ELASTIC

, an appellation given to all bodies endowed with the property of elasticity or springiness.

Elastic Body, is that which changes its figure, and yields to any impulse or pressure, but endeavours by its own nature and force to restore the same again; or, it is a springy body, which, when compressed or condensed, or the like, makes an effort to set itself at liberty, and to repel the body that constrained it. Such, for instance, as a bow, or a sword blade, &c, which are easily bent, but presently return to their former figure and extension. All bodies partake of this property in some degree, though perhaps none are perfectly elastic, as none are found to restore themselves with a force equal to that with which they are compressed.

The principal phenomena observable in Elastic bodies, are 1, That an elastic body (i. e. 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 sound, 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 incidence 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 cannot 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, according 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 relaxes them, has the opposite effect: but, on the contrary, air, and other elastic fluids, are expanded by heat, and rendered more elastic.—For the laws of Motion and Percussion in Elastic bodies, see Motion, and Percussion.

Elastic Curve. See Catenaria.

Elastic Fluids. See Air, Electricity, Gas, Elastic Vapours, &c.

Elastic Gum. The same as Caoutchouc, or Indian Rubber.

Elastic Vapours, or Fluids, are such as may be compressed mechanically into a less space, and which resume their former state when the compressing force is withdrawn. Such as atmospherical air, and all the aerial fluids, with all kinds of fumes raised by means of heat, whether from solid or fluid bodies.

Of these, some remain elastic only while a considerable degree of heat is applied to them, or to the substance which produces them; while others continue elastic in every degree of cold that has yet been observed. Of the former kind, are the vapours of water, spirit of wine, mercury, sal-ammoniac, and all kinds of sublimable salts: of the latter, those of spirit of salt, mixtures of vitriolic acid and iron, nitrous acid, and various other metals, and in short the several species of aerial fluids indiscriminately.

The elastic force with which any one of these fluids is endowed, has not yet been calculated, as being ultimately greater than any obstacle we can put in its wayThus, on compressing the atmospherical air, we find that for some little time at first it easily yields to any force applied; but at every succeeding moment the resistance becomes always the stronger, and a greater and greater force must be applied, to compress it farther. As the compression goes on, the vessel containing the air becomes hot; but no power whatever has yet been able in any degree to destroy the elasticity of the contained fluid; for, upon removing the pressure, it is always found to occupy the very same space that it did before. The case is the same with the steam of water, to which a sufficient heat is applied to keep it from condensing into water.

Elasticity

, or Elastic Force, that property of bodies by which they restore themselves to their former figure, after any external pressure.

The cause or principle of this important property, elasticity, is variously accounted for. The Cartesians ascribe it to their subtile matter making an effort to pass through pores that are too narrow for it. Thus, say they, in bending or compressing a hard elastic body, as a bow, for instance, its parts recede from each other on the convex side, and approach on the concave one: | consequently the pores are contracted or straitened on the concave side; and, if they were before round, are now perhaps oval: so that the materia subtilis, or matter of the second element, endeavouring to pass out of the pores thus straitened, must make an effort, at the same time, to restore the body to the state it was in when the pores were rounder, i. e. before the bow was bent: and in this consists its Elasticity.

Other later philosophers account for Elasticity-much after the same manner as the Cartesians; with this only difference, that instead of the subtile matter of the Cartesians, these substitute Ether, or a fine ethereal medium that pervades all bodies.

Others, setting aside the precarious notion of a materia subtilis, account for Elasticity from the great law of nature, Attraction, or the cause of the cohesion of the parts of solid and firm bodies. Thus, say they, when a hard body is struck or bent, so that the component parts are moved a little from each other, but not quite disjointed or broken off, or separated so far as to be out of the power of that attracting force by which they cohere; they must, on removing the external violence, spring back to their former natural state.

Others again resolve Elasticity into the pressure of the atmosphere: for a violent tension, or compression, though not so great as to separate the constituent particles of bodies far enough to let in any foreign matter, must yet occasion many little vacuola between the separated surfaces; so that on the removal of the force they will close again by the pressure of the aerial fluid upon the external parts.

Lastly, others attribute the Elasticity of all hard bodies to the power of resilition in the air included within them; and so make the elastic force of the air the principle of Elasticity in all other bodies. See Desaguliers's Exper. Philos. vol. 2, pa. 38, &c.

The Elasticity of Fluids is accounted for from their particles being all endowed with a centrifugal force; whence Sir Isaac Newton demonstrates, prop. 23, lib. 2, that particles, which naturally avoid or fly off from one another by such forces as are reciprocally proportional to the distances of their centres, will compose an elastic fluid, whose density shall be proportional to its compression; and vice versa, if any fluid be composed of particles that fly off or avoid one another, and have its density proportional to its compression, then the centrifugal forces of those particles will be reciprocally proportional to the distances of their centres.

Elasticity of the Air is the force with which that element endeavours to expand, and with which it does actually dilate itself, on removing the force that compressed it. See Air, and Atmosphere.

The Elasticity or spring of the air was first discovered by lord Bacon, and farther established by Galileo. Its existence is proved by this experiment of that philosopher: An extraordinary quantity of air being intruded, by means of a syringe, into a hollow ball or shell of glass or metal, till such time as the ball, with this accession of air, weigh considerably more in the balance than it did before; then, opening the mouth of the ball, the air rushes out, till the ball sink to its former weight. From hence we infer, that there is just as much air gone out, as compressed air had been crowded in. Air therefore returns to its former degree of expansion, upon removing the force that compressed or resisted its expansion; and consequently it is endowed with an elastic force. It may be added, that as the air is found to rush out in every situation or direction of the orifice, the elastic force acts every way, or in every direction alike.

The cause of Elasticity in air hath been usually ascribed to a repulsion between its particles; but what is the cause of that repulsion? The term repulsion, like that of attraction, requires to be defined; and probably it will be found in most cases to be the effect of the action of some other fluid. Thus, it is found that the Elasticity of the atmosphere is very considerably affected by heat. Supposing a quantity of air heated to such a degree as to raise Fahrenheit's thermometer to 212, it will then occupy a considerable space; but if it be cooled again to such a degree, as to sink the thermometer to o, it will shrink up to less than half the former bulk. The quantity of repulsive power therefore acquired by the air, while passing from one of these states to the other, is evidently owing to the heat added to it, or taken away from it. Nor does there seem to be any reason to suppose, that the quantity of Elasticity or repulsive power it still possesses, is owing to any other cause than the fire contained in it. The supposition that repulsion is a primary cause, independent of all others, has given rise to many erroneous theories, and very much embarrassed philosophers in accounting for the phenomena of Elasticity.

The Elasticity of the air is not only proportional to its density, but is always equal to the force which compresses it, because these two exactly balance each other. This Elasticity, in the atmospheric air, is meafured by the height of the barometer at any time, allowing for its heat or temperature, after this rate, viz, the 434th part for each degree of Fahrenheit's thermometer, above or below some mean temperature, as 55°; for by that part of the whole it is that air expands or contracts, or else increases or decreases in its Elasticity, for each degree of the thermometer. Sir Geo. Shuckburgh, in the Philos. Trans. for 1777, pa. 561.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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ECLIPTIC
EFFECT
EFFECTION
EFFERVESCENCE
EFFLUVIUM
* ELASTIC
ELECTIONS
ELECTRIC
ELECTRICITY
ELECTROMETER
ELECTROPHOR