curiosity, this air is yet very wonderful, both in itself and in its uses. Imperfect as the knowledge of the ancients was, they recognised its importance by giving it a place among what they regarded as the four primal elements of nature-fire, air, earth, and water.

Yet, though apparently pure and elementary, it is by no means either a simple or pure substance. It is a mixture of several different kinds of matter, each of which performs a beautiful and wise part in relation to animal and vegetable life. Four substances, at least, are known to be necessary to its composition. Two of these, oxygen and nitrogen, form nearly its entire bulk; the two others, carbonic acid. and watery vapour, being present only in minute quantities.

Oxygen is a kind of air or gas, which, like the atmosphere itself, is without colour, taste, or smell. A candle burns in it with much greater brilliancy and rapidity than in common air. Animals also breathe in it with an increase of pleasure; but it excites them, quickens their circulation,

Fig. 1.

throws them into a state of fever, and finally kills them, by excess of excitement. They live too rapidly in pure oxygen gas, and burn away in it like the fast-flaring candle.

This gas is easily prepared by mixing the chlorate of potash of the shops with a little sand, powdered glass, or oxide of manganese, and heating the mixture in a flask over a spirit-lamp. When it melts, the gas is given off, and will soon fill the flask. It cannot be seen by the

eye, or detected by any of the other senses. Its presence may be readily shown, however, by introducing a lighted taper or a bit of red-hot charcoal, or of kindled phosphorus at the end of a wire (fig. 1). The brilliancy of the burning will prove the presence of the gas.

Fig. 2.

Nitrogen is also a kind of air which, like oxygen, is void of colour, taste, and smell; but a lighted candle is instantly extinguished, and animals cease to breathe when introduced into it. We obtain this gas by put ting a bit of phosphorus into a small cup over water, kindling it, and inverting over it a bottle, dipping with its mouth into the

water (fig. 2). When the phosphorus has ceased to burn, and the bottle has become cool, it may be corked and removed from the water. If a lighted taper be now introduced into the bot

tle, it will immediately be extinguished, showing that only nitrogen remains (fig. 3). In this process, the burning phosphorus removes the oxygen from the air contained in the bottle, and leaves only the nitrogen.

Oxygen is one

ninth part heavier,

Fig. 3.

and nitrogen one thirty-sixth part lighter than common air.

Carbonic acid is a kind of air which, like oxygen and

Fig. 4.

nitrogen, is void of colour; but, unlike them, possesses a slight odour, and a perceptibly sour taste. Burning bodies are extinguished, and animals cease to breathe when introduced into it. It is one-half heavier than common air, and can therefore be poured through the air from one vessel to another (fig. 4). When passed through lime-water, it makes it milky (fig. 5), forming with the dissolved lime insoluble white powder,

an

which, because it contains carbonic acid, is called carbonate

Fig. 5.

of lime, and is the same thing as chalk. It is the escape of this gas which gives their sparkling briskness to fermented liquors, to soda-water, and to the waters of some mineral springs.

Carbonic acid is easily prepared by pouring vinegar upon common soda, or diluted spirit of salt (muriatic The gas rises in bubbles

through the liquid, and, in consequence of its weight, remains in the lower part of the vessel. As it collects it gradually ascends, driving the common air before it, and at

*Lime-water is formed by pouring water upon slaked lime, shaking them well together, and allowing the mixture to settle. The clear liquid contains a portion of the lime in solution, and is thereforo called lime-water.

last flows, as water would do, over the edge of the vessel. Its rise may be shown by introducing two lighted tapers, as in the figure (fig. 6), when the lower one will be seen to go out, while the upper one is still burning.

By watery vapour is meant the steam or vapour visible, or invisible, which ascends from a surface of water when exposed to the air. When water is spilt upon the ground in dry weather, it soon disappears it rises in invisible vapour, and floats buoyantly among the other constituents of the atmosphere.

Fig. 6.

These four substances the air every where and always contains. They are all necessary to the daily wants of animal and vegetable life; but the two gases, oxygen and ni trogen, form so large a proportion of the whole that we are accustomed to say of dry air, that it consists of nitrogen and oxygen only, in the proportion of 4 gallons of the former to 1 of the latter. More correctly, however, air, when deprived of the watery vapour and carbonic acid it contains, consists, in 100 gallons, of 79 of nitrogen mixed with 21 of oxygen; or of

Nitrogen,
Oxygen,

By measure.

79

21

100

The carbonic acid exists in the air in very small proporAt ordinary elevations there are only about 2 gallons of this gas in every 5000 of air-36th part of the whole. It increases, however, as we ascend, so that at heights of 8000 or 10,000 feet the proportion of carbonic acid is nearly doubled. Even this increased quantity is very small; and

yet its presence is essential to the existence of vegetable life on the surface of the earth.

But being heavier than common air, it appears singular that the proportion of this gas should increase as we ascend into the atmosphere. Its natural tendency would seem to be rather to sink towards the earth, and there to form a layer of deadly air, in which neither animal nor plant could live. But independent of winds and aërial currents, which tend to mix and blend together the different gases of which the air consists, all gases, by a law of nature, tend to diffuse themselves through each other, and to intermix more or less speedily, even where the utmost stillness prevails and no wind agitates them. Hence a light gas like hydrogen does not rise wholly to the utmost regions of the air, there to float on the heavier gases; nor does a heavy gas like carbonic acid sink down so as to rest permanently beneath the lighter gases. On the contrary, all slowly intermix, become interfused, and mutually intercorporated, so that the hydrogen, the carbonic acid, and the other gases which are produced in nature, may be found everywhere through the whole mass, and a comparatively homogeneous mixture uniformly overspreads the whole earth. In obedience to this law, carbonic acid in all places slowly rises or slowly sinks, as the case may be, and thus, on the whole, a uniform purity is maintained in the air we breathe. If it seems to linger in sheltered hollows like the deadly gas-lake of Java, it is because the fatal air issues from the earth as rapidly as it can diffuse itself upwards through the atmosphere; and if it rest more abundantly on the mountain top, it is because the leaves of plants, and the waters of the sea, absorb it from the lower layers of the air faster than it can descend to supply their demands.

The watery vapour varies in quantity with the climate and temperature of the place. It is less in cold seasons