Incidentally, also the exact values of the apparent mass (see equation (29)) show in both cases similarities in their form; however, they are not identical.
It is to be mentioned that we have tacitly assumed, that the quantity and thus is not (explicitly) dependent from the value of the translatory velocity; we will come back later to this.
For providing the values of quantity and thus the apparent mass just introduced, it was necessary to know the quantities and . Yet also without it, one can conclude the existence of them from expression (19); furthermore one immediately recognizes, that this expression must be proportional to in first approximation.
Since the heat content of every body partly consists of radiating heat, the things that we have demonstrated at a cavity, are true mutatis mutandis for every body whose temperature is different from 0° A.. In particular, every body must have an apparent mass determined by the inner radiation, and which is therefore above all dependent on the temperature.
The velocity changes just considered, and thus the state of radiation in our cavity, can be denoted as isothermic ones, since space was always in connection with the bodies and , whose heat capacity was assumed by us as infinitely great against that of space itself.
If we want to study the process at "adiabatic" changes, then it would be the most simple way, to consider a space filled by radiation energy of certain amount and which is enclosed at all sides by mirrors, and then giving to it different velocities. Though the execution of this thought gives the result, that radiation states are formed in this case, which are essentially different from the ones considered earlier, and which we can denote as unstable; because at the slightest change of the surface constitution of the boundary surfaces, the radiation state immediately becomes different, while the
