negative electrical masses constituting the electron would
necessarily be scattered under the influence of their mutual repulsions, unless there are forces of another kind operating between them, the nature of which has
hitherto remained obscure to us.The general theory of relativity renders it likely that the electrical masses of an electron are held together by gravitational
forces. If we now assume that the relative distances between the electrical masses constituting the electron remain unchanged during the motion of the electron (rigid connection in the sense of classical mechanics), we arrive at a law of motion of the electron which does not agree with experience. Guided by purely formal points of view, H. A. Lorentz was the first to introduce the hypothesis that the particles constituting the electron experience a contraction in the direction of motion in consequence of that motion, the amount of this contraction being proportional to the expression . This hypothesis, which is not justifiable by any electrodynamical facts, supplies us then with that particular law of motion which has been confirmed with great precision in recent years.
The theory of relativity leads to the same law of motion, without requiring any special hypothesis whatsoever as to the structure and the behaviour of the electron. We arrived at a similar conclusion in [chapter:XIII]Section XIII in connection with the experiment of Fizeau, the
result of which is foretold by the theory of relativity without the necessity of drawing on hypotheses as to the physical nature of the liquid.
The second class of facts to which we have alluded has reference to the question whether or not the motion