ELASTICITY, or SPRING. When an external force acts upon a solid body, it pro duces at first slight alterations in the relative positions of the particles; and if before these alterations exceed a certain limit, the force ceases to act, the particles return to their former position, and the disfigurement disappears. This power or property of recovering their previous form after alteration, is called E., and we are justified in ascribing it to all bOdieS,, though in' very different degrees. It was once believed that there were definite limits within which chatiges of form produced pressure or other forces disappeared completely. It was thought, for instance, that when a weight of no great magnitude is suspended from a metallic wire, the slight increase of length which the wire is observed to undergo, is completely lost when the weight is removed; and the limit to which the wire might thus be stretched, and still suffer no permanent increase of length, was called the limit of its elasticity. But recent more accurate experiments have shown that no such limits exist, at least in the case of metals; or, which is the same thing, that permanent lengthening results, however slightly the wire be loaded—it never contracts again quite so far as it was stretched. It is necessary, therefore, to fix the limit arbitrarily; and this is done by agreeing that it shall be held to begin when the metal in question suffers a permanent elongation of 0.00005 of its length. To get the elastic extensibility of a wire, then, we must compare its length suspended, with its length when the weight is removed. In this way it is found that the extensions pro duced are proportional to the extending forces or weights. From this law, then, we can calculate what weight it would require to stretch a wire or rod of a sq. in. in section to double its own length; supposing it possible to proceed so far without break ing it, and that the law of E. continued up to this point unaltered. This weight, which is different for every metal or kind of wood, is called the or modulus of elasticity of the particular substance; and is used in mechanics in calculating how far a given weight will extend a wire or rod of given diameter. This co-efficient is not con stant for the same metal; for all circumstances that increase the density of the metal, increase the modulus of elasticity. Bodies manifest E., not only when extended in length, but also when compressed, when bent, or when twisted. If an ivory ball be dropped from a height upon a marble slab smeared with fat and lampblack, when caught after the rebound, it is seen to have touched the marble, not in a point, but in a circle of several lines in diameter, and must therefore have lost for a time its spherical shape over that extent. In the same way the mark of a well-hit golf-ball is pretty broadly
shown upon the face of a club after the stroke. The E. shown by wires and threads of glass when twisted, has been turned to account in the torsion-balance (q.v.), for measuring other weak forces. Steel, ivory, caoutchouc, etc., are well known for their elastic properties, to which they owe much of their utility.
The propagation of waves of sound through solid bodies depends upon their E.; and from observations of this kind made with different substances, the modulus of E. for each may be deduced; the results, however, differ slightly from those arrived at by attaching weights, owing to the heat produced by the vibratory move ment.
All solid bodies are only imperfectly elastic—that is, they do not quite recover their form and volume when the disturbing force ceases. Liquids and gases, on the contrary, are perfectly elastic, or return exactly to their original bulk or volume when the pressure is removed. The elasticity of liquids and gases, however, acts only in expanding after compression, while that of solids acts also in contracting after extension. The expansive elasticity of liquids and gases is equal to the force used to compress them. Water and other liquids are easily seen to be compressible, by the fact of their conveying sound s sound-wave being merely a state of compression, propagated from each layer of the liquid to the next. The coefficient of elasticity of water determined by Colladon and Sturm, from the velocity of sound in the lake of Geneva, agrees very well with that determined by direct measurements in Oerstedt's apparatus. The discovery of the com pressibility of water is an English one, due to Canton, in 1762. Previous attempts, by Italian and Dutch philosophers, to compress water by hammering a silver shell filled with that fluid, had failed to give any certain result, as the water was forced through the pores of the metal. At a temperature of 50°, one atmosphere compresses water to about 0.999995 of its volume. From the existence of a maximum density temperature for water, some curious consequences arise with regard to the effects of pressure on the fluid. The volumes or bulks which a given quantity of anygas assumes under different pressures, are nearly in inverse proportion to the pressures. See MAItIOTTE'S LAW. The elasticity of gases is usually measured by the height of the column of mercury that they sustain. The elasticity of gases is a force much and variously employed in the arts of life. See AIR-GUN, Aru-rumr, GUNPOWDER, etc.