is no right and left to gravitation. Two pieces of matter always fit in the gravitational sense.
The bald statements of the laws of gravitation and electric force bear a strong resemblance to each other. The laws tell us how the forces vary, but reveal no hint of the machinery by which they act.
Gravitation was the first force man encountered and it is still the one he knows least about, for we have got no farther than where Newton left it two and a half centuries ago. We have some inkling of the possible machinery by which one electric charge acts upon another at a distance and we feel nearly as sure that the push or pull is carried by the ether as that the pull of a horse on a cart is through the traces which bind him to it. With gravitation the case is very different, for we have not as yet the slightest valid conception of how the pull of one mass upon another is conducted across the intervening space, nor what conducts it. We can get no farther until the speed with which gravitational disturbances travel has been measured, and no one at present seems to know how to go about making such an experiment.
One further difference between gravitation and electric force. The force of attraction or repulsion between two charges of electricity is diminished by replacing the free ether between them with any material medium, but the force of gravitation between two bodies remains constant as long as the distance remains constant, and intervening masses are powerless to shield or to alter it. Hence we can not yet attribute the gravitation of matter to any electricity which may be contained in it, nor prove the ether to be the medium through which the force acts.
Gravitation is still unconnected, unattached to anything else in nature; as independent as Mr. Kipling's "cat that walked by himself, and all places were alike to him." It is still the stumbling block to the physicist which it has been these many years. How can he explain a universe when he is unable to give a reasonable account of the cement which holds it together?
Of the intimate association of electricity with matter we have learned much from careful study of the processes of electric conduction in solutions and gases.
When a simple chemical compound (and it should here be borne in mind that the molecule of a compound is built up of atoms of at least two different kinds)—when a simple chemical compound, hydrochloric acid for example, is dissolved in water and an electric current is passed through the solution, the products hydrogen and chlorine of the decomposed acid appear in definite proportions at the points where the current enters and leaves the liquid—the chlorine where the current enters, the hydrogen where it leaves. We know this current to consist of processions of single charged atoms, a disorderly march, perhaps, with a crowd of bystanders obstructing the way, but the movement is always forward, each constituent of the broken molecule carrying a definite