Rutherford, _Phys. Zeit._ 3, No. 12, p. 254, 1902. _Phil. Mag._ Jan. 1903. Footnote 274: Miss Brooks, _Phil. Mag._ Sept. 1904. Footnote 275: Rutherford and Miss Brooks, _Phil. Mag._ July, 1902. Footnote 276: Curie and Danne, _C. R._ 136, p. 364, 1903. Footnote 277: Mme Curie, _Thèse_, Paris, 1903, p. 116. Footnote 278: Debierne, _C. R._ 138, p. 411, 1904. Footnote 279: Giesel, _Ber. d. D. Chem. Ges._ No. 3, p. 775, 1905. Footnote 280: Miss Brooks, _Phil. Mag._ Sept. 1904. Footnote 281: Rutherford, _Phys. Zeit._ 3, No. 12, p. 254, 1902. Footnote 282: F. von Lerch, _Annal. d. Phys._ 12, p. 745, 1903. Footnote 283: Pegram, _Phys. Review_, p. 424, Dec. 1903. Footnote 284: Miss Gates, _Phys. Review_, p. 300, 1903. Footnote 285: A more complete examination of the effect of temperature on the excited activity of thorium has been made by Miss Slater (section 207). Footnote 286: Rutherford, _Phil. Mag._ Feb. 1900. Footnote 287: Henning, _Annal. d. Phys._ 7, p. 562, 1902. Footnote 288: Rutherford, _Phil. Mag._ Feb. 1900. Footnote 289: Curie and Debierne, _C. R._ 132, p. 768, 1901. Footnote 290: Fehrle, _Phys. Zeit._ 3, No. 7, p. 130, 1902. Footnote 291: Rutherford, _Phil. Mag._ Jan. 1903. Footnote 292: Giesel, _Ber. d. D. Chem. Ges._ 36, p. 342, 1903. Footnote 293: Debierne, _C. R._ 136, pp. 446 and 671, 1903; 138, p. 411, 1904. CHAPTER IX. THEORY OF SUCCESSIVE CHANGES. =193. Introduction.= We have seen in previous chapters that the radio-activity of the radio-elements is always accompanied by the production of a series of new substances with some distinctive physical and chemical properties. For example, thorium produces from itself an intensely radio-active substance, Th X, which can be separated from the thorium in consequence of its solubility in ammonia. In addition, thorium gives rise to a gaseous product, the thorium emanation, and also to another substance which is deposited on the surface of bodies in the neighbourhood of the thorium, where its presence is indicated by the phenomenon known as “excited activity.” A close examination of the origin of these products shows that they are not produced simultaneously, but arise in consequence of a succession of changes originating in the radio-element. Thorium first of all gives rise to the product Th X. The Th X produces from itself the thorium emanation, and this in turn is transformed into a non-volatile substance. A similar series of changes is observed in radium, with the exception that there is no product in radium corresponding to the Th X in the case of thorium. Radium first of all produces an emanation, which, like thorium, is transformed into a non-volatile substance. In uranium only one product, Ur X, has been observed, for uranium does not give off an emanation and in consequence does not produce excited activity on bodies.