65.

The magnitude of the entropy $S$ of black radiation found by integration of the differential equation [eqn:(73)] (73) is $S=\frac{4}{3}a{T}^{3}V.\mathrm{\backslash Label}[eqn](80)\text{upshape (80)}$

In this equation the additive constant is determined by a choice that readily suggests itself, so that at the zero of the absolute scale of temperature, that is to say, when $u$ vanishes, $S$ shall become zero. From this the entropy of unit volume or the volume density of the entropy of black radiation is obtained, $\frac{S}{V}=s=\frac{4}{3}a{T}^3.\mathrm{\backslash Label}[eqn](81)\text{upshape (81)}$