We shall see later (chapter IV.) that similar effects are produced when the rays from radio-active substances impinge upon solid bodies.
In this chapter an account of the ionization theory of gases has been given to the extent that is necessary for the interpretation of the measurements of radio-activity by the electric method. It would be out of place here to discuss the development of that theory in detail, to explain the passage of electricity through flames and vapours, the discharge of electricity from hot bodies, and the very complicated phenomena observed in the passage of electricity through a vacuum tube.
For further information on this important subject, the reader is referred to J. J. Thomson's Conduction of Electricity through Gases, in which the whole subject is treated in a full and complete manner. A simple account of the effect of moving charges and the electronic theory of matter was given by the same author in the Silliman Lectures of Yale University and published under the title Electricity and Matter (Scribner, New York, 1904).
