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pump extracts the air, and he shall find that as the air on the inside is removed, the air on the outside will press his hand down with much violence. Nor will he be surprised at this pressure of the air, when he comes to learn that a column or pillar of air, of only one inch diameter, and thirty-two miles high, which is the depth of the atmosphere from its top above the clouds to the ground, weighs about fourteen pounds. If the hand of the person, which covers the top of the tube, measure ten inches square, the sure on it will be about one hundred and forty pounds-sufficient in all conscience to crush every bone in the hand. By the same token, a square foot of such a column of air would weigh near two thousand pounds, and as a common sized man measures about fourteen square feet, it is a fact, as curious as it is awful, that every such person bears constantly on his body a weight of fourteen tons, or twenty-eight thousand pounds of air. Some persons may doubt this from the conclusion that such a weight would crush every man to pieces. So it would, if it were to press solely on any particular part. But this conclusion instantly falls to the ground, when it is recollected, that this pressure of the air is uniform and equal all around him, the air pressing as strongly from below as from above; from one side as from another; thus causing the various pressures most exactly and admirably to counterpoise each other; of this we see a wonderful instance in the case of fishes in the ocean. One of these animals at a great depth under water, would be crushed to atoms, if all that heavy element pressed only on his back. But the God who made him has so kindly attended to his safety, as to cause the water that surrounds him from below to press upwards as strongly as that from above to press downwards. There is another reason why our bodies are not so sensible of the tremendous weight of air, which thirty-two miles deep presses upon us; it is simply this, all bodies are full of air; and the air within pressing against that without, preserves even the most delicate bodies uninjured. A bladder, or even a bag of oiled paper, if filled with water, remains perfectly unhurt, though a hundred fathoms below the surface of the sea; because the water within furnishes a full resistance to the water without. But take away this resistance

from within, and you shall find that the slightest pressure will bring the two sides together.

According to the late discoveries in Chymistry, the atmosphere consists of three different species of air-namely, pure, respirable, or dephlogisticated air; azotic, or phlogisticated air; and fixed, or carbonic acid air.

The proportion of the first, namely, pure or vital air, consists, according to the French Chymists, who have given it the name of Oxygen, of 27 or 28 in the hundred parts; the second, the Azote of the French, of 72 or 73 in the hundred; and the third, namely, the Carbonic acid air, of about one part only in the hundred.

Oxygen is much better adapted to the respiration of animals than common atmospheric air. If two animals be enclosed in vessels, one of which contains pure Oxygen and the other common atmospheric air, in proportions equal to the size of the animals, the former in the Oxygen will be found to live six or seven times as long as the latter in common air. It is properly this Oxygen which we inspire, and which is the grand support of animal life. Persons apparently dead, or in a state of suffocation, have been instantly restored to life, by its influence; and from the corresponding testimony of several respectable physicians, it appears to have been employed with advantage in many obstinate diseases. The celebrated Ingenhouz, therefore, gave it the name of vital air. It promotes combustion in a very high degree. A candle will burn in it from six to seven times longer than in common air, with a much greater degree of heat, and a more brilliant flame. Bodies in a glowing state are immediately inflamed, when put into Oxygen gas; and even metals, which are not very fusible, are melted in it with the greatest facility.

Azote, by others called phlogisticated, mephitic, corrupted, or suffocative air is absolutely unrespirable, and not miscible with water. It arises from the change which atmospherical air undergoes in every process of combustion, putrefaction and respiration, whether produced by nature or art.

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Azote enters into no combination with water, but may be rendered less hurtful by shaking it with that fluid. This accounts in some measure for the salubrity of the sea-air. It greatly promotes the growth of plants, and readily accumulates in apartments filled with people, or containing articles fresh painted with oil colours, or in which strongly fragrant flowers are kept without having any accession of fresh air. We should be extremely cautious in entering such places, as diseases of the breast and lungs are too frequently the consequence of neglect or ignorance.

The Carbonic acid air, or fixed air, is miscible with water, but in its pure state is equally unrespirable as the Azote. It derives its origin, partly from the vinous fermentation of vegetables and some animal substances, and partly from the mild alkaline salts and earths combined with acid. Much of this air abounds in mines, where it frequently distresses the workmen by its suffocating effect. It is also observed in most mineral waters, where a stratum of it sometimes floats upon the surface of the well.— These waters, as well as fermented liquors which contain a considerable portion of fixed air, receive from it that well known pungency so agreeable to the palate. Hence flat and spoiled beer or wine, may be corrected and restored to its former briskness, by the addition of fixed air evolved from chalk and vitriolic acid; or by mixing it with new beer, or wine in a state of fermentation.

