The laws of thermodynamics are amongst the most assured and wide-ranging of all scientific laws. They do not pretend to explain any observation in molecular terms but, by showing the necessary relationships between different physical properties, they reduce otherwise disconnected results to compact order, and predict new effects. This classic title, first published in 1957, is a systematic exposition of principles, with examples of applications, especially to changes of places and the conditions for stability. In all this entropy is a key concept.