Casimir's entropy

The classical `Kirchhoff's theorem' (the energy density of the radiation at equilibrium at high temperature, T, is a function of T only) is used to obtain the Casimir energy at zero temperature without recourse to regularization. The validity of `Kirchhoff's theorem' at the high-temperature limit for the case at hand is confirmed. The Casimir entropy is defined and its temperature dependence is displayed. The Casimir entropy at high temperatures is shown to approach a positive geometry-dependent but temperature-independent constant.