[21]. Runge, Astrophys. Journal, p. 1, 1900. Annal. d. Phys. No. 10, p. 407, 1903.
[22]. Exner and Haschek, Wien. Ber. July 4, 1901.
[23]. Crookes, Proc. Roy. Soc. 72, p. 295, 1904.
[24]. Runge and Precht, Annal. d. Phys. XIV. 2, p. 418, 1904.
[25]. Runge and Precht, Phil. Mag. April, 1903.
[26]. Watts, Phil. Mag. July, 1903; August, 1904.
[27]. Runge, Phil. Mag. December, 1903.
[28]. Debierne, C. R. 129, p. 593, 1899; 130, p. 206, 1900.
[29]. Giesel, Ber. d. D. Chem. Ges. p. 3608, 1902; p. 342, 1903.
[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.