Earth's Important Atmospheres. --The earth is surrounded by four important atmospheres--nitrogen, oxygen, vapor of water, and carbon dioxide--and others of less importance, each comporting itself, in accordance with Dalton's law, practically as it would do if the others were not present, except that its rate of diffusion is retarded by their presence. This composite is atmospheric air--usually called air. It can be easily compressed, because there is space between its molecules, which are in constant vibration. A doubling of its pressure reduces its volume to one half. This is in accordance with the law discovered independently by Boyle and Mariotte, which is as follows: The temperature remaining the same, the volume of a given quantity of gas is inversely as the pressure that bears upon it.
Constituent Gases of the Air. --Air is composed almost entirely of oxygen and nitrogen, mechanically mixed and not in chemical combination. Both by volume and by mass these are the two principal atmospheres. In general, the proportion by volume is about 21 parts of oxygen to 78 parts of nitrogen. But it should not be thought that because some of the other constituent gases are relatively small in amount they are not vitally important in the carrying on of the functions that Nature seems to have assigned to the air. In addition to oxygen and nitrogen, air contains small amounts of many other substances--vapor of water (aqueous vapor), carbon dioxide, argon, nitric acid, ammonia, ozone, hydrogen, helium, xenon, krypton and neon; as well as organic matter, germs, and dust in suspension. Over the land it contains sulphates in minute quantities, and over the sea and near the seashore salt left from the evaporated spray can always be detected. The relative proportions of the gases of the air are practically the same in all parts of the open country.
Since the molecular weight of each component of the atmosphere is different from that of any other, therefore its proportion by volume of the