All too easily, a book on thermal physics leaves the impression of a collection of applications: blackbody radiation, Debye theory, conduction electrons, van der Waals equation, and so on. Those applications are vital; they connect the theoretical and experimental worlds. Yet even more important is to come away from the book with a sense of its underlying theoretical structure. What is its equivalent of the equation F = ma in a mechanics text? Or of Maxwell's equations in a book on electromagnetism?

In this book, the key theoretical ideas are the Second Law of Thermodynamics, the canonical probability distribution, the partition function, and the chemical potential. Of these, the Second Law of Thermodynamics comes first logically and is most nearly the central organizing principle. The majority of the applications, however, found us using the latter three items: P(Ψj) = (1/Z)exp(−Ej/kT), Z, or µ. By comparison with mechanics or electromagnetism, thermal physics suffers in that its most basic principle, the Second Law of Thermodynamics, is separated by layers of secondary theory from the physical applications. Perhaps that reflects the inherent difficulty of coping with 1020 particles all at once, or perhaps the root lies in the diversity of topics that can be addressed.