  An isospin lattice-gas model, which is a spin-1 Ising model, is employed to
investigate the liquid-gas phase transition in asymmetric nuclear matter. We
consider nuclear matter as a lattice where each lattice site can be either
empty or occupied by a proton or a neutron, with a nearest-neighbor interaction
among the nucleons. With the Bragg-Williams mean field approximation, we
calculate various thermodynamic properties of nuclear matter for different
densities and different proton-neutron asymmetry parameter $s$. Our model
exhibits liquid-gas phase transition below a critical temperature $T_c$, and
predicts a monotonic decreasing of $T_c$ as the magnitude of $s$ is increased.
The dependence of the nuclear matter isotherms on the asymmetry parameter $s$
is discussed.
