Theories of the liquid state

A critical discussion is given of the various theories of the liquid state, which give an explanation of the liquid properties in terms of the intermolecular forces. In § 2 the general properties of the equation of state of monatomic liquids and the melting and vapourpressure curve are given using the principle of corresponding states with molecular units. In § 3 these experimental data are compared with the theory of Lennard-Jones and Devonshire. The influence of a smaller co-ordination number is investigated in § 4, and in § 5 the difficulties in explaining the additive constant in the vapour pressure curve is discussed. The generalizations of the theory of Lennard-Jones and Devonshire, discussed recently also by Rowlinson and Curtiss, are given in a somewhat simplified representation, showing that the decrease of the co-ordination number can be obtained as a straightforward result of the application of statistical mechanics to the ‘lattice model’ of the liquid state. The transition of the solid to the liquid phase is discussed in § 7, starting with Lennard-Jones and Devonshire’s theory of melting. It is shown that the disordered ‘liquid’ state corresponds to a liquid with a co-ordination number 9, and that an explanation can be given of the melting-point formula of Simon. Finally, in § 8 attempts are discussed to base a theory of liquids on a calculation of the molecular distribution function.