ELASTICITY. In Physics, that prop erty which certain bodies possess of re covering their primitive form and dimen sions after the external force by which they have been dilated or compressed or bent is withdraws.
The theory of elasticity must be deduct ed from some hypothesis respecting the constitution of matter. The simplest and most general view which can be taken of the subject is, that all matter is compo sed of indefinitely small parts or molecules acted upon by attractive and repulsive forces. The attractive forces result from the action of the molecules on each other ; the repulsive forces from the caloric with which the molecules are combined. From the combined action of these two forces the attraction of matter and the repulsion of caloric, the different forms ofmatter and its varied physical proper ties may he explained.
This view of the constitution of bodies supposes that the molecules are not in contact, but at a certain distance from each other, which, though it is to be re garded as indefinitely small in compari son of any distance appreciable by our senses, admits nevertheless of increase and diminution. When a body is in a state of rest, the opposite forces which any two of its contiguous molecules exer cise on each other are in equilibrium. The energy of the forces also depends on the distance between the two molecules, or, in mathematical language, is a func tion of that distance. if the distance be increased within the limits of the action of the forces, both forces are dim i n is h ed ; and if the distance is diminished, both are increased, but not in the same pro portion. If the interval at which the two forces balance each other be diminished, the repulsive force becomes stronger than the attractive force, and the two mole cules are repelled from each other; on the contrary, if the distance be increased, the attractive force acquires the superiority, and the molecules are drawn towards each other.
Elasticity is perfect when the body ex actly recovers its primitive form, after the force by which it is bent or compressed or dilated has been removed, in the same time as was required for the force to pro duce the alteration. This perfect elasti
city is, however, not found in any of the bodies of nature • the aeriform fluids or gases are those whose elasticity approach es the nearest to it. Hard bodies, even tempered steel and ivory, possess it in a less degree ; in fluid substances the elas tic force is greatly diminished ; and in soft bodies, as butter, moist clay, it en tirely disappears. In solid bodies the elastic force is, in general diminished by use, or by a long continued application of a straining force. A bow which has been long bent, or a spring which has been long compressed, will not entirely recover its original form. In many cases the elasticity of a body augmented by producing a closer aggregation of the molecules. The metals, for example, are rendered more elastic by hammering them cold, or by alloys. Iron and steel acquire a greater elasticity by tempering ; that is, by producing a sudden contraction of their volumes when they have been ex panded by heat.
The principal phenomena of elastic bodies are the following :-1, That an elastic body (the elasticity being sup posed perfect) exerts the same fOrce rn endeavoring to restore itself, as that with which it was compressed or bent. 2, The force of elastic bodies is exerted equally in all directions, but the effect chiefly takes place on the side on which the re sistance is the least. 8, When an elastic solid body is made to vibrate by a sudden stroke, the vibrations are performed in equal times, to whatever part of the body the stroke may be communicated. Thus, sonorous bodies always emit sounds of the same pitch; and the difference of the pitch depends on the greater or less fre quency of the vibrations of the sonorous body. 4, A body perfectly incompressi ble cannot be elastic, therefore bodies perfectly solid can have no elasticity ; and hence, also, the small degree of elasticity belonging to the liquids which are emi nently incompressible.