in Natural Philosophy, an appellation given to all bodies endowed with the property of elasticity. See Elasticity.

ELASTIC Fluids. See Air, Electricity, Gas, and Elastic Vapours below.

ELASTIC Resin. See CAOUTCHOUC.

ELASTIC Vapours are such as may, by any external mechanical force, be compressed into a smaller space than what they originally occupied; restoring themselves, when the pressure is taken off, to their former state with a force exactly proportioned to that with which they were at first compressed. Of this kind are all the aerial fluids without exception, and all kinds of fumes raised by means of heat whether from solid or fluid bodies.

Of these, some retain their elasticity only when a considerable degree of heat is applied to them or the substance which produces them; while others remain elastic in every degree of cold, either natural or artificial, 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 muriatic acid gas, hydrogen gas, nitrous gas, common air, &c.

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 way. Thus, if we compress the atmospherical air, we shall find that for some little time it will easily yield to the force we apply; but every succeeding moment the resistance will become stronger, and a greater and greater force must be applied in order to compress it further. As the compression goes on, the vessel containing the air becomes hot; but no power whatever has yet been able to destroy the elasticity of the contained fluid in any degree; 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 aqueous steam, to which a sufficient heat is applied to keep it from condensing into water. This will yield to a certain degree; but every moment the resistance becomes greater, until at last it will overcome any obstacles whatever. An example of the power of this kind of steam we have every day in the steam engine; and the vapours of other matters, both solid and fluid, have frequently manifested themselves to be endowed with an equal force. Thus the force of the vapours of spirit of wine has occasioned terrible accidents when the worm has been stopped, and the head of the still absurdly tied down to prevent an explosion; the vapours of mercury have burnt an iron box; and those of sal ammoniac, volatile salts, nitrous acid, marine acid, phosphorus, &c., have all been known to burst the chemical vessels which confined them with great force, in such a manner as to endanger those who stood near them. In short, from innumerable observations, it may be laid down as an undoubted fact, that there is no substance whatever capable of being reduced into a state of vapour, but what in that state is endowed with an elastic force ultimately superior to any obstacle we can throw in its way.

