use of dephlogisticated air might, perhaps, wear out the system much sooner than common air, in the same manner as it consumes fuel much faster than common air. The great quantity, however, even of the purest air, which is requisite to support animal life, and the expense and trouble of the most ready methods of procuring it, have hitherto prevented any fair trial from being made. Yet philosophers, considering the probability of this kind of air being salutary in many diseases, have bestowed some pains in attempting to find out methods of procuring it easily, and in a large quantity'.

This important discovery by Dr. Priestley had been preceded by the discoveries of many other ingenious philosophers. To Lord Bacon is due the discovery of artificial or factitious air 2; a term used by Mr. Boyle to express what the more antient chemists denominated gas, and which is now more generally known by the name of fixed air. It comprehends all those kinds of air, which are, originally, parts of some solid substance, and exist in an inelastic state; but which, by various analytical processes, are capable of being disengaged, and recovering their elasticity.

Sir Isaac Newton discovered the true permanent air arising from fixed bodies by heat and fermentation, and distinguishable from vapour, which is only apparent or transient air.-Dr. Hales discovered the air abounding in the Pyrmont waters, to which he attributed their spirit and briskness. He was the first, likewise, that produced the nitrous air from the Walton pyrites, by means of spirit of nitre; and to him our hospitals, prisons,

See Cavallo's Treatise on Air.

2 Sir John Pringle's Discourse on the different Kinds of Air,

and ships of war, owe the beneficial invention of ventilators.-Dr. Brownrigg discovered the quality of that air, which is of the mephitic or deadly kind; such as is found in damps, deep wells, caverns, and mines, and which is so often fatal to miners.—Dr. Black was the first who accurately investigated the chemical nature and effects of that particular kind of factitious air, to which he gave the name of fixable or fixed air, which adheres to all calcareous earths and alkaline salts, with different degrees of force, and which may be separated from these substances, and again combined with them.-Mr. Cavendish first discovered the acid air, produced from copper, by means of spirit of salt: it has a strong affinity with phlogiston; extracts it from other substances, such as inflammable spirits, expressed oils, &c. and is thereby converted into inflammable air. Mr. Cavendish also discovered inflammable air, of the nature of that found in neglected privies, common sewers, and chiefly in coal-pits, where it is called the fire-damp. This kind of air is surprisingly light, being only the tenth part of the weight of common air, and therefore totally different from the mephitic' or fixed air, which is found to be heavier. The inflammable air is that by which all the aërostatic machines (the Montgolfiers excepted) have been filled: it may be produced in abundance from zinc, iron, and tin, by dissolving them in the diluted vitriolic acid, or spirit of sea salt.

The term mephitic is applied to fixed air, to that which is inflammable, and to many other kinds, since they are equally noxious when breathed by animals; but it is usually appropriated to the former species of air..

2 As inflammable air is specifically lighter than common air, it occupies the upper parts of subterraneous places, and is called the fire-damp, because it is liable to take fire, and to explode like gunpowder. The mephitic, or fixed air, on the contrary, being specifically heavier than common air, is found at the bottom of pits: it extinguishes candles, &c. and kills animals that breathe it; and, on

The common, atmospherical, or heterogeneous air, is a coalition of corpuscles of various kinds, which together form one fluid mass, wherein we live and move, and which we are continually receiving and expelling by respiration. The whole assemblage of this makes what we call the atmosphere. Where this air, or atmosphere, terminates, there ether is supposed to begin; which is distinguished from the air, by its not making any sensible refraction of the rays of light, as air does. Air, in this popular sense, is acknowledged, by Boyle, to be the most heterogeneous body in the universe. Beside the matter of light or fire, which continually flows into it from the heavenly bodies, and probably the magnetic effluvia of the earth, whatever fire can volatilize is found in the air.-In a word, a late eminent author considers the atmosphere as a large chemical vessel, wherein the matter of all the kinds of sublunary bodies is copiously floating; and thus exposed to the continual action of that immense surface the sun; whence proceed innumerable operations, sublimations, separations, compositions, digestions, fermentations, putrefactions, &c.

The component parts of our atmosphere are, indeed, so various, and of such heterogeneous natures, that they do not admit of any kind of definition or analysis, one only excepted; namely, the electric fluid. This, we know, pervades the

this account, has been called the choke-damp. Among the most useful and important inventions of modern science, is the safety-lamp of Sir HUMPHRY DAVY, for preventing explosions in coal mines from fire-damp. The flame of the lamp or candle is surrounded with a wire sieve, the meshes of which amount at least to 250 in an inch. This sieve completely prevents the explosion from setting fire to the gas on the outside of it, even though the most inflammable mixtures of gases, as oxygen and hydrogen be present. This contrivance is expected completely to answer the purposes of the miner, and to prevent many dreadful accidents,

whole, but appears to be much more copious in the upper than in the lower atmospherical regions. To measure the absolute quantity of this fluid, either in the atmosphere or any other substance, is impossible. All that we can know on this subject is, that the electric fluid pervades the atmosphere; that it appears, as before observed, to be more abundant in the superior than in the inferior regions; that it seems to be the immediate bond of connection between the atmosphere and the water that is suspended in it; and that, by its various operations, hail, rain, snow, lightning, and various other kinds of meteors, are occasioned.

Various attempts have been made to ascertain the height to which the atmosphere is extended all round the earth. These commenced soon after it was discovered, by means of the Torricellian tube, that air is a gravitating substance. Thus it also became known, that a column of air, whose base is a square inch, and the height that of the whole atmosphere, weighs fifteen pounds; and that the weight of air is to that of mercury as 1 to 10,800; whence it follows, that if the weight of the atmosphere be sufficient to raise a column of mercury to the height of thirty inches, the height of the homogeneous aërial column must be 10,800 times as much, and consequently, a little more than five miles high.

It was not, however, at any time supposed, that this calculation could be just; for as the air is an elastic fluid, the upper parts must expand to an immense bulk, and thus render the barometrical calculations extremely erroneous. Philosophers, therefore, have had recourse to another method, in order to determine the height of the atmosphere; namely, by a calculation of the height from which the light of the sun is refracted, so as to become visible to us before he himself is seen in the heavens.

By this method it was determined, that, at the height of forty-five miles, the atmosphere had no power of refraction; and, consequently, beyond that distance, was either a mere vacuum, or the next thing to it, and not to be regarded.

This theory soon became very generally received; and the height of the atmosphere was spoken of as familiarly as the height of a mountain, and reckoned to be as well ascertained as the height of the greater part of mountains. But slow, progressive, and imperfect, must be the inquiries of the philosopher into the great secrets of Nature: to the most intelligent and penetrating of human minds much must ever remain mysterious and inscrutable; mysterious and inscrutable too, perhaps, even to beings of a superior order to man in the scale of creation. To this supposed height of the atmosphere very great objections (and objections that have never yet been removed) arise from the appearance of some meteors, like large globes of fire, not unfrequently to be seen at vast heights above the earth. One of this kind appeared in March 1719, whose height Dr. Halley computed to be about 70 miles, its diameter upward of a mile and a half, and its velocity about 350 miles in a minute. There was also a remarkable one in 1783, whose distance from the earth, computed by Dr. Blagden, could not be less than 50 miles, its diameter not less than the former, and its velocity not less than 1000 miles in a minute. These meteors were not only extremely bright, but were attended with explosions. But how was flame to be supported, or sound propagated, at a height where the air must be 300,000 times rarer than that we breathe? Appearances of this kind are, indeed, with some probability, attributed to electricity; but the difficulty is not thus removed. In a word, it appears that the absolute height of the atmosphere is not