142. But we must make one remark. If Le Sage’s theory, or anything of a similar nature, be at all a representation of the mechanism of gravitation, a fatal blow is dealt to the notion of the tranquil form of power we have called _potential_ energy. Not that there will cease to be a profound difference in kind between it and ordinary _kinetic_ energy; but that BOTH must come henceforth to be regarded as kinetic. What we now call kinetic energy is that of visible motions, also of motions of the smaller parts of bodies, and of the luminiferous ether, etc., each of these being more refined, as it were, than the preceding. But if Le Sage’s theory be true, potential energy of gravitation is a kinetic form still further refined than any of these. And the conservation of energy may perhaps once more be completely and accurately expressed as the conservation of _vis viva_, though the term will of course have then a meaning incomparably more extensive than its original one. 143. But, in speculations like these, we have soared far beyond that which may be called the first refinement on ordinary gross matter; _i.e._ the luminiferous, probably also the electric and magnetic, medium, provisionally the _Ether_. To the consideration of its principal properties we now turn our attention. These are, at first sight at least, of an apparently incongruous character; for, from one point of view, the ether appears as a fluid, from another as an elastic solid. Nothing is more certainly established in physical astronomy than the excessive minuteness of the resistance offered by the ether to the planetary motions, if, indeed, there be such a resistance at all appretiable, even when the velocity is, as in the case of the earth, somewhere about 100,000 feet per second! On the other hand, we learn from physical optics that light, transmitted with a velocity of 188,000 miles per second, depends upon transverse disturbances of some kind or other; while several optical phenomena indicate that a disturbance of the nature of compression (if such be possible) would be transmitted with velocity almost infinitely great, in comparison even with this enormous velocity. 144. Stokes, however, has given a very ingenious illustration which enables us to see that such an extraordinary combination of apparently irreconcilable properties is by no means without analogy, even in common matter. He takes the case of a solution of glue, or isinglass, or jelly, in different relative amounts of water. When the quantity of water is small, we have the elastic solid; when large, a liquid little different from water. And Stokes shows that it is excessively improbable that there is any definite intermediate stage which we could assign as that at which the transition from the solid to the liquid takes place. Of course, any such analogy must necessarily be excessively imperfect; but a great deal is gained by our being able to trace even a very imperfect analogy in a case like this.