Even the man of science often finds it convenient to discuss and deal with air as if it were a single substance, but he is well aware that it is nothing of the kind. He knows that it is, in fact, a jumble of gases having very different properties. Some are heavy, others light. Some are chemically very active, others extremely inactive. Some are abundant, others very rare. These gases constitute the earth’s atmosphere. Other planets have atmospheres that are quite different in composition from ours. The sun itself has a very complex atmosphere. The earth’s atmosphere is, then, a collection of gases, which are mixed but not chemically combined. Some of them are themselves chemical compounds. Each of these gases behaves very much the same as if the others were not present, and each of them has its separate business to perform in the economy of nature. For example, a tree draws upon the store of carbon dioxide gas in the atmosphere to build up its tissues. Presently the tree is cut down and its wood is burned for fuel. In this process a different atmospheric gas is brought into play. We often say that the “air” supports combustion--that we supply “air” with a bellows to make a fire burn more brightly--but it is not the air as a whole that enables things to burn. Four-fifths of the atmospheric substance takes no part in the process. We burn with oxygen alone. So it is with breathing. Oxygen and not air constitutes the breath of life. Near the surface of the earth the proportions of the more abundant gases mixed together in the air are remarkably constant. Ignoring a variable admixture of water vapor, oxygen is always about 21 per cent, by volume, and nitrogen about 78 per cent. The remaining 1 per cent is mainly argon. At great altitudes, however, these percentages no longer obtain. The atmospheric gases differ greatly among themselves in weight, and in the high atmosphere, where they are not mixed by the winds, as they are below, the heavier tend to settle to the bottom and the lighter to float on top, as oil floats on water. It is calculated that at a height of thirty miles above sea level the percentage of nitrogen is about 86½ and of oxygen only 10, while at the same altitude the gas hydrogen, which at low levels constitutes less than one-hundredth of 1 per cent of the atmosphere amounts to more than 2½ per cent. Going higher, the percentage of hydrogen is supposed to increase rapidly, until, at an altitude of forty-eight miles, the atmosphere is more than half hydrogen, and at eighty miles above the earth this gas forms 99 per cent of the whole. These figures are not necessarily final; for some authorities believe that the atmosphere contains an unknown gas lighter than hydrogen, while others think that the hydrogen found in the lower air enters into chemical combinations before it can reach the higher levels; but it is beyond doubt that the composition of the upper atmosphere is quite different from that of the lower.