  This article is a continuation of our previous works (see Yukhnovskii I.R. et
al., J. Stat. Phys, 1995, 80, 405 and references therein), where we have
described the behavior of a simple system of interacting particles in the
region of temperatures at and about the critical point, T \geqslant T_{c}. Now
we present a description of the behavior of the system at the critical point
(T_{c}, \eta_{c}) and in the region below the critical point. The calculation
is carried out from the first principles. The expression for the grand
canonical partition function is brought to the functional integrals defined on
the set of collective variables. The Ising-like form is singled out. Below
T_{c}, when a gas-liquid system undergoes a phase transition of the first
order, i.e., boiling, a "jump" occurs from the "extreme" high probability gas
state to the "extreme" high probability liquid state, releasing or absorbing
the latent heat of the transition. The phase equilibria conditions are also
derived.
