Fig. 4. Fig. 5.
39. Elasticity.—Closely allied with the compressibility of matter is its elasticity. We see this property strikingly exemplified in India-rubber. It occasions the rebounding of a ball of this substance when thrown down. Observe flow exactly what occurs in this case. The ball as it meets the resistance of the floor is flattened, as represented in Fig. 4. Then, as it assumes the round shape, as seen in Fig. 5, it pushes downward upon the floor. It is this sudden pushing downward that makes it rebound. It is as if there were a compressed spring between the ball and floor. It may be likened also to jumping. When one jumps he bends his limbs at the thigh and knee joints, and then, in straightening himself up, gives a sudden push, like that given by the ball as it assumes its round shape, and so is thrown forward or upward, according to the direction in which the pushing force is made. The same flattening occurs in an ivory ball, though not to the same degree. You can prove that it does occur by experiment. Let a marble slab be wet and drop the ball upon it. Quite a spot will be made dry by the blow of the ball, showing that it touched more of the marble than it does when it is merely placed upon it.
40. Elasticity Shown in Other Ways.—If a stick be bent, as in Fig. 2, as soon as the bending force is withdrawn the stick becomes straight again from its elasticity. It is this elastic force of the bow, straightening it, that speeds the arrow. Observe in this case that while the particles on the concave side of the bent bow are brought nearer together or compressed, those on the convex side are moved apart. This moving apart of the particles is often shown in India-rubber. You can see how very far apart particles that are in near neighborhood may be carried, if you will stick two pins close together in a strip of India-rubber before you stretch it.
41. Degrees of Elasticity in Different Substances.—Some substances have so very little elasticity that they are practically considered as having none. Lead is one of these. A rod of lead when bent remains so, and a leaden ball does not rebound. While aeriform substances are the most compressible of all, they are also the most elastic. Air compressed returns to its usual condition the moment that it is relieved from the pressure, and with a force proportioned to the amount of the pressure. So it is with steam and the gases. The varied results of this quality of aeriform substances will claim our attention more particularly in some other parts of this book.
42. Definition of Elasticity.—You see from the illustrations that have been given that elasticity is that property of matter by which its particles, when brought nearer together or carried farther apart by any force, return to their usual condition when the force is withdrawn.
43. Usefulness of Variety in Properties of Matter.—The various properties of matter brought to view in this chapter are providential adaptations to the necessities of man. Each substance has those properties which best fit it for his use. Iron, for example, designed by the Creator to be both the strongest and most extensively useful servant of man among the metals, is therefore provided in great abundance, and has those strong, decided, and various qualities which fit it for the services it is to perform. Gold and silver, on the other hand, designed for services less extensive, lighter, and in a great measure ornamental, are provided in very much less quantity, and have properties admirably adapting them to the services for which they are so manifestly intended. The same can be substantially said of all other substances, and especially of those very abundant ones, air and water. And it may be remarked also that the ingenuity of man is continually discovering new modes of bringing the various properties of matter into his service. I will give but a single illustration—the tempering of steel. "This discovery," says Dr. Arnot, "is perhaps second in importance to few discoveries which man has made; for it has given him all the edge-tools and cutting-instruments by which he now moulds every other substance to his wishes. A savage will work for twelve months with fire and sharp stones to fell a great tree and to give it the shape of a canoe, where a modern carpenter, with his tools, could accomplish the object in a day or two."