At any rate, theoretical physics has in recent years made an advance of fundamental importance and widest reach in our knowledge of nature, in that it has come nearer to a knowledge of this cosmic ether, and has forced the question of its essence, its structure, and its motion into the foreground of monistic nature-philosophy. Only a few years ago the cosmic ether was to the majority of scientists an imponderable something, of which, strictly speaking, absolutely nothing was known, and which could be admitted provisionally only as a precarious working hypothesis. All this was changed when Heinrich Hertz (1888) demonstrated the nature of electrical energy, by his beautiful experiments establishing the conjecture of Faraday that light and heat, electricity and magnetism, are closely related phenomena of one single set of forces, and depend on transverse vibrations of the ether. Light itself--whatever else it be--is always and everywhere an electrical phenomenon. The ether itself is no longer hypothetical; its existence can at any moment be demonstrated by electrical and optical experiment. We know the length of the light wave and the electric wave. Indeed, some physicists believe that they can even determine approximately the density of ether. If by means of the airpump we remove from a bell-jar the atmospheric air (except an insignificant residue), the quantity of light within it remains unchanged; it is the vibrating ether we see.[9] These advances in our knowledge of the ether mean an immense gain for monistic philosophy. For they do away with the erroneous ideas of empty space and _actio in distans_; the whole of infinite space, in so far as it is not occupied by mass-atoms ("ponderable matter"), is filled by the ether. Our ideas of space and time are quite other than those taught by Kant a hundred years ago; the "critical" system of the great Koenigsberg philosopher exhibits in this respect, as well as in his teleological view of the organic world and in his metaphysics, dogmatic weaknesses of the most pronounced kind.[8] And religion itself, in its reasonable forms, can take over the ether theory as an article of faith, bringing into contradistinction the mobile cosmic ether as creating divinity, and the inert heavy mass as material of creation.[11] From this successfully scaled height of monistic knowledge there open up before our joyously quickened spirit of research and discovery new and surprising prospects, which promise to bring us still nearer to the solution of the one great riddle of the world. What is the relation of this light mobile cosmic ether to the heavy inert "mass," to the ponderable matter which we chemically investigate, and which we can only think of as constituted of atoms? Our modern analytical chemistry remains for the present at a standstill, in presence of some seventy irreducible elements, or so-called primary substances. But the reciprocal relation of these elements, the affinity of their combinations, their