Physical Chemistry is a difficult and diversified subject. Based on a good long spell of university teaching, this book lays emphasis on the structure and continuity of the whole subject and tries to show the relation of its various parts to one another. Certain themes or, one might almost say, leitmotifs run through physical chemistry, and these have been used to unify the composition. The treatment is neither historical nor formally deductive, but at each stage the author tries to indicate the route by which an inquiring mind might most simply and naturally proceed in its attempt to understand that part of the nature of things included in physical chemistry.