This book can be described as a student's edition of the author's Dynamical Theory of Gases. It is written, however, with the needs of the student of physics and physical chemistry in mind, and those parts of which the interest was mainly mathematical have been discarded. This does not mean that the book contains no serious mathematical discussion; the discussion in particular of the distribution law is quite detailed; but in the main the mathematics is concerned with the discussion of particular phenomena rather than with the discussion of fundamentals.