amount of energy E[0], then its inertial mass increases by an amount

eq. 22: file eq22.gif


the inertial mass of a body is not a constant but varies according to
the change in the energy of the body. The inertial mass of a system of
bodies can even be regarded as a measure 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

eq. 23: file eq23.gif

we see that the term mc^2, which has hitherto attracted our attention,
is nothing else than the energy possessed by the body ** before it
absorbed the energy E[0].

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 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.

eq. 22: file eq22.gif


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