The first edition of this text established a new conceptual structure for thermodynamics. This now classical structure is maintained in the present edition which includes a simple descriptive account of recent advances in critical phenomena and a fully compatible but logically distinct introduction to statistical mechanics. The text concludes with an interpretive chapter on the conceptual roots of thermostatistics. Many examples are explicitly solved, and answers to many problems are given. The previous edition of this book was published in 1966.