It hath been a kind of desideratum among philosophers to give a satisfactory reason for this astonishing Elastic vapours, being power of elasticity in vapour, seemingly so little capable of accomplishing any great purpose when in an unconfined state. As air is that fluid in which, from the many experiments made upon it by the air-pump and otherwise, the elastic property has most frequently been observed, the researches of philosophers were at first principally directed towards it. The causes they assigned, however, were very inadequate; being founded upon a hypothesis concerning the form of the particles of the atmosphere itself, which they supposed to be either rolled up like the springs of watches, or that they consisted of a kind of elastic flakes. This was followed by another hypothesis concerning their substance, which was imagined to be perfectly elastic, and so strong that they could not be broken by any mechanical power whatever; and thus they thought the phenomenon of the elasticity of the air might be explained. But an insuperable difficulty still attended their scheme, notwithstanding both these suppositions; for it was observed, that the elastic power of the air was augmented not only in proportion to the quantity of pressure it was made to endure, but in proportion to the degree of heat applied to it at the time. Sir Isaac Newton was aware of this difficulty; and justly concluded, that the phenomena of the air's elasticity could not be solved on any other supposition but that of a repulsive power diffused all around each of its particles, which became stronger as they approached, and weaker as they removed from each other. Hence the common phenomena of the air-pump and condensing-engine received a satisfactory explanation; but still it remained to account for the power shown in the present case by heat, as it could not be denied that this element had a very great share in augmenting the elasticity of the atmosphere, and seemed to be the only cause of elasticity in other vapours. It does not appear that Sir Isaac entered into this question, but contented himself with attributing to heat the property of increasing repulsion, and ascribing this to another unexplored property called rarefaction. Thus matters stood till the great discovery made by Dr Black, that some bodies have the power of absorbing in an unknown manner the element in question, and parting with it afterwards, so that it flows out of the body which had absorbed it with the very same properties that it had before absorption. Hence many phenomena of heat, vapour, and evaporation, were explained in a manner much more satisfactory than had ever been attempted or even expected before. One of these was that remarkable property of metals becoming hot by hammering; during which operation, in the Doctor's opinion, the element of heat is squeezed out from between the particles of the metal, as water is from the pores of a sponge by pressing it between the fingers. Of the same nature is the phenomenon above-mentioned, that air when violently compressed becomes hot, by reason of the quantity of more subtle element squeezed out from among the particles. In this manner it appears that heat and the repulsive power of Sir Isaac Newton are the very same; that by diminishing the heat of any quantity of air, its elasticity is effectually diminished, and it will of itself shrink into a smaller space as effectually as by mechanical pressure. In one case we have what may be called ocular demonstration of the truth of this doctrine, viz. that by throwing the focus of a strong burning lens upon a small quantity of charcoal in vacuo, the whole will be converted into inflammable air, having even a greater power of elasticity than common air in an equal degree of heat. Here there is nothing else but heat or light to produce the elastic power, or cause the particles of charcoal which before attracted now to repel each other. In another case we have evidence equally strong, that the element of heat by itself, without the presence of that of light, is capable of producing the same effect. Thus when a phial of ether is put into the receiver of an air pump, and surrounded by a small vessel of water, the ether boils violently, and is distilled in vapour, while the water freezes, and is cooled to a great degree. The distillation of this vapour shows that it has an elastic force; and the absorption of the heat from the water shows, that this element not only produces the elasticity, but actually enters into the substance of the vapour itself; so that we have not the least reason to conclude that there is any other repulsive power by which the particles are kept at a distance from one another than the substance of the heat itself. In what manner it acts, we cannot pretend exactly to explain, without making hypotheses concerning the form of the minute particles of matter, which must always be very uncertain. All known phenomena, however, concur in rendering the theory just now laid down extremely probable. The elasticity of the steam of water is exactly proportioned to the degree of heat which flows into it from without; and if this be kept up to a sufficient degree, there is no mechanical pressure which can reduce it into the state of water. This, however, may very easily be done by abstracting a certain portion of the latent heat it contains; when the elastic vapour will become a dense and heavy fluid. The same thing may be done in various ways with the permanently elastic fluids. Thus the purest dephlogisticated air, when made to part with its latent heat by burning with iron, is converted into a gravitating substance of an unknown nature, which adheres strongly to the metal. If the decomposition is performed by means of inflammable air, both together unite into a heavy, aqueous, or acid fluid: if by mixture with nitrous air, still the heat is discernible, though less violent than in the two former cases. The decomposition indeed is slower, but equally complete, and the dephlogisticated air becomes part of the nitrous acid, from which it may be again expelled by proper means: but of these means heat must always be one; for thus only the elasticity can be restored, and the air be recovered in its proper state. The same thing takes place in fixed air, and all other permanently elastic fluids capable of being absorbed by others. The conclusion therefore which we can only draw from what data we have concerning the composition of elastic vapours is, that all of them are formed of a terrestrial substance, united with the element of heat in such a manner that part of the latter may be squeezed out from among the terrestrial particles; but in such a manner, that as soon as the pressure is taken off, the surrounding fluid rushes in, and expands them to their original bulk: and this expansion or tendency to it will be increased in proportion to the degree of heat, just as the expansion of a sponge would be exceedingly augmented, if we could contrive to convey a stream of water into Elasticity into the heart of it, and make the liquid flow out with violence through every pore in the circumference. In this case, it is evident that the water would act as a power of repulsion among the particles of the sponge, as well as the fire does among the particles of the water, charcoal, or whatever other substance is employed. Thus far we may reason from analogy; but in all probability the internal and essential texture of these vapors will forever remain unknown. Their obvious properties, as well as some of their more latent operations in many cases, are treated of under Chemistry.

It has been imagined by some, that the artificial elastic fluids have not the same mechanical property with common air, viz. that of occupying a space inversely proportional to the weights with which they are pressed; but this is found to be a mistake. All of them likewise have been found to be non-conductors of electricity, though probably not all in the same degree. Even aqueous vapor, when intimately mingled with any permanently elastic fluid, refuses to conduct this fluid, as is evident from the highly electrical state of the atmosphere in very dry weather, when we are certain that aqueous vapor must abound very much, and be intimately mixed with it. The colour of the electric spark, though it may be made visible in all kinds of permanently elastic vapors, is very different in different fluids. Thus in inflammable and alkaline air it is red or purple, but in fixed air it appears white.