With the discovery and study of Cathode rays, Rontgen rays and Radio-activity a new era has begun in Physics, in which the electrical properties of gases have played and will play a most important part; the bearing of these discoveries on the problems of the Constitution of Matter and the Nature of Electricity is in most intimate connection with the view we take Of the processes which go on when electricity passes through a gas. I have endeavoured to show that the view taken in this volume is supported by a large amount of direct evidence and that it affords a direct and simple explanation of the electrical properties Of gases. The pressure of my other duties has caused this book to be a considerable time in passing through the press, and some important investigations have been published since the sheets relating to the subjects investigated were struck off. I have given a short account of these in a few Supplementary Notes.
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