what electricity is, any more than we shall know what energy is. What we shall be able probably to discover is, the relationship between electricity, magnetism, light, heat, gravitation, and the attracting force which manifests itself in chemical changes. We have one great guiding principle which, like the pillar of cloud by day, and the pillar of fire by night, will conduct us, as Moses and the Israelites were once conducted, to an eminence from which we can survey the promised scientific future. That principle is the conservation of energy. To-day we see clearly that there are not different kinds of forces; that light is not one thing and heat another; that, in truth, we should blot the word light from our physical text-books; that electricity and magnetism have their equivalents in heat, and heat in mechanical work. The ancients had a god for every great manifestation of Nature—a god of peace, a god of war, a god of the land, a god of the sea. Fifty years ago scientific men were like the ancients. There was a force attached to every phenomenon of Nature. Thus, there were the forces of electricity and magnetism, the vital forces, and the chemical forces. Now we accept treatises on mechanics which have the one word "Dynamik" for a title; and we look for a treatise on physics, which shall be entitled "Mechanical Philosophy," in which all the phenomena of radiant energy, together with the phenomena of energy, which we entitle electricity and magnetism, shall be discussed from the point of view of mechanics. It is true that Mascart and Joubert have entitled their treatise on electricity and magnetism "The Mechanical Theory of Electricity and Magnetism"; but what we are to have in the future is a treatise which will show the mechanical relations of gravitation, of so-called chemical attracting force and electrical attracting force, and the manifestations of what we call radiant energy.
When we survey the field of modern physics, we see that there is a marked tendency to simplify our conceptions. The question is sometimes asked. How shall the man of the future be able to make any advance, since it now takes one until middle age to gain familiarity with the vortex theories, with quaternions, and the more or less complicated mathematical analysis which characterizes our mechanical theory of electricity to-day? It is evident that much of our complicated scaffolding is to be taken down, and the student of electricity in the future will start with, perhaps, the laws of vortices as axioms, just as the student in physics to-day starts with the truth that the energy which we receive from the sun does not exist either as light or heat in the space between us and the sun, but may be electro-magnetic, or even in an unsuspected form; and that light and heat are merely manifestations of waves of energy differing only in length.
We have reduced our knowledge of electricity and magnetism to what may be called a mechanical system, so that, in a large number
