  Previously, we established a connection between the macroscopic classical
laws of gases and the quantum mechanical description of molecules of an ideal
gas (T. Yarman et al. arXiv:0805.4494). In such a gas, the motion of each
molecule can be considered independently on all other molecules, and thus the
macroscopic parameters of the ideal gas, like pressure P and temperature T, can
be introduced as a result of simple averaging over all individual motions of
the molecules. It was shown that for an ideal gas enclosed in a macroscopic
cubic box of volume V, the constant, arising along with the classical law of
adiabatic expansion, i.e. PV5/3=constant, can be explicitly derived based on
quantum mechanics, so that the constant comes to be proportional to h^2/m; here
h is the Planck Constant, and m is the relativistic mass of the molecule the
gas is made of. In this article we show that the same holds for a photon gas,
although the related setup is quite different than the previous ideal gas
setup. At any rate, we come out with PV5/3 hc=constant, where c is the speed of
light. No matter what the dimensions of the constants in question are different
from each other, they are still rooted to universal constants, more
specifically to h2 and to hc, respectively; their ratio, i.e. V1/3 h/mc,
interestingly pointing to the de Broglie relationship's cast.
