The importance of the atmosphere to the animal economy, is so very great, and its derangements so very hurtful to health, that it appears that a few observations respecting it may be useful to some non-professional readers. It may enable them to understand better the observations that are to follow on its vitiation.
The atmosphere is a fluid of an elastic nature, encompassing the globe, occupying the space comprehended from its surface, to the distance of twenty or thirty miles therefrom. It possesses weight, and it is by this property that water rises in pumps, and that mercury is sustained in the barometer. It is in constant motion, going, as it does, with the globe itself, revolving on its axis, and rushing, in counter streams, from the tropics to the poles, and from the poles to the tropics.
That portion nearest the sun becomes rarefied and lightened with the heat which it acquires:—it then rushes, by virtue of its comparative lightness, to the poles, and that in temperate regions presses forward to occupy its place.
By means of this motion, the temperature of the earth is kept pretty uniform, and it is corrected of any impure taint it may acquire.
The atmosphere is composed of two gases, oxygen and nitrogen, a small quantity of watery vapour, and a fraction of carbonic acid gas, or fixed air.
Oxygen is the agent on which its more active properties depend. The other component, viz. nitrogen, serving to dilute it.
They are united in the proportion of about seventy-seven of nitrogen by volume, and twenty-one of oxygen, the rest being made up of watery vapour, and carbonic acid gas.
The atmosphere supports combustion,—oxygen gas being the essential agent. During combustion, it is consumed, and at the end of the process, it will be found wanting,—the other gas being undiminished.
This may be seen, at least the diminution in the volume of air, by burning a candle in a large wide mouthed bottle, inverted over coloured water. As it continues to burn, the water ascends in the bottle, and occupies the place of the oxygen consumed.
Atmospheric air supports respiration, a process essential to the continuance of life. Oxygen gas here, too, is the agent on which it depends. Air, which has been once respired, is found to be deprived of part of its oxygen, from ten to twelve per cent.