or Elastic Force, that property of bodies wherewith they restore themselves to their former figure, after any external pressure.
The cause or principle of this important property elasticity, or springiness, is variously assigned. The Cartesians account for it from the materia subtilis making an effort to pass through pores that are too narrow for it. Thus, say they, in bending, or compressing, a hard elastic body, e.g., a bow, its parts recede from each other on the convex side, and approach on the concave; consequently the pores are contracted or tightened on the concave side; and if they were before round, are now, for instance, oval; so that the materia subtilis, or matter of the second element, endeavouring to pass out of those pores thus tightened, must make an effort, at the same time, to restore the body to the state it was in when the pores were more patent and round, i.e., before the bow was bent; and in this consists its elasticity.
Other later and more wary philosophers account for elasticity much after the same manner as the Cartesians; with this only difference, that in lieu of the subtle 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 broke off, or separated so far as to be out of the power of that attractive force whereby they cohere; they must certainly, on the cessation of the external violence, spring back to their former natural state.
Others 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 vacuums between the separated surfaces; so that upon the removal of the force they will close again by the pressure of the aerial fluid upon the external parts. See Atmosphere.
Lastly, others attribute the elasticity of all hard bodies to the power of reflection in the air included within them; and so makes the elastic force of the air the principle of elasticity in all other bodies.
The Elasticity of Fluids is accounted for from their particles being all endowed with a centrifugal force; when Sir Isaac Newton, prop. 23, lib. 2, demonstrates, that particles, which naturally avoid or fly off from one another by such forces as are reciprocally proportioned to the distances of their centre, 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 and avoid one another, and hath its density proportional to its compression, then the centrifugal forces of those particles will be reciprocally as the distances of their centres.
Elasticity of the Air, is the force wherewith that element dilates itself, upon removing the force whereby it was before compressed. See Air and Atmosphere.
The elasticity or spring of the air was first discovered 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 glass or metal ball, till such time as the ball, with this accession of air, weighs considerably more in the balance than it did before; upon opening the mouth thereof, the air rushes out, till the ball sinks to its former weight. From hence we argue, 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; consequently it is endowed with an elastic force. It must 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.
The elasticity of the air makes a considerable article in Pneumatics.
The cause of the elasticity of the atmosphere hath been commonly ascribed to a repulsion between its particles; but this can give us only a very slight idea of the nature of its elasticity. The term repulsion, like that of attraction, requires to be defined; and in all probability will be found in most cases to be the effect of the action of some other fluid. Thus, we find, that the elasticity of the atmosphere is very considerably affected by heat. Supposing a quantity of air heated to such a degree as is sufficient to raise Fahrenheit's thermometer to 212°, it will then occupy a considerable space. If it is cooled to such a degree as to sink the thermometer to 0°, it will shrink up into less than half the former bulk. The quantity of repulsive power power therefore acquired by the air, while passing from one of these states to the other, is evidently owing to the heat added or taken away from it. Nor have we any reason to suppose, that the quantity of elasticity or repulsive power it still possesses is owing to any other thing than the fire contained in it. The supposing repulsion to be a primary cause, independent of all others, hath given rise to many erroneous theories, and been one very great mean of embarrassing philosophers in their accounting for the phenomena of Electricity.