This is the seventh edition of a book that was first published by Professor Klotz in 1950. He died while we were preparing this edition, and it is dedicated to his memory. Many friends have asked why a new edition of a thermodynamics text is necessary, because the subject has not changed basically since the work of J. Willard Gibbs. One answer is given by the statement of Lord Rayleigh in a letter to Gibbs, The original version, though now attracting the attention it deserves, is too condensed and too difficult for most, I might say all, readers. This statement follows a request for Gibbs to prepare a new edition of, or a treatise founded on, the original. Those of us who still have difficulty with Gibbs are in good company. Planck wrote his famous textbook on thermodynamics independently of Gibbs, but subsequent authors were trying to make the work of Gibbs more easily understood than the Gibbs original. Similarly, each new edition of an established text tries to improve its pedagogical methods and bring itself up to date with recent developments or applications. This is the case with this edition