Let us consider the atom of hydrogen, the lightest and simplest of the gases. We know that it consists of one planet only, one single negative electron revolving round the minute central sun, the positive nucleus. We know also that the mass of the electron is two thousand times as small as that of the hydrogen atom. It follows, as we can calculate, that the _positive nucleus_ must have a radius two thousand times smaller than that of the electron. Now, the experiments of the English physicists have proved that the large Alpha particles of the radium emanation can pass through hundreds of thousands of atoms without being appreciably diverted by the positive nucleus. We conclude that the latter is in reality much smaller than the electron, as theory predicted. All this irresistibly compels us to think that the inertia of the various component parts of atoms—that is to say, of all matter—is exclusively electro-magnetic in origin. There is now no matter. There is only electrical energy, which, by the reactions of the surrounding medium upon it, leads us to the fallacious belief in the existence of this substantial and massive something which hundreds of generations have been wont to call “matter.” And from all this it also follows, by calculation and by the simple and elegant reasoning of Einstein, of which I here convey only the faintest adumbration, that mass and energy are the same thing, or are at least the two different sides of one and the same coin. There is, then, no longer a material mass. There is nothing but energy in the external universe. A strange—in a sense, an almost spiritual—turn for modern physics to take! According to all this the greater part of the “mass” of bodies must be due to a considerable and concealed internal energy. It is this energy which we find gradually dissipated in radio-active bodies, the only reservoirs of atomic energy which have as yet opened externally. If this is true, if energy and mass are synonymous, if mass is merely energy, it follows that free energy must possess the property of mass. As a matter of fact, light, for instance, has mass. Careful experiments have shown that when a ray of light strikes a material object, it exerts upon it a pressure which has been measured. Light has mass; therefore it has weight, like all masses. When we come to consider the new form given by Einstein to the problem of gravitation, we shall see a further and beautiful proof that light has weight. We can calculate that the light received from the sun by the earth in the space of a year is rather more than 58,000 tons. It seems very little when one thinks of the formidable weight of coal that would be needed to maintain our globe at the temperature at which the sun keeps it—in the event of a sudden extinction of our luminary.