  The recently discovered scaling law for the relaxation times, tau=f(T,V^g),
where T is temperature and V the specific volume, is derived by a revision of
the entropy model of the glass transition dynamics originally proposed by
Avramov [I. Avramov, J. Non-Cryst. Solids 262, 258 (2000).]. In this
modification the entropy is calculated by an alternative route, while retaining
the approximation that the heat capacity is constant with T and P. The
resulting expression for the variation of the relaxation time with T and V is
shown to accurately fit experimental data for several glass-forming liquids and
polymers over an extended range encompassing the dynamic crossover. From this
analysis, which is valid for any model in which the relaxation time is a
function of the entropy. we find that the scaling exponent g can be identified
with the Gruneisen constant.
