kinetic motion of a body from its internal state, it follows immediately that a constant with the properties of inertial mass may not exist. The reason for this lies again in the internal energy of thermal radiation which contributes to the inertia of a body in a small but determinable way, i.e., by a term which depends on the radiation density or temperature. If, however, we want to define mass rather by momentum than by kinetic energy, namely as the ratio of momentum by velocity, we obtain no different result. For according to the investigations of H.A. Lorentz, H. Poincaré and M. Abraham, the internal heat radiation of a moving body, as well as in general any electromagnetic radiation, has a certain finite momentum included in the whole momentum of the body. However, it depends (as the radiation energy) on temperature, and consequently also on the mass defined by it.
The alternative, which is to distinguish between "real" and "apparent" mass and to attribute constancy only to the former, represents the same facts only in a modified formulation. While the "real" mass would now remain constant, it loses on the other side its previous significance for kinetic energy and momentum.
After this consideration a third example immediately follows, namely the question of the identity of inertial and ponderable mass. The thermal radiation in a fully evacuated space, bounded by reflecting walls, surely has inertial mass; but has it also ponderable mass? If this question is to be denied, which surely should be the obvious choice, then it seems that the identity of inertial and ponderable mass, which was confirmed by all previous experiences and was generally accepted, must be abolished. We must not object that the inertia of black cavity radiation is imperceptibly small compared to that of the limiting material walls. On the contrary: by a sufficiently large cavity volume, the inertia of radiation can be made arbitrarily great against that of the walls. Such a freely moving cavity radiation, separated form outer space by thin rigid reflective walls, provides a good example of a rigid body, whose laws of motion completely differ from those of ordinary mechanics. Although it differs, considered superficially, in no way from other rigid bodies
