Page:Popular Science Monthly Volume 45.djvu/520

502 resistance of metals would disappear; cooled to the temperature of liquid oxygen, the red oxide of mercury becomes yellow, and both sulphur and bichromate of potash turn white.

Surprising as are the figures which denote the molecular motion due to the temperature of water, more surprising still are the computations which declare the chemical energy in the gases which unite to form water. Measuring the heat liberated in their union, it is found that the molecules of hydrogen and oxygen possess as chemic motion energy equal to lifting the masses involved some eleven hundred miles from the surface of the earth. It is imagined that the molecular motions representing temperature, chemical affinity, electrification, or other energy, coexist without confusion, just as air sustains, in perfect order, the superposed harmonies of an orchestra and chorus. The extremely rapid motion of molecules, acting at their comparatively vast surfaces, must immensely exalt forces which, between masses, are but feeble. A simple model may help to make this clear. Let two cylindrical wheels, similar in all respects and covered with rubber, be brought into contact on a floor—they will in a slight degree adhere; let the wheels be swiftly turned in a direction toward each other, and they will press each other with considerable force—force proportionate to their speed, which force at high speed will exalt their weak adhesion to somewhat of the intensity of cohesion as manifested between molecules. The model can illustrate something more: as a unit it does not change its place, albeit that its halves are in rapid motion; were its dimensions too small for microscopic view, the motion of its parts would be undetected, and, because the motions would balance each other, a mass built up of such pairs of molecules would be in seeming rest.

While the chemists are busy disentangling the orbits in which swim the atoms and molecules of the laboratory, the physicists are equally active in endeavoring to reduce to mechanics the various phases of energ5^ Here the first and decisive step was taken when the revelations of form and color to the eye were explained as borne by ethereal waves, which follow one another at a rate so prodigious as to yield the impression of rest; which explanation, indeed, had long been suggested in the phenomena of sound where air-waves are palpably concerned. The notable points of agreement in both spheres of action are that a medium can transfer motion as perfectly as if the two bodies connected by it were in immediate contact; moreover, that the efficiency of the medium increases as its density diminishes; and that the medium itself exacts no toll whatever, relapsing when its work is done into the seeming rest from which its task awakened it. With apparatus acoustic in model, the late Dr. Hertz, of Bonn,