  A new theory on gas-liquid phase transition is given. The new idea is that
the total intermolecular potential energy for a classical system in equilibrium
is relative with the average distance of molecules. A new space homogeneity
assumption is postulated, a new formulation -- the mean distance expansion,
instead of Mayer's Cluster Expansion, is introduced, a new explanation on
liquid-gas phase transition is given, and the physical quantities in a system
of coexistent vapor and liquid -- the densities of the coexistent vapor and
liquid, the vapor pressure and the latent heat, are calculated.
