Air - The air consists chiefly of two substances, only one of which can keep up the process of burning. This substance is known as oxygen. The other, in which nothing can burn, is known as nitrogen. Besides these the air contains smaller quantities of other substances, particularly water vapor, carbonic acid (carbon dioxid), and ammonia.

Composition of air.

Oxygen - Oxygen is the most common element in nature. It forms between forty and fifty per cent of the solid crust of the earth, eight-ninths of all the water on the globe, and one-fifth of all the air around the globe.

We have oxygen around us in great abundance, but it is mixed with nitrogen, and it is difficult to separate the two so as to secure the oxygen for any practical or commercial use.

Distribution of oxygen.