229.  The general method of regarding the subject would be unchanged, even if it were proved that the radio-activity of thorium is not due to thorium at all but to a small constant amount of a radio-active impurity mixed with it.

230.  Rutherford and Soddy, Phil. Mag. Sept. 1902.

231.  Owens, Phil. Mag. p. 360, Oct. 1899.

232.  Rutherford, Phil. Mag. p. 1, Jan. 1900.

233.  Rossignol and Gimingham, Phil. Mag. July, 1904.

234.  Bronson, Amer. Journ. Science, Feb. 1905.

235.  Phil. Mag. April, 1904.

236.  Dorn, Abh. der. Naturforsch. Ges. für Halle-a-S., 1900.

237.  P. Curie, C. R. 135, p. 857, 1902.

238.  Rutherford and Soddy, Phil. Mag. April, 1903.