1. J. J. Thomson, Phil. Mag., 5, 346 (1903)
2. see also J. J. Thomson, The Electron in Chemistry, Franklin Institute Lectures, Chapman and Hall, London (1923).
3. W. Crookes (1879), quoted by S. Glasstone in Textbook of Physical Chemistry, Van Nostrand, New York (1940).
4. R. A. Millikan, Phys. Rev., 2, 143 (1913).
5. J. S. Townsend, Electricity in Gases (1879), quoted by Glasstone as in Ref. 2.