  We study the Casimir effect with different temperatures between the plates
($T$) resp. outside of them ($T'$). If we consider the inner system as the
black body radiation for a special geometry, then contrary to common belief the
temperature approaches a constant value for vanishing volume during isentropic
processes. This means: the reduction of the degrees of freedom can not be
compensated by a concentration of the energy during an adiabatic contraction of
the two-plate system. Looking at the Casimir pressure, we find one unstable
equilibrium point for isothermal processes with $T > T'$. For isentropic
processes there is additionally one stable equilibrium point for larger values
of the distances between the two plates.}
