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returns to its former position. In such cases, we say that the body is elastic. When I take a small quantity of wool into my hand, and compress it, upon opening my hand, it recovers its former bulk, by the natural spring of its fibres; and hence we conclude that this substance possesses a certain degree of elasticity. In like manner, if I take a bladder and fill it with air, and apply a force to the sides of it, so as to compress it into a smaller space, when the force is removed it immediately expands, and fills the same space as before, which clearly proves that the air contained in the bladder is of an elastic nature.
In consequence of this elastic property, the air always endeavours to expand itself, and to occupy more space. This is proved by taking a bladder, containing only a small quantity of air, tying its neck close, so as to prevent the escape of the air, and then placing it under the receiver of an air-pump. So long as the bladder is exposed to the pressure of the atmosphere, it will remain in the same state ; but, when the air is exhausted from the receiver, and the external pressure removed, the side of the bladder, which was flabby and lax, stretches itself out, swells, and becomes tight, being raised by the elastic power. And, if the air be again let into the receiver, the bladder returns to its former shape. By a similar experiment it is shown, that the expansive power of the small quantity of air in the bladder is capable of raising leaden weights of
a considerable size. In consequence of this strong elastic power of the air, a person, by blowing into a pipe connected with several bladders, has been able sensibly to raise a mill-stone, which was placed upon the bladders; which demonstrates the very strong expansive power of a very small quantity of air.
On the same principle, were a bladder, containing a very small quantity of air, taken to the higher regions of the atmosphere, it would gradually expand the higher it was carried, in consequence of the pressure of the atmosphere being gradually diminished, till, at length, it would burst the bladder, by the expansive force with which it is endued. In like manner, heat increases the elasticity of air. If a bladder, containing a small quantity of air, be placed before a strong fire, the small portion of air it contains will expand, till the bladder appears quite full, and ready to burst. There is another striking experiment which demonstrates this elastic force of the air. When a thin bottle with flat sides is firmly corked, so as to prevent the included air from escaping, is placed under the receiver of an air-pump, and the air exhausted, the spring of the air within it will dilate with so much violence, as to break the bottle to pieces. In like manner, were the pressure of the external air completely removed from our bodies, and the escape of the internal air prevented, the elastic force of the air within us would immediately tear the lungs and other vessels to pieces, force the blood through the
arteries and veins, and put an end to all the functions of the animal machine. If an animal, as a cat, mouse, or bird, be put under a receiver, and the air exhausted, the animal will be at first oppressed as with a great weight, then grow convulsed, their bodies will swell, and if they are allowed to remain only for a few minutes, they inevitably die. Were we to take a shrivelled apple, and put it under the receiver of the air-pump, and exhaust the air, the skin will gradually swell as the pressure of the air diminishes, the wrinkles will be filled up, and the apple will appear as if freshgathered. When the air is let in, it returns again to its former withered state. The effect now stated, is owing to the elasticity of the air in the inside of the apple, which expands when the atmospheric pressure is removed.
From a variety of experiments it is demonstrated, that the spring of the air is equal to its weight, and produces the same effects as its pressure; for, action being equal to re-action, the force which the elasticity of the air exerts, in endeavouring to expand itself, is equal to the force with which it is compressed, just as it is in the spring of a watch, which exerts no force, but in proportion as it is wound up. If a quantity of air, therefore, is included in a vessel, and is of the same density with the surrounding air, its pressure against the sides of the vessel is equal to that produced by the external atmosphere. Hence it is that we can break a square glass bottle, either by the
direct pressure of the atmosphere, after the air has been extracted from it, or by removing the pressure of the atmosphere, and allowing the elasticity of the air within to exert its expansive force.
It is owing to the elasticity of the air that it is susceptible of dilatation and compression. To what degree air of the same density which it possesses at the surface of the earth is capable of being compressed, has not yet been fully ascertained. Dr. Halley informs us that he has seen it compressed, so as to be sixty times denser than in its natural state. Some have supposed that no bounds can be fixed to the condensation of air. But it some experiments made in London, and by the Academy of Florence, that no force whatever is able to reduce air into eight hundred times less space than that which it naturally possesses at the surface of the earth. It is owing to the power of being artificially condensed, that forcing-pumps produce their effects, and that an air-gun is enabled to discharge a ball to a considerable distance with great violence. The air is forced into a certain compartment of the gun by means of a syringe or condenser, which drives the air in, and suffers none of it to come back till it be sufficiently condensed. When the valve which confines the air is opened, the air by its elastic power rushes in behind the ball, and forces it out of the barrel with great violence. It would be better for mankind, however, that no such
instruments were ever constructed. Science ought always to have for its object the construction of instruments and machines which have a tendency to promote the comforts of mankind, not those which may be employed by unprincipled men as weapons of destruction; and, therefore, the construction of this instrument is alluded to merely as an illustration of the powerful effect of the elasticity of the air. Would to God that guns, and cannons, and warlike instruments of all descriptions were for ever unknown among men; that swords were beaten into ploughshares, and spears into pruning-hooks; that nation might no longer lift
up sword against nation, but delight themselves in peace!
The dilatation or expansion of air, in virtue of its elastic force, is found to be very surprising. In several experiments made by the honourable Mr. Boyle, it dilated first into nine times its former space, then into thirty-one times, then into sixty, and then into one hundred and fifty. Afterwards, it was brought to dilate 8,000 times its space, then into 10,000 times; and, at last, into 13,679 times the space it originally occupied, and all this was effected by its own expansive force, without the help of fire, or the principle of heat. Hence it appears that the air we breathe near the surface of the earth is compressed by its own weight into at least the 13,000th part of the space it would occupy in vacuo. And, as it has been found that it may be compressed into