  This paper addresses the interplay between vacuum and thermal local averages
for massless scalar radiation near a plane wall of a large cavity where the
Dirichlet boundary condition is assumed to hold. The main result is that stable
thermodynamic equilibrium is possible only if the curvature coupling parameter
is restricted to a certain range. In more than three spacetime dimensions such
a range contains the conformal coupling, but it does not contain the minimal
coupling. Since this same range for possible values of the curvature coupling
parameter also applies to massive scalar radiation, it may be relevant in
settings where arbitrarily coupled scalar fields are present.
