97.

We may go one step further, and, from the entropy $𝗌$ and the temperature $T$ of an unpolarized monochromatic radiation which is uniform in all directions, draw a certain conclusion regarding the entropy and temperature of a single, plane polarized, monochromatic pencil. That every separate pencil also has a certain entropy follows by the second principle of thermodynamics from the phenomenon of emission. For since, by the act of emission, heat is changed into radiant heat, the entropy of the emitting body decreases during emission, and, along with this decrease, there must be, according to the principle of increase of the total entropy, an increase in a different form of entropy as a compensation. This can only be due to the energy of the emitted radiation. Hence every separate, plane polarized, monochromatic pencil has its definite entropy, which can depend only on its energy and frequency and which is propagated and spreads into space with it. We thus gain the idea of entropy radiation, which is measured, as in the analogous case of energy radiation, by the amount of entropy which passes in unit time through unit area in a definite direction. Hence statements, exactly similar to those made in [sect:14.] Sec. 14 regarding energy radiation, will hold for the radiation of entropy, inasmuch as every pencil possesses and conveys, not only its energy, but also its entropy. Referring the reader to the discussions of [sect:14.] Sec. 14, we shall, for the present, merely enumerate the most important laws for future use.