Rediscussion on gas-liquid phase transition

Liquid-gas phase transition in statistical mechanics is a long-standing dilemma not yet well explained. In this paper we propose a novel approach to this dilemma, by: 1). Putting forth a new space homogeneity assumption. 2). Giving a new formulation-- the mean distance expansion, instead of Mayer's Cluster Expansion, to calculate the intermolecular potential and partition function for a classical system. 3). Explaining how the separation of two phases occurs below the critical temperature T_{c} and what is the gap between two phases. 4). Calculating the physical quantities in a system of coexistent vapor and liquid, and comparing them with the experimental results. Qualitative, and some quantitative, consistencies are obtained. So the statistical explanation on first order liquid-gas phase transition is solved in principle.