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system. For another system the energy flows in general through the surface. (We find an instance for this fact in the outer surface of the body, mentioned in the last paragraph but one, which is in rest relatively to . For no energy current passes through the surface, it does, however, in and . But this representation fails altogether when absorption takes place, because then inside such a surface the energy would gradually diminish indefinitely. Therefore it seems to me that no great importance can be attributed to the conception of the velocity of the energy.
Institute for theoretical physics.