ELASTICITY, that disposition in bo die:, by which they endeavour to restore themselves to the posture from whence they were displaced by any external force. The principal phenomena ob servable in elastic bodies are : 1. That an elastic body (i.v. a body perfectly elastic, if any such there be) endeavours to restore itself with the same force with which it is pressed or bent. 2. An elastic body exerts its force equally towards all sides, though the effect is chiefly found on that side where the resistance is weakest, as is evident in the case of a gun exploding a ball, a bow shooting out an arrow, &C. 3. Elastic bodies, in what manner soever struck, or impelled, are inflected, and rebound after the same manner : thus a bell yields the same musical souhd, in what manner or on what side soever it be struck ; the same of a tense or musical chord ; and a body rebounds from a plane in the same angle in which it meets or strikes it, making the angle of inci dence equal to the angle of reflection, whether the intensity of the stroke be greater or less. 4. A body perfectly fluid, if any such there be, cannot be elastic, if it be allowed that its parts be compressed. 5. A body perfectly solid, if any such there be, cannot be elastic ; because, having no pores, it is incapable of being compressed. 6. The elastic' properties of bodies seem to differ, ac cording to their greater or less density or compactness, though not in an equal degree : thus metals are rendered more compact and elastic by being hammered; tempered steel is much more elastic than soft steel; and the density of the former is to that of the latter as 7809 to 7738 : cold condenses solid bodies, and renders them more elastic ; whilst heat, that re laxes them, has the opposite effect '; but, on the contrary, air and other 'elastic fluids, are expanded by heat, and ren dered more elastic.
Some philosophers account for elastici ty from the principles of corpuscular at traction and repulsion : thus, if a steel spring, -wire, or piece of very thin glass, be bent out of its natural position, the particles on the convex part are forced from the intimate union they had before; and, on the concave part, they are forced nearer together, or harder upon each other, thy n in the natural state : in both which cases there will be a considerable resistance to overcome, and consequently require a superior force. During this
state of the particles, they may be said to be under a sort of tension on one 'side, and compression on the other : and since by this force they are not drawn out of each other's attraction, as soon as the force is remitted, or ceases to act, the at tractive power reduces the particles and unbends the wire. Now it is well known that many substances are composed of such fibrous parts or filaments which re semble fine wires, and are interwoven and disposed in such a manner, as in sponge, for instance, that they cannot be compressed without being bent or wrest ed from their natural position ; whence all bodies will in such cases exert a spring or force to restore themselves, in the same manner that the bent wire did. Others attribute the elasticity of all hard bodies to the force of the air included within them: and so they make the elastic force of the air the principle of elasticity in all other bodies. See PNEUMATICS.
All substances that we know of are in some degree or other elastic, but none of them perfectly so ; such are most metals, semi.metals, stones, and animal and vege table substances, however they may differ in degree.
We may consider all elastic bodies to be made up of such strings or fibres as A B (Plate IV. Miscel. fig. 9.) or rather of elastic strata parallel to each other, re presented by A B in the ball D C. If this ball be struck at D by a hard or elastic body, all the strata will be bent in towards C, , as expressed by the dotted lines, whilst the ball is flattened or dented at D. But the strata quickly restoring them selves, the surface of the ball re-assumes its figure, and that more or less ex actly, according as the elasticity is more or less perfect.