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 and the two laws of thermodynamics in its application on reversible processes. The fact that the same principle is also contained in the laws of moving cavity radiation, I have especially shown in the following (see below (II (12)). But the principle of least action is not sufficient for a complete foundation of the dynamics of ponderable bodies; because by itself it provides no replacement - which has proven above to be untenable and which should not be introduced here - for splitting the energy of a body in kinetic and internal energy. On the other hand, such a replacement is in prospect in a complete way for the introduction of another theorem: the "principle of relativity as expressed by H. A. and in the most general form by A.  . Although only a single direct confirmation of the validity of this principle, yet very important, is to be mentioned: the result of the experiments of Michelson and Morley, yet on the other hand no fact is known so far, directly preventing us to ascribe general and absolute accuracy to that principle. On the other hand, the principle proves to be so pervasive and fertile that a possible in depth investigation appears desirable, and this can obviously only be done by examining the consequences which it contains.

Following this consideration, I felt it a worthwhile task to develop the conclusions, leading to a combination of the principle of relativity with the principle of least action for any ponderable body. On that occasion some other views have been obtained, as well as some conclusions that may be accessible for direct experimental confirmation.

First Section. Dynamics of a moving black cavity radiation.
§ 1. The black-body radiation in pure vacuum is of all physical systems the only one, in which thermodynamic, electrodynamic and mechanical properties can be specified with absolute precision, independent of the opposition of special theories. Its treatment is therefore placed in front of the other systems. Imagine a radiation surrounded by vacuum and