ELASTICITY

From Agepedia

ELASTICITY ; the peculiar property of bodies, by virtue of which, the particles of which they are composed, when moved out of their positions by an external force, or pressed into a narrower space, tend to return to their former position, as soon as the external force ceases to act. A bow, bent by the tension of the string, recovers its previous form when the tension is relaxed. Let an ivory ball fall upon a plate of marble, it is partially flattened by the impulse, but becomes immediately round again as soon as the force of the blow is destroyed. Here we see the cause of its rebounding from the hard surface. Feathers are in a high degree elastic. This property of elasticity is particularly observable in atmospheric air. If it is enclosed in a vessel, and pressed with a piston, as soon as the force is removed from the piston, the air throws it up violently. This is the principle of the airgun. There is an important difference between the elasticity of solids and fluids ; the former tend to recover their previous form; the latter to expand into a greater space, whence the term expansibility is applied to them. For the sake of distinction, the elasticity of solid bodies may be termed attractive, and that of fluids, expansive. The degree of it is very different in different bodies, and in many it is increased by art. Those bodies in which it cannot be perceived at all are called unelastic. The elasticity of a solid body is greater the more its particles are expanded. If all the ]>articles of a body are so far expanded that their elasticity is just equal to the expansive power, the expansion can be carried no further without separating the particlis. The weights, necessary to produce a given degree of extension, must be proportionate to the extension already existing. If three cords, of the same size and substance, stretched in proportion to the numbers 1, 2, 3, are to receive each a given amount of additional extension, the weight* necessary to produce this extension are as 1,2,3. The laws of elasticity in fluids am different from those in solids. In heavy elastic fluids, the inferior layers support the weight of the superi<M*; in a cylindrical vessel,therefore,the bottom suffers th?pressure of tiie whole mass of elastic fluid, and the lower strata are sensibly denser than the upper. A difference is made, too, between absolute and specific elasticity. By the former is understood the peculiar property of bodies to repel a pressing force, in itself, and without regard to ternperature and density. This must be always equal to the pressing force. But as different kinds of matter may press with equal force under unequal densities and temperatures, that is called specifically most elastic, which with a less density presses with a force equally strong, and with an equal density stronger. In all elastic fluids, the specific elasticity increases with the temperature; it is likewise augmented by greater density: if air is confined, and made more dense, its specific elasticity is greater in proportion to its increase of density.