||In view of the large number of books on thermodynamics it may seem surprising that there should be any need for yet another. A cursory survey of all the existing books will however show that only very few are at all comparable. The total number is considerably reduced if we reject the ones which G. N. Lewis so aptly described as containing 'cyclical processes limping about eccentric and not quite completed cycles' and consider only those which present thermodynamics as an exact science. Many of these, including some of the best, are out of date. No book written before 1929 even attempts an account of any of the following matters: the modern definition of heat given by Born in 1921; the quantal theory of the entropy of gases and its experimental verification; Debye's formulae for the activity coefficients of electrolytes; the use of electrochemical potentials of ions; the application of thermodynamics to dielectrics and to paramagnetic substances. The first textbook on thermodynamics to include any of these matters is that of Schottky published in 1929. The number of textbooks on thermodynamics written since then is in single figures and of these fewer than half a dozen are in English. The only two available bearing any appreciable resemblance to this book are Zemansky's 'Heat and Thermodynamics' and Macdougall's 'Thermodynamics and Chemistry'. I have a great admiration for both these books, but they are quite different from each other and from this book. Zemansky's book is supremely good on the fundamentals of thermodynamics and should be equally useful to physicists, chemists and engineers. It includes especially thorough discussions on the meaning of heat, on calo-rimetry, on thermometry, on steam engines and on refrigerators. On the other hand there are important applications to physical chemistry, such as solutions, interfaces, electrochemistry, the third principle, entropy constants which are dealt with sketchily or not at all. Macdougall's book on the other hand is, as its title indicates, devoted mainly to applications of thermodynamics to chemistry. Less attention has been paid to a logical formulation of the fundamental principles and there is no application to dielectrics or to paramagnetics.