In earlier papers – already cited multiple times – I have tried to calculate the energy content and apparent mass of a moving cavity. There I assumed isotropic distribution (in all directions) of true radiation. A contradiction with the second thermodynamic main-theorem arising at that occasion, could be solved by two different hypotheses. Either by the assumption of a change of the emission capability of the black body, or by the hypothesis of a change of the dimensions of matter due to motion. I have confined myself to the study of the latter hypothesis, though I have explicitly emphasized the possibility of the first assumption.[1] The isotropy of the distribution of true radiation was assumed by me, so that the mutual radiation of two elements is equal. If one employs Lorentz's contraction hypothesis, as it was done by me, then this assumption must of course be modified, so that the isotropy of the true radiation is related to the contracted system. This modification was indicated by me,[2] though I have confined myself to note, that the term of the apparent mass which is independent of velocity is not affected by this. The reason for this was, that a way of calculating the emission capability of the black body as a function of velocity was unknown to me at this time, thus I had to confine myself to the first term of the corresponding expansions. Since then, this gap was filled by the work of v. Mosengeil. By that and the fortunate thought of this author, to assume a change of temperature with velocity, it became possible to definitely determine the processes in a moving cavity. Based on the concept of true radiation, I did this in § 9 of the present paper; then one comes to the same result as v. Mosengeil.
