65.  Helmholtz and Richarz, Annal. d. Phys. 40, p. 161, 1890.

66.  Wilson, Phil. Trans. p. 265, 1897; p. 403, 1899; p. 289, 1900.

67.  Thomson, Phil. Mag. p. 528, Dec. 1898.

68.  Wilson, Phil. Trans. A, 193, p. 289, 1899.

69.  Thomson, Phil. Mag. p. 528, Dec. 1898, and March, 1903. Conduction of Electricity through Gases, Camb. Univ. Press, 1903, p. 121.

70.  Wilson, Phil. Mag. April, 1903.

71.  Townsend, Phil. Trans. A, p. 129, 1899.

72.  Townsend, loc. cit. p. 139.

73.  Some difference of opinion has been expressed as to the value of V required to produce ions at each collision. Townsend considers it to be about 20 volts; Langevin 60 volts and Stark about 50 volts.

74.  Rutherford, Phil. Mag. Jan. 1899.

75.  Rutherford, Phil. Mag. Jan. 1899.

76.  Strutt, Phil. Trans. A, p. 507, 1901 and Proc. Roy. Soc. p. 208, 1903.

77.  McClung, Phil. Mag. Sept. 1904.

78.  Eve, Phil. Mag. Dec. 1904.

79.  Rutherford, Phil. Mag. p. 137, Jan. 1899.