28. The atmosphere is composed of several bodies of gas, which exist independently of one another. They are thoroughly mingled by diffusion, but they are not united or combined in any way. The principal gases in the atmosphere are oxygen, nitrogen, water vapor or steam, and carbonic acid, the last in very small proportion.

The weight and density of the oxygen and nitrogen are nearly equal; consequently, it is not practicable to separate them by mechanical means. Wherever the oxygen is permitted to go, the nitrogen will certainly accompany it, because no means are known by which it may be held back.

The chemical properties of these gases are very different. Oxygen supports combustion, and is required by all living things to sustain life. In breathing air, nitrogen is taken into the lungs along with the oxygen, because it cannot be avoided. It is not known to be of any use to animals, but it is absorbed from the air and converted into useful substances by several varieties of plants, such as peas and beans, and notably by the common red clover. Nitrogen operates to retard combustion, mainly by absorbing heat. Pure oxygen may be breathed with impunity, and persons may live in it for a short time without serious harm. But as the vital energy is so much greater than that to which the lungs are accustomed, a person breathing it would become feverish and intoxicated.

In the ordinary atmosphere the proportions of oxygen and nitrogen vary slightly at times, but not to any important extent. The average composition of 1 pound of dry air is as follows: