Page:Popular Science Monthly Volume 34.djvu/198

186 bodies. At length the vibrations become so energetic that the fixed order in which the particles are arranged can no longer be maintained. They begin to collide and interfere with each other; the whole artificial edifice of the crystal collapses. The molecules can not regain their equilibrium—the snow has changed into water.

In liquids the molecules move about in all directions, yet none of them can voluntarily separate itself from the main body. Just as in a vessel, completely filled with live eels, each individual fish may wriggle and move about among the others, yet it can not detach itself or swim away from them. The liquid particles are not yet sufficiently potent or energetic to overcome the pressure exerted upon them by their surroundings. Above the water, even in the open vessel, they have to encounter the pressure of the atmosphere, the molecules of which keep up a constant and vigorous bombardment against the liquid particles, forcing them down. Still, this can not prevent that here and there a favorably situated water-molecule pushes itself between the air-molecules; the water evaporates. Now, if the temperature is heightened—that is, if the vibrations of the water-molecules increase to such an extent that they can hold their own against the pressure of the air—then a condition of things is brought about familiar to us under the name of boiling. The water-particles shoot about very rapidly; they are no longer crowded together, they force their way through the air-molecules and disperse—the water evaporates; we have no longer liquid but gaseous water.

In gases—as, for instance, the air, carbonic acid, etc.—the molecules are in a state of vibration so violent that they fly about with marvelous rapidity in all directions.

Now, we are in possession of information—of pretty accurate information—respecting these molecule-movements. The researches of men like the late Prof. Kingdon Clifford, Prof. Helmholtz, and, above all, Sir William Thomson—one of the ablest physicists and beyond comparison the greatest living mathematician as well as one of the subtlest thinkers the world has ever produced—the researches of men like these have thrown quite a flood of light on this important and highly interesting subject. Nor is our information merely confined to molecular movements and vibrations; the dimensions of the molecules themselves have been approximately ascertained, because, from known facts of pressure, friction, and heat-conducting capacity, very reliable conclusions can and have been drawn.

The air which surrounds us is a chaos of innumerable minute solid bodies, flying rapidly about in all directions. Our skin is perpetually bombarded by them, and it is this bombardment which causes us to experience atmospheric resistance or