Air is now known to be a mixture of many colorless gases. It is composed chiefly of nitrogen (about 78 per cent, or nearly four fifths), oxygen (about 21 per cent, or about one fifth), and argon (nearly 1 per cent).
Air contains very small quantities of the gases carbon dioxide and ozone. It contains also traces of ammonia gas and the rare gases helium, neon, krypton, and xenon. In addition, it has in it still smaller quantities of the gases which escape from factories, natural-gas wells, coal mines, cooking rooms, and similar sources. Water vapor is always found in the air. So also are solid substances in the form of fine dust.
It must be understood that the different parts of the air are not combined with one another. They are merely mixed together. Dust may be present sometimes only in a trace and sometimes in such large quantities as to form a dark cloud. Thus the amounts of dust, and also of water vapor, in the air vary much more widely than the proportions of the principal gases composing the air.