der Waals theory did provide the basis of a satisfactory theory of liquids but this was not understood until recently. In any case, until the 1960’s the thinking was, since the density of a liquid is not too different from that of a solid, a theory of the solid state would be a promising starting point. Eyring, and others, developed the cell or lattice theory of liquids. In reality this is a classical (as opposed to quantum) theory of a solid, due to the higher temperatures of most liquids. Eyring, and probably others, realized that the entropy of the cell theory lacked a factor of Nk . Eyring coined the term, communal entropy , and added the missing entropy arbitrarily. Although arbitrary, this is preferable to ignoring the issue and does give a liquid a different free energy than a solid. He went one step further and developed the idea that when a molecule evaporated, it left a hole or vacancy in the quasi-lattice of the liquid. Thus, for every molecule in the vapor phase, there would be a vacancy in the liquid that mirrored the gas molecule. If this were literally true the sum of the densities of the liquid and vapor would be a constant, equal to the critical density. This is not quite correct. The average density of the two phases is a linear function of the temperature but is not a constant and decreases somewhat as the temperature increases. Nonetheless, this reasoning provides a simple qualitative explanation of the law of rectilinear diameters. He ‘formalized’ his reasoning into the significant structure ‘theory’ 10 , 11 at Utah. Using the idea that a liquid is a mixture of molecules and vacancies that mimic the vapor molecules, the partition function, Z , could be written as where Z$_{s}$ and Z$_{g}$ are the partition functions of the solid and vapor phases, respectively, and V and V$_{s}$ are the volumes of the liquid and solid phases, respectively. Eyring used the Einstein theory and ideal gas theory for Z$_{s}$ and Z$_{g}$ . The Einstein parameter, Θ$_{E}$, and V$_{s}$ are taken from experiment. The significant structure theory is a description rather than a theory. Conventionally, a theory in statistical mechanics relates the properties of a system to the forces between the molecules whereas Eyring’s description relates the properties of the liquid to those of the solid and vapor without obtaining either from the intermolecular forces. This said, Eyring by focussing on the volume on the important variable was on the right track and anticipated later developments, such as perturbation theory of liquids. One consequence of Eq. (5) is that the heat capacity, C , of monatomic liquid, such as Z = Z V s /V s Z ( V − V s ) /V g , (5) 7