The following presentation of Theoretical Chemistry is a development of an "introduction" prepared by the writer two years ago for the Handbook der Anorganischen Cliemie (Handbook of Inorganic Cliemidry), edited by Dr. O. Bammer (vol. i. pp. 1 to 358). In keeping with the broader requirements of an independent text-book — and not merely an introduction to a special work—it has been considerably rearranged and extended. It aims to consider the very latest investigation, and while this requires no change in the fundamental nature of the more recent theories, yet it implies a surprising development of them.