Statistical mechanics forms the basis of the study of the properties of matter in bulk from a molecular or microscopic viewpoint. This 1966 edition of the late Sir Ralph Fowler's monumental exposition of the subject was one of the first to adopt a quantum mechanical viewpoint, and it rapidly became recognised as the classic work on the subject. The fundamental principles of statistical mechanics are treated in the first few chapters and the rest of the book contains comprehensive accounts of their application to a wide range of branches of chemistry and physics: thermionics, electric and magnetic effects, the theory of metals, application to liquids and solutions, to atmospheric problems, stellar interiors and chemical kinetics. Detailed comparisons between theoretical predictions and experimental data are given throughout. This 1966 edition of the late Sir Ralph Fowler's exposition of statistical mechanics was one of the first to adopt a quantum mechanical viewpoint.