IT is well known that a knowledge of the second virial coefficient of a gas does not suffice to specify the intermolecular potential of the constituent molecules. In fact, even if a simple formula of Mie–Lennard-Jones type is used as an approximation to the potential, the parameters are not uniquely determined by the second virial coefficient. On the other hand (as mentioned in the preceding communication) certain elementary properties of the solid state are sufficient to determine these parameters uniquely, and further observations on the solid and gaseous states can then be used as a test of the region of validity of this type of approximation to the potential. The recent development of experimental technique enabling data on the solid state to be determined with accuracy over a wide range of pressures and temperatures has provided valuable material for theoretical analysis.