1899. Footnote 39: Giesel, _Ber. d. D. Chem. Ges._ p. 3775, 1901. Footnote 40: Hofmann and Strauss, _Ber. d. D. Chem. Ges._ p. 3035, 1901. Footnote 41: Hofmann, Gonder and Wölfl, _Annal. d. Phys._ No. 13, p. 615, 1904. Footnote 42: Hofmann and Zerban, _Ber. d. D. Chem. Ges._ No. 12, p. 3093, 1903. Footnote 43: Baskerville and Zerban, _Amer. Chem. Soc._ 26, p. 1642, 1904. CHAPTER II. IONIZATION THEORY OF GASES. =25. Ionization of gases by radiation.= The most important property possessed by the radiations from radio-active bodies is their power of discharging bodies whether positively or negatively electrified. As this property has been made the basis of a method for an accurate quantitative analysis and comparison of the radiations, the variation of the rate of discharge under different conditions and the processes underlying it will be considered in some detail. In order to explain the similar discharging power of Röntgen rays, the theory[44] has been put forward that the rays produce positively and negatively charged carriers throughout the volume of the gas surrounding the charged body, and that the rate of production is proportional to the intensity of the radiation. These carriers, or ions[45] as they have been termed, move with a uniform velocity through the gas under a constant electric field, and their velocity varies directly as the strength of the field. [Illustration: Fig. 1.]