Snowballs

CHAPTER I
THE ANATOMY OF THE ATMOSPHERE

Two quite different conceptions of the substance called “air” are current in the world. One has prevailed from time immemorial. The other is wholly modern. One is the popular view, the other the scientific.

Ancient philosophers regarded air as one of the four “elements” of which all things were supposed to be made. Average humanity, though it did not concern itself with philosophy, must have begun, almost as soon as it realized the existence of air at all, to think of it as something that, however it changed its state from hot to cold, dry to moist, pure to impure, was fundamentally uniform—a single entity. Certainly this idea is in full vigor today. The air that we breathe, supply to our fires, stir with fans, pump into bicycle tires, fly in—the air that asserts its independence of our will in the wind and the weather—gives us the impression of individuality. We instinctively rank it with water among the simple, definite things in the repertory of nature.

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.

Of course almost any gas may be found locally and occasionally in the atmosphere, but there are several that are always found wherever a refined analysis of the air is made, and others that are generally present. The following is a fairly complete list: Nitrogen, oxygen, water vapor, argon, carbon dioxide, hydrogen, helium, neon, krypton, xenon, niton (radium emanation), ozone, hydrogen dioxide, ammonia and other compounds of nitrogen.

A number of these substances have only become known to science within the last quarter of a century. Argon, though it constitutes nearly 1 per cent of the atmosphere, escaped detection until the year 1894. The investigation of argon led to the discovery of some of the others. In 1895 it was found that the air, as well as certain minerals, contains helium. This substance was not new to science, but it had never before been found on earth. It was discovered in the atmosphere of the sun, by means of the spectroscope, as early as 1868. Terrestrial helium, neon, krypton, and xenon were all discovered by Sir William Ramsay, who also shared with Lord Rayleigh the distinction of discovering argon.

Ramsay has published the following figures for the proportions in which some of the rare gases exist in the atmosphere: