  The viability of the variable generalised Chaplygin gas (VGCG) model is
analysed from the standpoint of its thermodynamical stability criteria with the
help of an equation of state, $P = - \frac{B}{\rho^{\alpha}}$, where $B =
B_{0}V^{-\frac{n}{3}}$. Here $B_{0}$ is assumed to be a positive universal
constant, $n$ is a constant parameter and $V$ is the volume of the cosmic
fluid. We get the interesting result that if the well-known stability
conditions of a fluid is adhered to, the values of $n$ are constrained to be
negative definite to make $ \left(\frac{\partial P}{\partial V}\right)_{S} <0$
\& $ \left(\frac{\partial P}{\partial V}\right)_{T} <0$ throughout the
evolution. Moreover the positivity of thermal capacity at constant volume
$c_{V}$ as also the validity of the third law of thermodynamics are ensured in
this case. For the particular case $n = 0$ the effective equation of state
reduces to $\Lambda$CDM model in the late stage of the universe while for $n
<0$ it mimics a phantom-like cosmology which is in broad agreement with the
present SNe Ia constraints like VGCG model. The thermal equation of state is
discussed and the EoS parameter is found to be an explicit function of
temperature only. Further for large volume the thermal equation of state
parameter is identical with the caloric equation of state parameter when $ T
\rightarrow 0$. It may also be mentioned that like Santos et al our model does
not admit of any critical points. We also observe that although the earlier
model of Lu explains many of the current observational findings of different
probes it fails to explain the crucial tests of thermodynamical stability.
