When a solid body comes into collision with another solid body, the energy of motion is instantly converted into heat, except such portion of it as may be consumed in fragmentation, and retained in the motion of the flying pieces. If two armor-plates, twelve inches in thickness, could be brought together face to face, each with a velocity equal to that of a modern 12-inch projectile, the energy of the impact would be sufficient to melt both plates. New suns are created by the occasional collision of great celestial bodies in their flight through space. The heat generated by such collisions is, however, vastly greater than that developed by the collision of a projectile against armor-plate, for the reason that the velocity of celestial bodies is so much greater, being commonly from thirty-five to fifty miles per second, and sometimes as high as two hundred miles per second, instead of but three-quarters of a mile per second. The heat developed by the collision of worlds is sufficient not only to fuse them, but also to gasefy them, and reduce them to their ultimate elements. All the suns that emblazon the evening sky have been created in this manner, and the heat generated by their natal impact is sufficient to maintain their radiant energy for hundreds of millions of years. Planets are born, some of them to become inhabited worlds, finally to grow old and die, with the extinguishment of all life upon them, while their parent sun is still blazing hot. The earth is being constantly bombarded with meteorites, usually of very small size. But the earth is armor-plated with its envelope of air. The impact of meteorites upon this envelope, at the enormous speed at which they are traveling through space, is fatal to them, and they are dashed to pieces and consumed upon it, as though it were a solid shield of hardest tempered steel. It is seldom, indeed, that a meteorite has sufficient size and mass to penetrate through the atmosphere to the earth's surface. Were it not for the protection offered by the earth's envelope of air, every living thing upon its surface would be very soon destroyed by the meteoric bombardment from the heavens. A minute particle of meteoric dust, traveling at celestial velocity, would be more deadly than a bullet from a shoulder-rifle.