We are now very far from the ideas which were once prevalent. Lavoisier taught us that matter can neither be created nor destroyed. It remains always the same. What he meant was that mass is invariable, as he proved by means of scales. Now it appears that, perhaps, bodies have no mass at all—if it is entirely of electro-magnetic origin—and that, in any case, mass is not invariable. This does not mean that Lavoisier’s law has now no meaning. There remains something that corresponds to mass at low velocities. Our idea of matter is, however, revolutionised. By matter we particularly meant mass, which seemed to us to be at once the most tangible and most enduring of its properties. Now this “mass” has no more reality than the time and space in which we thought we located it! Our solid realities were but phantoms. The reader must pardon me for whatever difficulties he finds in this exposition. The new mechanics opens out to us such strange new horizons that it is worth far more than a rapid and superficial glance. If you want to see a vast prospect in an unexplored world, you must not hesitate to do some rough climbing, however breathless it may leave you for the time. * * * * * There is, in fine, another fundamental idea of mechanics, that of _energy_, which takes on a new aspect in the light of Einstein’s theory: an aspect which, in turn, is largely justified by experiment. We saw that a body charged with electricity and in motion makes a certain resistance to interference, on account of the electrical inertia which is known as self-induction. Calculation and experiment show that, if we reduce the dimensions of a body that is charged with a certain quantity of electricity, without altering the charge, the electrical inertia increases. As a matter of fact, in our hypotheses, and if the inertia is entirely electro-magnetic in origin, the electrons are now merely a sort of electric trails moving in the propagating medium of electrical and luminous waves which we call ether. The electrons are no longer anything in themselves. They are merely, in the words of Poincaré, a sort of “holes in ether,” round which the ether presses much as a lake makes eddies which check the progress of a boat. In that case, however, the smaller the holes in the ether are, the more important will be the agitation of the ether round them; and, consequently, the greater will be the inertia of the “hole in ether” which represents the corpuscle under investigation. What will follow? We know from measurements we have made that the mass of the tiny sun of each atom, the _positive nucleus_, round which the planet-electrons revolve, is greater than that of an electron. If this mass and the corresponding inertia are electro-magnetic in origin, it follows that the positive nucleus of the atom is much smaller than the electron.