This monograph is an attempt to give an account of the foundations of thermodynamics which is more than usually rigorous, not only in its logical structure but also in the "rules of interpretation" in which physical meaning is assigned to the theoretical terms. It has not been my aim to discuss the applications of thermodynamics; only the fundamentals of the subject are treated, such examples as are discussed being introduced purely for illustrative purposes. Another important aspect of thermodynamics which is not treated in this book is its relation to statistical mechanics. Statistical mechanics provides a mechanical explanation of thermodynamic concepts and it might be supposed that the most logically satisfactory treatment of thermodynamics would be one based on statistical mechanics. However this book offers a formulation of pure thermodynamics, in which no such explanation is contemplated.