The necessity for a revision of dynamical theory.

The decision between the two theories of the electron is not so important as the result, about which there is no doubt, namely that it is impossible to deal with the inertia of the electron as if it were represented by a simple Neiutonian mass.

We are not necessarily bound to admit the conclusion of Abraham and Kaufmann in 1902 that “the mass of the electron is of purely electromagnetic origin “—this might be only approximately true, and we have seen too that pure electromagnetic theory is not sufficient to determine the motion of an electron. But we have at least a glimpse of a region in which Newtonian dynamics is not fundamental or universal Further, with the growth of our knowledge of the complexity of the material atom, it seems possible that the sum of the variable parts of the masses of the electrons within an atom is comparable with the whole mass of the atom, and indeed that the same may be true of an extended material body. Without admitting the ambitious programme of the completely electromagnetic constitution of matter, it must at once be recognized that the mass of a material body becomes a statistical quantity depending partly on the motions of the multitude of electrons within it, and that only the fact of the smallness of the relative velocities of all bodies of which ordinary dynamics treat, compared with that of light, has made it possible to think for so long of the constant mass-ratio of two bodies as a universal property of matter.