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WE have before observed, that the air on the tops of high mountains

is colder than on the surface of the earth. This has been ascribed by some philosophers to the igneous fluid, or elementary fire, being more rare in those elevated situations than on the plains. This fluid, they have supposed, is rarefied above by the action of the air, and below it is condensed by its own proper weight. They have considered fire as a fluid in motion, the parts of which are separable, and which is rarefied when its velocity is accelerated. Thus they have invented an hypothesis to account for this fact.

But the celebrated Bouguer has demonstrated, by principles the most simple, and most universally adopted, that it is not necessary, in order to account for the diminution of heat on mountains, to have recourse to hypotheses that are at best but doubtful. He has shewn, that the heat of the sun, on the tops of the highest mountains, is as great as in the plains; but that the rays of the sun, falling obliquely upon the former, and continuing but a few hours in the day, are not reflected with the same force as from the plains, where they fall more perpendicularly, and continue much longer: to which he adds, that the facility with which the air moves around a mountain is greater than that of the air in the plains, and consequently the greater cold of the air on mountains is easily and rationally thus accounted for.

Philosophers have invented different instruments whereby to measure and determine the various properties of the air, and which serve for several useful purposes. The barometer serves to measure its weight, to tell when it is heavier, and when lighter. It is not necessary for us to describe this instrument; but by its means we can, with some VOL. IV.

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exactness, determine the weight of the air, and, of consequence, tell beforehand the changes of the weather. Before fine dry weather the air is charged with a variety of vapours, which float in it unseen, and render it extremely heavy; then the quicksilver in the barometer rises. In moist rainy weather the vapours are washed down, or there is not sufficient density in the asr for them to rise, so that the air is then sensibly lighter; in this case the quicksilver is seen to fall. Our bodies also, especially those of persons afflicted with nervous complaints, seem to correspond with the changes of the weather-glass; they are braced, strong, and vigorous with a large body of air upon them; they are languid, relaxed, and feeble when the air is light, and refuses to give our fibres their proper tone.

The thermometer is an instrument whereby to measure the degrees of heat and cold in the air. By this instrument also the various degrees of heat and cold are determined in all other fluids.

The barometer is also serviceable in measuring the height of mountains; for, as the higher we ascend from the surface of the earth the air becomes lighter, so the quicksilver in the barometer will descend in proportion. It is found to sink at the rate of the tenth part of an inch for every ninety feet of perpendicular height; so that in going up a mountain, if I find the quicksilver fallen an inch, I conclude that I have ascended near nine hundred feet. In this, indeed, there has been some variation found; but it is not our province to enter particularly into this.

But notwithstanding the general uses of the barometer, it is subject to many irregularities, which no exactness of the instrument can remedy, nor any theory account for. When high winds blow, the quicksilver generally falls: it rises higher in cold weather than in warm, and is usually higher at morning and evening than at mid-day; and it generally descends lower after rain than before it. There are also frequent changes in the air, without any sensible alteration in the barometer.

In order to determine the fact of the elasticity of the air, the windgun has been invented. This is an instrument variously made; but in all its forms the principle is the same, which is, to compress a large quantity of air into a tube, in which there is an ivory ball, and then giving the compressed air free power to act, and drive the ball as directed. Thus driven, the ball will pierce a thick board, and will be as fatal, at small distances, as if driven with gunpowder. This instrument has been assisted by heat-which we know has the power of expanding air to an unknown degree-by the aid of which a force has been given, to the airr-gun which gunpowder itself is not possessed of.

The air-pump is an instrument contrived to exhaust the air from a vessel adapted to that purpose, called a receiver. By this instrument the air is so totally exhausted, that animals die almost immediately in the

yacuum, and even vegetables languish, and soon also die. Hence we are shewn what are the benefits and effects of air, in supporting animal life, and promoting vegetation,

The digester is another instrument of very great power, and sufficiently discovers the amazing force of air, when its elasticity is augmented by fire. A common tea-kettle, if it was strong enough, with the spout closed up, and the lid put firmly down, would serve as à digester. But the proper instrument used for this purpose is a strong metal pot, with a lid to screw close on, so that, when down, no air can get in or go out into this pot meat and bones are put, with a small quantity of water, and then the lid is screwed close, a lighted lamp is put underneath, and, what is very extraordinary, yet equally true, in eight or ten minutes, the whole mass, bones and all, are dissolved into a jelly-so great is the force and elasticity of the air contained within, struggling to escape, and breaking in pieces all the substances with which it is mixed. Care, however, must be taken not to heat this instrument too violently, for then the inclosed air would become irresistible, and burst the vessel with an explosion which might, perhaps, be fatal.

On the same principle as the foregoing does that culinary utensil act which is also called a digester: only this latter has a small vent in the lid, which, when the vessel is fully heated, the force of the air within lifts up, whereby the steam escapes: the power of this instrument is' consequently much less than the other, but sufficient, however, to extract all the marrow, and every nutritive juice from all the bones which are stewed in it. Such is the economical utility of this vessel, especially in these times of scarcity, that every prudent family, which knows its service, will not be without one.

There are many other useful instruments made, which depend on the weight, the elasticity, or th fluidity of the air, which do not come within the compass of our work-as our design is not to give an account of the inventions which curious men have made to determine the nature and properties of air, but a mere narrative of its effects. The description of the pump, the forcing pump, the fire engine, the steam engine, the syphon, and an hundred others, belong not to the naturalist, but to the experimental philosopher: the one gives an history of nature as he finds she presents herself to him, and he draws the obvious picture the other pursues her with close investigation, tortures her by experiments to give up her secrets, and measures her latent qualities with laborious precision.

