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[12] As judged from a co-ordinate system moving with the body.

A direct comparison of this relation with experiment is not

possible at the present time (1920; see[Note], p. 48), owing to

the fact that the changes in energy E0 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.

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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.

[Note] The equation E = mc2 has been thoroughly proved time and

again since this time.