All these forms of energy may exist, as in the case of visible energy, either as energies of motion or of position, and the actual constitution of the universe is due in a great measure to the alternation of these two energies. Thus all wave-motion, whether it be of the waves of the sea grinding down a rocky coast, of the air transmitting sound, or of ether transmitting light and heat, are instances of energies of motion and of position, conflicting with one another and alternately gaining the victory. So also a pound of gunpowder or dynamite has an immense energy of position, which, when its atoms are let loose from their mutual unstable connection by heat or percussion, manifests itself in an enormous energy of motion, which is more or less destructive according to the rapidity with which the atoms rush into new combinations. Let us consider these different energies a little more in detail. The energy of visible motion is manifested principally by the law of gravity, under which all matter attracts other matter directly as the mass and inversely as the square of the distance. It is a universal and uniform law of matter, and can be traced without change or variation from the minutest atom up to the remotest double star. The energy of living force might, at first sight, be considered as another of the commonest causes of visible motion; but, when closely analysed, it will be found that what appears as such is only the result of molecular energy of position stored up in the living body by chemical changes during the slow combustion of food, and that nothing has been added by any hypothetical vital force. The conscious will seems to act in those cases simply as the signalman who shows a white flag may act on a train which has been standing on the line waiting for it. The energy which moves the train is due entirely to the difference of heat, which has been developed by the combustion of coal, between the steam in the boiler and the steam when allowed to escape into the air; and this energy came originally from the sun, whose rays enabled the leaves of growing plants to decompose carbonic dioxide and store up the carbon in the coal. Of this force of gravity causing visible motion we may say that it is comparatively a very weak force, which acts uniformly over all distances great or small.