[Sidenote: Composition of air]
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), ammonia, and carburetted
hydrogen.
[Sidenote: Distribution of oxygen]
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.