=24.= If elements heavier than uranium exist, it is probable that they will be radio-active. The extreme delicacy of radio-activity as a means of chemical analysis would enable such elements to be recognized even if present in infinitesimal quantities. It is probable that considerably more than the three or four radio-elements at present recognized exist in minute quantity, and that the number at present known will be augmented in the future. In the first stage of the search, a purely chemical examination is of little value, for it is not probable that the new element should exist in sufficient quantity to be detected by chemical or spectroscopic analysis. The main criteria of importance are the existence or absence of distinctive radiations or emanations, and the permanence of the radio-activity. The discovery of a radio-active emanation with a rate of decay different from those already known would afford strong evidence that a new radio-active body was present. The presence of either thorium or radium in matter can very readily be detected by observing the rate of decay of the emanations given out by them. When once the existence of a new radio-element has been inferred by an examination of its radio-active properties, chemical methods of separation can be devised, the radiating or emanating property being used as a guide in qualitative and quantitative analysis. Footnote 1: Niewenglowski, _C. R._ 122, p. 385, 1896. Footnote 2: Becquerel, _C. R._ 122, p. 559, 1896. Footnote 3: Troost, _C. R._ 122, p. 564, 1896. Footnote 4: Arnold, _Annal. d. Phys._ 61, p. 316, 1897. Footnote 5: Le Bon, _C. R._ 122, pp. 188, 233, 386, 462, 1896. Footnote 6: Becquerel, _C. R._ 122, pp. 420, 501, 559, 689, 762, 1086, 1896. Footnote 7: Mme Curie, _Thèse présentée à la Faculté des Sciences de Paris_, 1903. Footnote 8: _Nature_, 56, 1897; _Phil. Mag._ 43, p. 418, 1897; 45, p. 277, 1898. Footnote 9: Rutherford, _Phil. Mag._ Jan. 1899. Footnote 10: _Ibid._ Footnote 11: Le Bon, _C. R._ 130, p. 891, 1900. Footnote 12: Lenard, _Annal. d. Phys._ 1, p. 498; 3, p. 298, 1900. Footnote 13: Schmidt, _Annal. d. Phys._ 65, p. 141, 1898. Footnote 14: Mme Curie, _C. R._ 126, p. 1101, 1898. Footnote 15: Owens, _Phil. Mag._ Oct. 1899. Footnote 16: Rutherford, _Phil. Mag._ Jan. 1900. Footnote 17: M. and Mme Curie and G. Bemont, _C. R._ 127, p. 1215, 1898. Footnote 18: Giesel, _Phys. Zeit._ 3, No. 24, p. 578, 1902. Footnote 19: Giesel, _Annal. d. Phys._ 69, p. 91, 1890. _Ber. d. D. Chem. Ges._ p. 3608, 1902. Footnote 20: Demarçay, _C. R._ 127, p. 1218, 1898; 129, p. 716, 1899; 131, p. 258, 1900. Footnote 21: Runge, _Astrophys. Journal_, p. 1, 1900. _Annal. d. Phys._ No. 10, p. 407, 1903. Footnote 22: Exner and Haschek, _Wien. Ber._ July 4, 1901. Footnote 23: Crookes, _Proc. Roy. Soc._ 72, p. 295, 1904. Footnote 24: Runge and Precht, _Annal. d. Phys._ XIV. 2, p. 418, 1904.