lungs, and is absorbed into the blood: it appears essentially necessary to vitality.

An animal can only live for a limited time in a given portion of air. If a mouse or a bird be confined under a glass that is closed, they will soon die; a candle, also, will burn only a short time. In crowded rooms, where there is not a free circulation of air, the oxygen is diminished by the respiration, of so many persons, and the air is rendered unhealthy. The lights, also, are observed to burn dim, and contribute much to exhaust the oxygen. This points out the importance of ventilating all kinds of apartments, but particularly public places.

It has been found that in 100 parts by measure, of atmospheric air, there are 21 parts of oxygen gas and 79 of nitrogen gas.

From the property of oxygen as being essential to respiration and animal life, it had been thought that the salubrity of air must depend upon the quantity of oxygen which it contained; but, although the airs of various places have been examined, as that of towns, prisons, the country, tops of hills, the ocean, &c., it appeared that the proportion of oxygen did not sensibly differ in them all. The healthiness of certain airs, therefore, must depend upon some other circumstances.

Although the great mass of the atmosphere is to be considered as consisting of oxygen and hydrogen, yet it contains a small quantity of many other gases, and also water, and a variety of exhalations and substances dissolved in it. It always contains a portion of carbonic acid gas, perhaps 1 part in 1000, for alkalies become effervescent when exposed to it, and lime water acquires a pellicle on being

exposed a sufficient time to the action of the air, even upon the highest mountains.

Since the oxygen of the atmosphere is continually abstracted from it by various decomposing processes, it would appear that nature must have some mode of renewing a principle so important. Vegetables have been supposed to perform this office, since they always exhale oxygen gas in the day, and particularly when the sun shines. This circumstance may be easily observed by putting some leaves into an inverted tumbler of water placed in the sun-shine. Minute globules of air will be seen rising from the leaves, which, collected at the top and examined, will be found to be oxygen.

HYDROGEN.

Hydrogen is so called from two Greek words signifying the generator of water, because it is one of the constituent principles of this fluid. It is also one of the ingredients of bitumen, of oils, fat, ardent spirits, ether, and of all the proximate component parts of animal and vegetable bodies: it forms a constituent part of all animal and vegetable acids: it is one of the elements of ammonia, and of various compound gases.

It possesses so great an affinity for caloric, that it is impossible to procure it in the concrete or liquid state, independent of combination. Hydrogen united to caloric forms hydrogen gas.

Hydrogen gas is the lightest substance whose weight we are able to to estimate; when in its purest state it is about thirteen times lighter than atmospheric air. It is unfit for respiration; ani

mals, when obliged to breathe in it, die almost instantaneously. It has a peculiar and disagreeable smell.

It is decomposed by living vegetables, and its base is one of the constituents of oil, resin, &c. It is highly inflammable, and burns rapidly when kindled in contact with atmospheric air, or oxygen gas, by means of the electric spark, or by an inflamed body, exhibiting a blue lambent flame. Water absorbs about one thirteenth of its bulk. It dissolves carbon, sulphur, phosphorus, arsenic, and many other bodies. When its basis combines with that of oxygen gas, water is formed, and with nitrogen it forms ammonia.

An easy method of obtaining hydrogen gas consists in subjecting water to the action of a substance which is capable of decomposing this fluid.

For this purpose, let sulphuric acid, diluted with four or five times its weight of water, be poured on iron filings or bits of zinc, in a small retort or glass bottle; as soon as the diluted acid comes in contact with the metal, a violent effervescence takes place, and hydrogen gas escapes, without external heat being applied. It may be collected in the usual manner over water, taking care to let a certain portion escape, on account of the common air contained in the disengaging vessel.

Hydrogen gas is often found in great abundance in mines and coal-pits, where it is sometimes generated, and becomes mixed with the atmospheric air of these subterraneous cavities. If a lighted candle be brought into this mixture, it explodes, and produces the most dreadful effects. It is called by miners the fire damp. It generally forms a cloud

in the upper part of the mine, on account of its lightness, but does not mix there with atmospheric air, unless some agitation takes place. The miners frequently set fire to it with a candle, laying, at the same time on their faces, to escape the violence of the shock. An easier and safer method of clearing the mine is, by leading a long tube through the shaft of it to the ash-pit of a furnace; by this means the gas will be conducted to feed the fire.

Hydrogen gas, though itself inflammable, extinguishes burning bodies, and is incapable of maintaining combustion. Bring an inverted jar filled with hydrogen gas over the flame of a candle, and depress the jar, so that the lighted wick may be wholly surrounded by the gas; the candle will be immediately extinguished.

Hydrogen gas is only inflammable when in contact with atmospheric air or oxygen gas. Fill a small phial with hydrogen gas, and take it from the pneumatic trough, placing the thumb on the mouth thereof, to prevent the gas from escaping; if a lighted taper be applied to the mouth of the phial, the gas will take fire, and burn with a lambent flame. The gas will only burn where it is in contact with the atmospheric air; the flame will descend gradually, till all the gas is consumed.

If the hydrogen gas be pure, the flame will be of a blue colour; but if the gas holds any substance in solution, which is generally the case, the flame is tinged. of different colours, according to the substance. It is usually reddish, because the gas holds in solution a little charcoal.

On this principle is constructed the philosophical candle, which cannot be easily blown out. Fill with hydrogen gas a bell glass, furnished with a

capillary tube; compress the gas, by making the bell descend below the surface of the water in the pneumatic trough; then apply a lighted taper to the upper extremity of the tube; the gas will take fire, and exhibit a candle, which will burn till all the gas is exhausted.

Artificial fire-works may be constructed by filling bladders with hydrogen gas, and connecting them with revolving jets, tubes, &c., bent in different directions, and formed into various figures pierced with holes of different sizes. The air which is forced through these holes by pressing the bladders, will, when inflamed, exhibit a curious firework, without either noise or smoke.

By the inflammable property of hydrogen gas, and the effects of electricity, a curious lamp has been invented by Volta, which, by turning a stop cock only, may instantly be lighted, and that many hundred times.

Hydrogen gas burns more readily in proportion as it is surrounded with a larger quantity of atmos pheric air. Hydrogen gas and atmospheric air, or, what is better, oxygen gas, may be mixed together, so that every particle of each gas shall be contiguous to a particle of the other, in which case they will burn with great rapidity.

Into a strong bottle, capable of holding about four ounces of water, put one part of hydrogen gas and two of amospheric air. On applying a lighted taper, the mixture will explode with a loud report, and the inside of the bottle will become moist. It will be prudent to wrap a handkerchief round the bottle, to prevent it from doing any injury if it should burst.

The same experiment may be made with oxygen gas, instead of atmospheric air, changing the pro