Professor Buchdahl presents a systematic exposition of classical thermodynamics, against a background of general physical theory and on a purely phenomenological (i.e. non-statistical) level. Although particular attention is paid to the meaning of the various concepts introduced, Professor Buchdahl is not afraid of making simplifications where these are likely to enhance the reader's understanding of the subject and the relationships between the principal and ancillary laws. The emphasis throughout is on meaning and physical significance. Specific applications of the general theory are discussed in two final chapters. This book, first published in 1966, is intended for the student who has taken a first course in analytical, though not axiomatic, development of the subject. It will supplement rather than replace, the many familiar introductory treatments of thermodynamics.