All the facts thus predicted by the theory are confirmed by experiment.  There is no known process which permits the direct measurement of the mass of an electron, but it is possible, as we have seen, to measure simultaneously its velocity and the relation of the electric charge to its mass.  In the case of the cathode rays emitted by radium, these measurements are particularly interesting, for the reason that the rays which compose a pencil of cathode rays are animated by very different speeds, as is shown by the size of the stain produced on a photographic plate by a pencil of them at first very constricted and subsequently dispersed by the action of an electric or magnetic field.  Professor Kaufmann has effected some very careful experiments by a method he terms the method of crossed spectra, which consists in superposing the deviations produced by a magnetic and an electric field respectively acting in directions at right angles one to another.  He has thus been enabled by working in vacuo to register the very different velocities which, starting in the case of certain rays from about seven-tenths of the velocity of light, attain in other cases to ninety-five hundredths of it.

It is thus noted that the ratio of charge to mass—­which for ordinary speeds is constant and equal to that already found by so many experiments—­diminishes slowly at first, and then very rapidly when the velocity of the ray increases and approaches that of light.  If we represent this variation by a curve, the shape of this curve inclines us to think that the ratio tends toward zero when the velocity tends towards that of light.

All the earlier experiments have led us to consider that the electric charge was the same for all electrons, and it can hardly be conceived that this charge can vary with the velocity.  For in order that the relation, of which one of the terms remains fixed, should vary, the other term necessarily cannot remain constant.  The experiments of Professor Kaufmann, therefore, confirm the previsions of Max Abraham’s theory:  the mass depends on the velocity, and increases indefinitely in proportion as this velocity approaches that of light.  These experiments, moreover, allow the numerical results of the calculation to be compared with the values measured.  This very satisfactory comparison shows that the apparent total mass is sensibly equal to the electromagnetic mass; the material mass of the electron is therefore nil, and the whole of its mass is electromagnetic.

Thus the electron must be looked upon as a simple electric charge devoid of matter.  Previous examination has led us to attribute to it a mass a thousand times less that that of the atom of hydrogen, and a more attentive study shows that this mass was fictitious.  The electromagnetic phenomena which are produced when the electron is set in motion or a change effected in its velocity, simply have the effect, as it were, of simulating inertia, and it is the inertia due to the charge which has caused us to be thus deluded.