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Much more might, therefore, be said of the mechanical effects of air, and of the conjectures that have been made concerning the form of its parts; how some have supposed them to resemble little hoops coiled up in a spring; others like fleeces of wool; others that the parts are endued with a repulsive quality, by which, when squeezed together, they endeavour to fly off and recede from each other.

We have already noticed the disputes relative to the height to which this body of air extends above us, concerning which we see there is no agreement. The disputes and conjectures of philosophers would furnish us with many other ideas upon the same theme, as, How much of the air we breathe is elementary, and not reducible to any other

substance? and, Of what density would it become if continued down to the centre of the earth? Then, by the help of figures and a bold imagination, we might shew, that, at the centre, it would be twenty thousand times heavier than its bulk in gold. By the same method we might also prove, that, when raised to the surface of the atmosphere, it is millions of times purer than upon earth. But such speculations do not belong to natural history, and they have hitherto been but of little service to experimental philosophy.

It may be of more service to remark something on the different degrees of salubrity in the atmosphere. The air on the tops of hills is generally more salubrious than that in vallies. Dense air is more proper for respiration than such as is more rare, especially for persons afflicted with phthisic or asthma; yet the air on mountains, though much more rare, is more free from phlogistic vapours than that of vallies and lower grounds. Hence it has been found, that people can live very well on the tops of mountains, where the barometer sinks to fifteen or sixteen inches.

M. de Saussure, in his journey on the Alps, having observed the air at the foot, at the middle, and on the summits of various mountains, observes, that the air of the very low plains seems to be less salubrious; that the air of the very high mountains is neither very pure, nor, upon the whole, seems so fit for the lives of men, as that of a certain height above the level of the sea, which he estimates to be about two or three hundred toises, that is about four hundred and thirty or six hundred and fifty yards.

The air of a bed room has been examined at night, and in the morning after sleeping in it, and it has been generally found, that after sleeping in it, the air has been less pure than at any other timeHence appears the injury to health from very close chambers, curtains, and shutters.

The air of privies, even in calm weather, has not been found to be so much dephlogisticated as might have been expected from its disagreeable smell. From this, and other observations, it has been concluded that the exhalations from human excrements are very little injurious, except when they become putrid, or proceed from a diseased body; in which case they greatly infect the air, and very quickly too.

Dr. Ingenhousz found by experiment that air at sea, and, in general, air along the shore, is purer, and fitter for animal life, than air on land. -Hence, perhaps, the practice of modern physicians in sending consumptive persons to the coasts, or prescribing to them a voyage.

On the whole, notwithstanding the experiments of philosophers, and all their conjectures, Providence has given us air generally fit for respiration, and all the common purposes of life, every where, and at every season, and has allowed us a certain latitude, or power of living and being in health in qualities of air which differ to a considerable degree. We do not mean to deny the existence of certain kinds of noxious airs in some particular places, but only say, in general, that the air is good every where, and that the small existing differences are not to be feared so much as some people would make us believe.

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But it will be necessary for us to take a further view of the air; and we shall find by inspection, that the fluid, in which we breathe is far from being a pure elementary substance. Air, such as we find it, is one of the most compounded bodies in all the compass of nature. Philosophers have considered our atmosphere as one large chemical vessel, in which an infinite number of various operations are constantly performing. In it all the bodies of the earth are continually sending up a part of their substance by evaporation, to mix in this great alembic, and to float awhile in common. Here minerals, from the lowest depths, ascend in noxious or in warm vapours, to make a part of the general nass: seas, rivers, and subterraneous springs, furnish their copious supplies; plants receive and return their share, and animals, that, by living upon, consume this general store, are found to give it back in greater quantities when they die. Water may be reduced to a fluid every way resembling air, by heat, which, by cold, becomes water again. Every thing we see gives off its parts to the air, and has a little atmosphere of its own floating around it. The rose is encompassed with a sphere of its own odorous particles, while the hemlock and the night-shade infect the air with scents of a more ungrateful nature. The perfume of musk flies off in such abundance, that the quantity remaining becomes sensibly lighter by the loss. A thousand, substances that escape our senses we know to be there: the powerful emanations of ̈ the loadstone, the effluvia of electricity, the rays of light, and the insinuations of fire. Such are the various substances through which we move, in which we breathe, and which we are constantly taking in at every pore, and returning again with imperceptible discharge.

This great solution, or mixture of earthly bodies, is continually operating upon itself, which, some have thought, is the cause of its unceasing motion: but it operates more visibly upon such gross substances as are exposed to its influence, for scarce any substance is found capable of resisting the corroding qualities of the air. The air, say the chemists, is a chaos, furnished with all kinds of salts and menstruums, and therefore it is capable of dissolving all kinds of bodies.

It is well known that copper and iron are quickly covered and eaten with rust; and that in the climates near the equator, no art can keep them clean. In those countries the instruments, knives, and keys that are kept in the pocket, are nevertheless quickly incrusted; and the great guns, with every precaution, after a few years, become useless by excessive rust. Stones, as being less hard, may be readily supposed to be more easily soluble. The marble of which the noble monuments of Italian antiquity are composed, altho' in one of the finest climates in the world, nevertheless shew the impressions which have been made upon them by the air. In many places they seem worm-eaten, and, in others, they appear crumbling into dust. Gold alone seems to be exempted from this general state of dissolution; it is never found to contract rust, though exposed to the air ever so long: the reason of this seems to be that sea salt, which is the only menstruum capable of acting upon and dissolving gold, is but very little mixed with the air; for salt being

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