Although, however, monism is on the one hand for us an indispensable and fundamental conception in science, and although, on the other hand, it strives to carry back all phenomena, without exception, to the mechanism of the atom, we must nevertheless still admit that as yet we are by no means in a position to form any satisfactory conception of the exact nature of these atoms, and their relation to the general space-filling, universal ether. Chemistry long ago succeeded in reducing all the various natural substances to combinations of a relatively small number of elements; and the most recent advances of that science have now made it in the highest degree probable that these elements or the (as yet) irreducible primitive materials are themselves in turn only different combinations of a varying number of atoms of one single original element. But in all this we have not as yet obtained any further light as to the real nature of these original atoms or their primal energies. A number of the acutest thinkers have, so far in vain, endeavoured to grapple more closely with this fundamental problem of the philosophy of nature, and to determine more exactly the nature of atoms as well as their relation to the space-filling ether. And the idea steadily gains ground that no such thing as empty space exists, and that everywhere the primitive atoms of ponderable matter or heavy "mass" are separated from each other by the homogeneous ether which extends throughout all space. This extremely light and attenuated (if not imponderable) ether causes, by its vibrations, all the phenomena of light and heat, electricity and magnetism. We can imagine it either as a continuous substance occupying the space between the mass-atoms, or as composed of separate particles; in the latter case we might perhaps attribute to these ether-atoms an inherent power of repulsion in contrast to the immanent attracting power of the heavy mass-atoms, and the whole mechanism of cosmic life would then be reducible to the attraction of the latter and the repulsion of the former. We might also place the "vibrations of the cosmic ether" alongside of the "operation of space in general," in the sense in which these words are used by Professor Schlesinger.