As this species of air quickly extinguishes fire, animals cannot

live in it.

These three aerial bodies, though blended together, arrange themselves, in some degree, according to their specific gravities; that is, the proportion of azotic air, which is the lighter body of the three, will be found most in the upper part, the oxygen air in the middle, and the fixed air will be found most in the lower part of the apartment. This occasions a circulation in the air, the rarified air will ascend, the fixed air sink, and the colder and purer air rush into the apartment through every crevice. To render the circulation of the air plain to sense, if the air of a room be heated by a fire, whilst the air in the next room is cold,

and the door between opened, the hot air of one room being rarified, will pass through the upper part of the opening of the door into the cold room; and on the contrary, the cold air of the other room being heavier, will pass into the former through the lower part of the opening. This may be proved by applying a candle at the upper and lower openings between the two rooms. The direction of the flame of the candle will point out the contrary currents of air. It is for this reason, that when a fire is lighted in a chimney, a strong current of air enters the room, which may be felt by applying the hand near the key-hole, or other small openings if the door and windows be shut. A fire is said to purify a room; but this it does partly by drying the dampness of the room, and chiefly by promoting the circulation of the air. The fire does not perform such service by purifying the bad air, but by removing it, and substituting that which is fresh and wholesome. Hence it appears that those persons are mistaken, who are over anxious to keep air from the apartments of convalescent persons, studiously stopping, by list, linings, and sand bags, all the smallest openings that admit fresh air.

Unless the air were constantly renewed, persons would be exposed to the most fatal accidents in large assemblies or crowded

rooms.

A rout was lately given at a celebrated bathing-place, or spring. The room was small and the company very numerous. They had not been long scated at the card tables, before a young gentleman and lady, both in delicate health fell into a swoon. The doors and windows were immediately thrown open, to afford fresh air, which quickly dissipated the alarm, by reviving the young invalids. A physician present, telling one of his medical companions how severely he himself had suffered from the air of that vile oven, and that he had made up his mind to write a bitter phillippic against Routs, was archly answered by his friend:"Let them alone doctor, how otherwise should twenty-six physicians subsist in this place?"

A further illustration: Take a room thirty feet by twenty-five, and thirty feet high, capable of containing one hundred persons. Now, since each person consumes about five cubic feet of air in an hour, that is, deprives such a quantity of air of its oxygen, or vital

principle, it would follow, that, as such a room could contain only twenty-two thousand five hundred cubic feet of air, unless the air was constantly renewed, it would be rendered completely mephitic or noxious in about four hours and a half, and it is probable that the greater part of the company would be seriously incommoded, or even perish long before that time.

The following affecting narrative is a melancholy confirmation of this fact. In the summer of 1756, the British settlement of Calcutta, in India, was attacked by the natives under the viceroy Rajah Doulah, a young man of the most violent passions, and without the least sense of honour or humanity. After a most obstinate resistance, the little garrison surrendered themselves prisoners of war, on a solemn promise from the Rajah of the most honourable treatment. But no sooner had the monster got them in his power, than, utterly regardless, of that due to honour, humanity, and a brave enemy, he barbarously drove them all into a dark shallow vault under ground, called the black hole, only eighteen feet square. The number of the unfortunate men, thus cruelly immured, was one hundred and forty-six, with their gallant commander, Colonel Holwell, the historian of the following tragedy. The humane reader may form some idea of one hundred and forty-six poor fellows, many of them badly wounded and bleeding, and all worn out with the fatigue, and covered with the dust and sweat of a hard day's fighting, crammed together, on a bot sultry evening, into a small dirty hole, eighteen feet square, with only two little windows, and those obstructed by strong iron bars.

A profuse sweat quickly broke out on every individual, attended with an insatiable thirst, which became the more intolerable as the body was drained of its moisture. It was in vain they stripped off their clothes, or fanned themselves with their hats.

A difficulty in breathing was next observed, and every one panted for breath. Colonel Holwell, who was placed at one of the windows, called to the sergeant of the guard, and after striving to excite his compassion by drawing a pathetic picture of their sufferings, promised him a thousand rupees in the morning, provided he could find means to remove some of his people into another place of confinement. The sergeant, allured by the pro