70.

If the process of irreversible adiabatic expansion of the radiation from the volume $V$ to the volume $V^{\prime }$ takes place as just described with the single difference that there is no carbon particle present in the vacuum, after the stationary state of radiation is established, as will be the case after a certain time on account of the diffuse reflection from the walls of the cavity, the radiation in the new volume $V^{\prime }$ will not any longer have the character of black radiation, and hence no definite temperature. Nevertheless the radiation, like every system in a definite physical state, has a definite entropy, which, according to the second principle, is larger than the original $S$, but not as large as the $S^{\prime }$ given in [eqn:(82)] (82). The calculation cannot be performed without the use of laws to be taken up later (see [sect:103.] Sec. 103). If a carbon particle is afterward introduced into the vacuum, absolutely stable equilibrium is established by a second irreversible process, and, the total energy as well as the total volume remaining constant, the radiation assumes the normal energy distribution of black radiation and the entropy increases to the maximum value $S^{\prime }$ given by [eqn:(82)] (82).