46.

We shall now add, without further proof, another general law of reciprocity, which is closely connected with that stated at the end of [sect:43.] Sec. 43 and which may be stated thus:

When any emitting and absorbing bodies are in the state of thermodynamic equilibrium, the part of the energy of definite color emitted by a body $A$, which is absorbed by another body $B$, is equal to the part of the energy of the same color emitted by $B$ which is absorbed by $A$.

Since a quantity of energy emitted causes a decrease of the heat of the body, and a quantity of energy absorbed an increase of the heat of the body, it is evident that, when thermodynamic equilibrium exists, any two bodies or elements of bodies selected at random exchange by radiation equal amounts of heat with each other.

Here, of course, care must be taken to distinguish between the radiation emitted and the total radiation which reaches one body from the other.