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of their air, are easily crushed to pieces by the weight of the external air. Two small hollow segments of a sphere four inches in diameter, exactly fitting each other, being emptied of air, are pressed together by a force equal to 188 pounds, by the weight of the ambient air; and that they are kept together by the pressure of the air is evident by suspending them in an exhausted receiver, when they will separate of themselves. Moreover, if a tube, close at one end, be filled with mercury, and the other immerged in & basin of the same fluid, and thus erected, the mercury in the tube will be suspended at the height of about thirty inches above the surface of that in the basin. The reason of this suspension is, that the mercury in the tube cannot fall lower, without raising that in the basin; which being pressed upon by the weight of the incumbent atmosphere, cannot give way, unless the weight of the mercury exceeds that of the air without it. That this is the case is evident; because, if the whole apparatus be included in the receiver of an air-pump, the mercury will fall in proportion as the air is exhausted; and, on gradually letting in the air again, the mercury re-ascends to its former height. This makes what is usually called the Torricellian Experiment. The gravity of air may be further demonstrated, from the consideration that it can actually be weighed. Its weight is continually varying, according to the different degrees of heat and cold, and the concurrence of other causes. Its weight to that of water has been differently estimated by philosophers. Dr. Jurin thinks, that at a medium between heat and cold, when the barometer is thirty inches high, the proportion between the two fluids may be taken as 1 to 800; which agrees with the observation of Mr. Cavendish, the thermo

meter being at 50 degrees, and the barometer at 292 inches.

Air, then, being heavy and fluid, the laws of its gravitation, or pressure, may be inferred to be the same as in other fluids; and, consequently, its pressure must be as its perpendicular altitude. This is also confirmed by experiment; for, if the Torricellian tube be removed to a more elevated place, where the incumbent column of air is shorter, a proportionably shorter column of mercury is sustained. For, proceeding from the law given by observation, that the air is compressed in the ratio of the weights with which it is charged, it is proved that when the heights are in arithmetical progression, the corresponding densities are in geometrical progression; and it is manifest that these densities are in their turn proportional to the depressions of the mercury in the barometrical tube. On this principle, then, depend the structure and office of the barometer.

It follows hence, also, that the air, like all other fluids, must press equally every way. This is confirmed by observing, that soft bodies sustain this pressure without any change of figure, and brittle bodies without breaking; although the pressure upon them is equal to that of a column of mercury thirty inches high, or a column of water of thirty-two feet. It is obvious that no other cause can preserve such bodies unchanged, but the equable pressure on all sides, which resists as much as it is resisted. And hence, upon removing or diminishing the pressure on one side only, the effect of the pressure is soon perceived on the other.

From this gravity of the air, considered in connection with its fluidity, may be deduced several

Phil. Trans. vol. lvi. page 152.

of its uses and effects. And first, by means of the weight and fluidity of the air, it closely invests the earth, with all the bodies upon it, and constringes and binds them down with a force amounting, according to the computation of M. Pascal, to 2232 pounds weight upon every square foot, or upward of fifteen pounds upon every square inch. Hence, it prevents, for instance, the arterial vessels of plants and animals from being too much distended by the impetus of the circulating juices, or by the elastic force of the air so plentifully contained in the blood. Thus we see, in the operation of cupping, that, upon a diminution of the pressure of the air, the parts of the body grow tumid; which necessarily alters the manner of the circulation through the capillaries, &c. The same cause prevents the juices oozing and escaping through the pores of their containing vessels.

The weight of the air likewise promotes the mixture of contiguous fluid bodies. Hence many liquids, as oils and salt, which readily and spontaneously mix in air, remain, on a removal of it, in a state of separation. The gravity of air, moreover, in some cases, determines the action of one body upon another; and to the same principle are chiefly owing our winds, which are only air put in motion by some alteration in its equilibrium.

Elasticity, the third property of the air which I have mentioned, is the power of yielding to an impression by contracting its dimensions; and, upon removing or diminishing the impressive cause, of returning to its former space or figure. This elastic force has been long accounted the distinguishing property of air; the other properties, hitherto enumerated, being common to it with other fluids'. This property of air was first dis

I From some late experiments, however, it is more than

covered by some experiments of Lord Bacon, who, upon this principle, constructed his vitreum calendare, the first thermometer.

Of this elastic power we have numerous proofs: thus, a blown bladder being squeezed in the hand, we find the inclosed air sensibly resist; so that, upon ceasing to compress, the cavities, or impressions made in its surface, are readily expanded, and filled up again. On this property of elasticity depend the structure and office of the air-pump.

Every particle of air always exerts this endeavour to expand, and thus strives against an equal endeavour of the ambient particles; and when the resistance of the latter happens, by any means, to be weakened, it immediately diffuses into an immense extent. Hence it is, that thin glass bubbles, or bladders, filled with air, and exactly closed, being included in the exhausted receiver of an air-pump, burst, by the force of the included air. In like manner, a bladder, quite flaccid, containing the smallest quantity of air, swells in the receiver, and appears quite full. The same effect is also found, by carrying the flaccid bladder to the top of a high mountain.

The compression of the air by its own weight is surprisingly great; and it may be still further compressed by art. The utmost limits to which air, of the density which it possesses at the surface of the earth, may be compressed, have not been ascertained. Dr. Halley thinks, that no force whatever is able to reduce air into 800

probable that the capacity of being compressed and expanded is not peculiar to air. Mr. Canton, in particular, infers, from several experiments, that spirit of wine, oil of olives, water and mercury, are not only compressible, but elastic. See Phil. Trans. vol. lii. art. 103, and vol. liv. art. 47. And it is now well known 1816) that all the gases are permanently elastig

times less space than that which it naturally possesses on the surface of the earth'. On the contrary, M. Amontons, in the Memoirs of the French Academy, maintains, that there is no affixing any bounds to its condensation; that greater and greater weights will still reduce it into less and less compass; that it is elastic in virtue only of the fire which it contains; and that, as it is impossible ever absolutely to drive all the fire out of it, it is impossible ever to make the utmost con

densation.

The dilatation of the air by virtue of its elastic force, is also found to be very surprising. Sir Isaac Newton thinks it capable of diffusing into above a million of times more space than it before possessed; and Dr. Wallis suggests, that we are far from knowing the utmost of which it is capable. This immense compression and dilatation, Sir Isaac Newton observes, cannot be accounted for in any other way, than by a repelling force, with which the particles of air are endued; by virtue of which, when at liberty, they naturally fly each other; and this repelling power, he adds, is stronger and more sensible in air than in other bodies.

This elastic power of the air is the second great source of the effects of this important fluid. By means of this, it insinuates into the pores of bodies, possessing this prodigious faculty of expanding, which is so easily excited, that it must necessarily put the particles of bodies, into which it insinuates, into perpetual oscillations. Indeed, the degree of heat, and the gravity and density of the air, and consequently its elasticity and expansion, never remaining the same for the least space of

Philos. Trans. Abridg. vol. ii. p. 17.

2 Optics, p. 371.

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