Critical and co-operative phenomena. V. Specific heats of solids and liquids

In the first two papers in this series (Lennard-Jones and Devonshire 1937-8) we developed a simple method of calculating the free energy of a dense gas or a liquid in terms of interatomic forces. We used this to calculate critical temperatures and also vapour pressures and boiling-points. In later papers (Lennard-Jones and Devonshire 1939) we showed that the model used in the earlier papers was more appropriate to a solid than to a liquid, and that to obtain a satisfactory theory for a liquid we must modify it by introducing the concept of disorder. In this way we were able to account satisfactorily for the phenomenon of melting. In this paper we propose to use the expression for the free energy obtained in the earlier papers to calculate the specific heats of solids and liquids, and also the coefficients of thermal expansion and compressibilities. As before, we confine ourselves to the case when quantum effects are negligible.