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of its energy. The law of the conservation of the mass of a system becomes identical with the law of the conservation of energy, and is only valid provided that the system neither takes up nor sends out energy. Writing the expression for the energy in the form

${\frac {mc^{2}+E_{0}}{\sqrt {1-{\tfrac {v^{2}}{c^{2}}}}}}$,

we see that the term $mc^{2}$, which has hitherto attracted our attention, is nothing else than the energy possessed by the body^[1] before it absorbed the energy $E_{0}$. A direct comparison of this relation with experiment is not possible at the present time, owing to the fact that the changes in energy $E_{0}$, to which we can subject a system are not large enough to make themselves perceptible as a change in the inertial mass of the system. ${\tfrac {E_{0}}{c^{2}}}$is too small in comparison with the mass $m$, which was present before the alteration of the energy. It is owing to this circumstance that classical mechanics was able to establish successfully the conservation of mass as a law of independent validity.

Let me add a final remark of a fundamental nature. The success of the Faraday-Maxwell

1. As judged from a co-ordinate system moving with the body.