From the preceding chapters we are now able to realise what are the ultimate elements of the material universe, and it remains to show how they are put together. The elements are ether, energy, and matter. First, ether: a universal, all-pervading medium, imponderable or infinitely light, and almost infinitely elastic, in which all matter, from suns and planets down to molecules and atoms, float as in a boundless ocean, and whose tremors or vibrations, propagated as waves, transport the different forms of energy, light, heat, and electricity, across space. Secondly, energy: a primitive, indestructible something, which causes motion and manifests itself under its many diversified forms, such as gravity, mechanical work, molecular and atomic forces, light, heat, electricity, and magnetism, all of which are merely Protean transformations of the one fundamental energy, and convertible into each other. Thirdly, matter: the ultimate elements of this are atoms, which combined form molecules, or little pieces of ordinary matter with all its qualities, which are the bricks used in building all the varied structures of the organic and inorganic worlds. Of these atoms some seventy have never yet been divided, and therefore, although we may suspect that they are merely combinations or transformations of one original matter, we must be content for the present to consider them as elementary. In like manner we may suspect that matter is in reality only another form of energy, and that the impression of solidity is given by the action of a repellent force which is very energetic at short distances. If this were established we might look forward to the generalisation that energy was the one reality of nature; but for the present it is a mere speculation, and we must be content with over seventy elementary atoms as ultimate facts. In any case this much is certain, that matter, like energy, is indestructible. We have absolutely no experience of either of them being created or annihilated. Nay, more, we have no faculties to enable us even to conceive how something can be made out of nothing, and all we know, or can ever know, about these primitive constituents of the universe is of their laws of existence, their evolutions and their transformations. Minute as the atoms and molecules are, we must conceive of them not as stationary and indissolubly connected, but rather as little solar systems in which revolving atoms form the molecule, and revolving molecules form the matter, held together as separate systems by their proper energies and motions, until some superior force intruding breaks up the system and sets its components free to form new combinations. What is the principle which thus forms, un-forms, and re-forms the various combinations of atomic and molecular systems by which the world is built up from its constituent elements? It is polarity.