Hartmann, _Phys. Zeit._ 5, No. 18, p. 570, 1904. Footnote 195: In a recent paper, Giesel (_Ber. d. D. Chem. Ges._ No. 3, p. 775, 1905) has shown that the bright lines are due to didymium, which is present as an impurity. Exposure of didymium to the radium rays also causes the appearance of the lines. Footnote 196: Wiedemann and Schmidt, _Wied. Annal._ 59, p. 604, 1895. Footnote 197: Wiedemann, _Phys. Zeit._ 2, p. 269, 1901. Footnote 198: Elster and Geitel, _Annal. d. Phys._ 69, p. 673, 1899. Footnote 199: Willows and Peck (_Phil. Mag._ March, 1905) found that under some conditions, especially for long sparks, the rays of radium hindered the passage of the spark. Footnote 200: Hemptinne, _C. R._ 133, p. 934, 1901. Footnote 201: Himstedt, _Phys. Zeit._ p. 476, 1900. Footnote 202: Henning, _Annal. d. Phys._ p. 562, 1902. Footnote 203: Kohlrausch and Henning, _Verh. Deutsch. Phys. Ges._ 6, p. 144, 1904. Footnote 204: Kohlrausch, _Verh. Deutsch. Phys. Ges._ 5, p. 261, 1904. Footnote 205: P. Curie, _C. R._ 134, p. 420, 1902. Footnote 206: Becquerel, _C. R._ 136, p. 1173, 1903. Footnote 207: Becquerel, _C. R._ 133, p. 199, 1901. Footnote 208: P. Curie, Société de Physique, March 2, 1900. Footnote 209: Joly, _Phil. Mag._ March, 1904. Footnote 210: S. and P. Curie, _C. R._ 129, p. 823, 1899. Footnote 211: Giesel, _Verhandlg. d. D. Phys. Ges._ Jan. 5, 1900. Footnote 212: Salomonsen and Dreyer, _C. R._ 139, p. 533, 1904. Footnote 213: Elster and Geitel, _Phys. Zeit._ p. 113, No. 3, 1902. Footnote 214: Becquerel, _C. R._ 133, p. 709, 1901. Footnote 215: Hardy and Miss Willcock, _Proc. Roy. Soc._ 72, p. 200, 1903. Footnote 216: Hardy, _Proc. Physiolog. Soc._ May 16, 1903. Footnote 217: Whetham, _Phil. Mag._ Nov. 1899; _Theory of Solution_, Camb. 1902, p. 396. Footnote 218: Curie and Debierne, _C. R._ 132, p. 768, 1901. Footnote 219: Giesel, _Ber. D. d. Chem. Ges._ 35, p. 3605, 1902. Footnote 220: Ramsay and Soddy, _Proc. Roy. Soc._ 72, p. 204, 1903. Footnote 221: Danysz, _C. R._ 136, p. 461, 1903. Footnote 222: Aschkinass and Caspari, _Arch. d. Ges. Physiologie_, 86, p. 603, 1901. Footnote 223: Himstedt and Nagel, _Drude’s Annal._ 4, p. 537, 1901. Footnote 224: Hardy and Anderson, _Proc. Roy. Soc._ 72, p. 393, 1903. CHAPTER VI. CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER.