Strutt, Phil. Mag. June, 1903. Nature, Feb. 19, 1903.

McLennan and Burton, Phys. Rev. No. 4, 1903. J. J. Thomson, Nature, Feb. 26, 1903.

Cooke, Phil. Mag. Aug. 6, 1903. Rutherford, Nature, April 2, 1903.

Eve, Nature, March 16, 1905.

See article in Le Radium, No. 3, p. 81, Sept. 15, 1904.

J. J. Thomson, Proc. Camb. Phil. Soc. 12, p. 391, 1904.

Wood, Phil. Mag. April, 1905.

Campbell, Nature, p. 511, March 31, 1904. Phil. Mag. April, 1905.

An apparent exception has been observed by Danne in the case of certain lead minerals which occur under peculiar conditions at d’Issy-l’Évêque, France. See p. 465.