M. Crémieux has recently undertaken experiments directed, as he thinks, to showing that the divergences between the phenomena of gravitation and all the other phenomena in nature are more apparent than real. Thus the evolution in the heart of the ether of a quantity of gravific energy would not be entirely isolated, and as in the case of all evolutions of all energy of whatever kind, it should provoke a partial transformation into energy of a different form. Thus again the liberated energy of gravitation would vary when passing from one material to another, as from gases into liquids, or from one liquid to a different one. On this last point the researches of M. Crémieux have given affirmative results: if we immerse in a large mass of some liquid several drops of another not miscible with the first, but of identical density, we form a mass representing no doubt a discontinuity in the ether, and we may ask ourselves whether, in conformity with what happens in all other phenomena of nature, this discontinuity has not a tendency to disappear. If we abide by the ordinary consequences of the Newtonian theory of potential, the drops should remain motionless, the hydrostatic impulsion forming an exact equilibrium to their mutual attraction. Now M. Crémieux remarks that, as a matter of fact, they slowly approach each other. Such experiments are very delicate; and with all the precautions taken by the author, it cannot yet be asserted that he has removed all possibility of the action of the phenomena of capillarity nor all possible errors proceeding from extremely slight differences of temperature. But the attempt is interesting and deserves to be followed up. Thus, the hypothesis of the ether does not yet explain all the phenomena which the considerations relating to matter are of themselves powerless to interpret. If we wished to represent to ourselves, by the mechanical properties of a medium filling the whole of the universe, all luminous, electric, and gravitation phenomena, we should be led to attribute to this medium very strange and almost contradictory characteristics; and yet it would be still more inconceivable that this medium should be double or treble, that there should be two or three ethers each occupying space as if it were alone, and interpenetrating it without exercising any action on one another. We are thus brought, by a close examination of facts, rather to the idea that the properties of the ether are not wholly reducible to the rules of ordinary mechanics. The physicist has therefore not yet succeeded in answering the question often put to him by the philosopher: "Has the ether really an objective existence?" However, it is not necessary to know the answer in order to utilize the ether. In its ideal properties we find the means of determining the form of equations which are valid, and to the learned detached from all metaphysical prepossession this is the essential point. CHAPTER VII