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Page:Popular Science Monthly Volume 34.djvu/197

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of snow; not a sound to be heard, not a breath of wind stirring the branches.

And yet we know that all this is an illusion! Those bright points of light on the dark firmament are solar systems, whirling through space two hundred times faster than a cannon-ball, and under our feet the delicate snow-crystals are groups of atoms, which tremble with billions of vibrations every second.

The outer forms of the little snow-stars appear to us fixed and rigid like those of the bright constellations above. But from their surfaces a pale light enters our eyes, acts on the retina, and excites the optic nerve—an infallible proof that there is something active even in these—and we know of only one kind of activity in this universe, namely, that of motion.

The particles of which the snow-crystals are composed are not closely joined or cemented to each other like the stones in a wall. They are perpetually acting and reacting on the bodies which surround them, through the medium of an exceedingly fine substance which we term world-ether. It is the bearer of the movements, the effects or modifications of which we know as light, heat, electricity, chemical affinity, etc.; it also keeps the particles of the snow-crystals separate, maintains them in a state of mutual equilibrium, and regulates their vibrations. These vibrations we term the "mechanical cause of heat," because we are aware that every increase of temperature is represented by an increase in the rapidity of the vibrations.

These minute particles, of which, as we know, all bodies consist, are called molecules. In solids, as, for instance, the snow-crystals, they are arranged in a certain fixed order, and their vibration is limited to a given space. Now, let the sun shine on the snow.

The sun is a vast center of activity. From every point of its surface an enormous number of impulses are continually acting on the atoms of the surrounding ether, which are sent through space in every direction, with lightning-like rapidity. The number of these impulses has been estimated at from four to eight hundred billions per second. They give rise to a wave-like motion which traverses about two hundred thousand miles of space per second, and requires eight minutes to reach our earth.

If these ether-waves happen to come into contact with our optic nerve, we experience the sensation of light; if they impinge upon our skin, we feel warmth; if they strike the snow, they set its molecules, as well as the ether between them, into a livelier state of motion. The vibration of the molecules increases in violence, and the result is an increase in temperature in the snow. The particles have to move further away from each other, as their vibrations require more space—we say the heat expands the