  The thermodynamical properties of a quantized electromagnetic field inside a
box with perfectly conducting walls are studied using a regularization scheme
that permits to obtain finite expressions for the thermodynamic potentials. The
source of ultraviolet divergences is directly isolated in the expression for
the density of modes, and the logarithmic infrared divergences are regularized
imposing the uniqueness of vacuum and, consequently, the vanishing of the
entropy in the limit of zero temperature. We thus obtain corrections to the
Casimir energy and pressures, and to the specific heat that are due to
temperature effects; these results suggest effects that could be tested
experimentally.
