Soddy, _Trans. Chem. Soc._ 81, p. 460, 1902. Footnote 297: Rutherford and Grier, _Phil. Mag._ Sept. 1902. Footnote 298: Becquerel, _C. R._ 131, p. 137, 1900. Footnote 299: Meyer and Schweidler, _Wien Ber._ Dec. 1, 1904. Footnote 300: Meyer and Schweidler, _Wien Ber._ 113, July, 1904. Footnote 301: Rutherford, _Phil. Trans._ A. 204, pp. 169–219, 1904. Footnote 302: Pegram, _Phys. Rev._ p. 424, December, 1903. Footnote 303: Miss Slater, _Phil. Mag._ 1905. Footnote 304: von Lerch, _Ann. de d.] Phys._ November, 1903. Footnote 305: The ‘rayless change’ certainly does not give out α rays, and special experiments showed that no appreciable amount of β rays were present. On the other hand, the second change gives out all three types of rays. Footnote 306: Miss Brooks, _Phil. Mag._ Sept. 1904. Footnote 307: Rutherford and Soddy, _Trans. Chem. Soc._ 81, p. 837, 1902. _Phil. Mag._ Nov. 1902. Footnote 308: Miss Brooks, _Phil. Mag._ Sept. 1904. Footnote 309: Rutherford, _Phil. Trans._ A. p. 169, 1904. Footnote 310: Giesel, _Ber. d. D. Chem. Ges._ p. 775, 1905. Footnote 311: Godlewski, _Nature_, p. 294, Jan. 19, 1905. Footnote 312: Debierne, _C. R._ 138, p. 411, 1904. Footnote 313: Miss Brooks, _Phil. Mag._ Sept. 1904. CHAPTER XI. TRANSFORMATION PRODUCTS OF RADIUM. =215. Radio-activity of radium.= Notwithstanding the enormous difference in their relative activities, the radio-activity of radium presents many close analogies to that of thorium and actinium. Both substances give rise to emanations which in turn produce “excited activity” on bodies in their neighbourhood. Radium, however, does not give rise to any intermediate product between the element itself and the emanation it produces, or in other words there is no product in radium corresponding to Th X in thorium. Giesel first drew attention to the fact that a radium compound gradually increased in activity after preparation, and only reached a constant value after a month’s interval. If a radium compound is dissolved in water and boiled for some time, or a current of air drawn through the solution, on evaporation it is found that the activity has been diminished. The same result is observed if a solid radium compound is heated in the open air. This loss of activity is due to the removal of the emanation by the process of solution or heating. Consider the case of a radium compound which has been kept for some time in solution in a shallow vessel, exposed to the open air, and then evaporated to dryness. The emanation which, in the state of solution, was removed as fast as it was formed, is now occluded, and, together with the active deposit which it produces, adds its radiations to that of the original radium. The activity will increase to a maximum value when the rate of production of fresh emanation balances the rate of change of that already produced.