Chapter XX: The Atmosphere

The atmosphere or the air of our globe is the vast ocean of gaseous matter at the bottom of which human beings, as well as other land animals, dwell. While it is so thin that a vessel full of it is spoken of in ordinary language as being empty, it yet possesses a reality which it often displays in a very serious manner. Its presence is made gently evident to human beings by the moderate resistance it offers to them when they are in motion; but when itself is in motion with the force of the hurricane or tornado, no solid matters can stand in its path. Heavy railroad trains and massive buildings are hurled from their positions and turned into miserable masses of wreckage, while even strongly rooted forests are swept out of place by its vigorous breath. The terribly destructive power of air at one moment and its mild and subtle efficiency at another are very suggestive of the wonderful adjustment of the forces residing in it. It is by the restrained action of these forces that the atmospheric air is so admirably fitted to perform its varied functions in connection with animal life. At the same time it is so unobtrusive in its workings that its very existence is at first scarcely noted. When at rest, it peacefully wraps the earth about as in a gossamer veil, but when in angry agitation it scourges country and city alike as with a whip of gigantic cables.

The height to which the atmospheric air extends above the earth is not exactly known. But carefully devised experiments have shown that going upward, its compactness or density diminishes very rapidly. Indeed calculations based upon exact experiment show that at a height of fprty miles, or thereabouts, from the surface of the earth, the air is so highly rarified that practically it there comes to an end. In other words, at this height a given bulk of space contains no more air than exists in the so-called vacuum produced by a superior air-pump.

Weight of Air.

Notwithstanding the extreme tenuity of the gaseous medium in which we live, it is capable of buoying up on its wings a multitude of living beings of vast aggregate weight. It is firm enough to support the millions of birds that sail in it, and the myriad of millions of insects who yet more freely navigate it in search of food and warmth, and in answer to the various needs of their existence.

One of the most striking evidences of the fact that air is indeed a material substance is very easily discovered by showing that it possesses weight. Thus suppose a properly constructed glass globe is almost entirely emptied of air by the action of an efficient air-pump. Suppose then that the globe is weighed. Next if it be connected with a bell-glass containing ordinary atmospheric air over a pneumatic trough, it may be readily seen that the air leaves the bell-glass in order to pass into the globe. If this globe is now weighed again, it is found to manifest a decided increase of weight. This increase is due to the air it has received. By such means it may be easily shown that a cubic yard of air weighs not far from two pounds.

Composition of Air.

The principal constituents of air are the two gases oxygen and nitrogen; and of these the oxygen makes up about one-fifth and the nitrogen about four-fifths of the whole. In addition to these principal substances, however, certain others are always present, of which may be specified vapor of water, carbon dioxide and ammonia gas; while more minute quantities of a vast multitude of other gaseous substances find a reservoir in the air. It is an unquestioned fact that the atmosphere is likewise charged most of the time with still more minute quantities of solid dust materials of various kinds. An example is found in the common salt, blown up into the atmosphere from the ruffled surface of the oceans. Now the oceans are spread over fully three-fourths of the earth’s surface, and the winds, blowing upon the crested waves, not only diffuse the salt over the oceans themselves but also carry it far inland; accordingly spectrum analysis reveals the presence of salt in almost all atmospheric air.

Just as the rivers of water flow to the ocean and bear along to it debris of every kind—pulverized rock and earthy materials and other washings from the soil, leaves of forests, impure products of civilization thrown in from houses and manufacturing establishments—and all these materials make their relatively minute contributions to the impurities in the great ocean itself, so it is with the atmospheric ocean. Thousands of millions of living animals pour out, with every breath from the lungs, materials exhaled from their bodies. And so wherever fuel is burned, or wherever manufacturing establishments liberate gases or vapors, or even finely pulverized solids, these are cast forth from the mouths of their reeking chimneys; and they all flow into the great aerial sea. So then it is no unexpected circumstance that the air should be a reservoir in which, in minute quantity, is likely to exist every gaseous substance produced.

Offices of the Several Constituents of the Air.

The oxygen of the air is its most active constituent. This is the substance that has already been described as essential for all ordinary combustion and all animal respiration. By a great variety of characteristics it is well fitted for these important offices.

The chief duty of the nitrogen appears to be to dilute the oxygen and moderate the excessive activities that would be manifested if the atmosphere consisted entirely of the active gas. Since iron and other metals burn in pure oxygen, it is plain that in an atmosphere of oxygen—containing no moderating gas like nitrogen—a fire once kindled in a stove would not confine itself to its proper fuel, but would soon spread to the metal of the stove itself, and so initiate conflagrations that could hardly be restrained.

The _moisture_ in the air adds a number of wonderful and serviceable characteristics to it. Thus it helps to retain the heat received from the sun and so materially contributes to the sustenance of animal and vegetable life. The heat of the sun penetrates our atmospheric coverlet with great readiness and this heat is received by the surface of the earth and thence is imparted to the layer of air immediately upon it. Now the moisture contained in the atmosphere—and in principal quantity in the portions of air closest to the earth—is one of the chief agencies that prevent the immediate escape of that heat that the solid earth has secured from the sun’s rays. And it is in the warm layer of air so produced that animals and plants chiefly flourish. Ascend a mountain’s side and a height is soon reached at which eternal snow and cold prevail, where animal life cannot penetrate and even the lowest forms of vegetable life can hardly make their residence. What has thus far been said points out a valuable office of watery vapor and one that is entirely in addition to that which this same material performs as it floats in the clouds, ready to fall as beneficent showers and then to proceed to the other steps in the progress of that useful circulation which it performs as a liquid. But it may not be out of place to mention here that the aqueous vapor in the atmosphere appears to serve in another way for man’s pleasure, even though in this particular no utility can be claimed. Thus the glories of sunrise and sunset, which have delighted intelligent beings for so many ages, are paintings upon the drapery of the firmament which the pencil of light has been enabled to produce through the medium of the refractive power of those gathering drops of water which float about in various forms and combinations in the morning or the evening sky.

It has already been more than once declared that _carbon dioxide_ is poured out into the atmosphere by all the ordinary processes of combustion. This is not only true of combustions such as those of coal and wood and similar highly carbonaceous materials; it applies with equal force to the animal body itself, which has been properly likened to a furnace. The air taken into the lungs at each breath inspired, supports during life a continual combustion, by reason of which, minute fragments of the animal tissue are burned in all parts of the system. One of the products of this burning is carbon dioxide, which is carried to the lungs, thence to be exhaled as a waste product into the atmosphere. It might at first be expected that this carbon dioxide would accumulate and form a constantly increasing proportion of the air. But it is one of the proper foods of vegetable life; for nature has wonderfully provided that plants should thrive by the absorption or inhalation of this particular gas. And so all the leaves in the forest are continually cleansing the air of that carbon dioxide that living animals have cast aside as a useless thing. And by a magnificent alchemy, the result of a wonderful and beneficent plan, they turn this waste matter of the animal frame into food for themselves, and they cast out into the air as _their_ refuse that oxygen gas which living animals demand. So then the two forms of living beings exist in a harmonious partnership by reason of which each one is benefited.

An example similar to that just given with respect to carbon dioxide is found in _ammonia gas_. This substance is one of the commonest products of the decay and decomposition of animal matters. Wherever animal waste is deposited upon the surface of the earth it quickly evolves ammonia gas. This gas diffuses itself through the atmosphere under the influence of conditions whereby it may perform an important service; for it is always extremely soluble in water. And so as soon as rain is condensed, whether in a gentle shower or in abundant torrents, each drop in passing through the air gathers ammonia and carries it down to the earth. Again, ammonia is one of the chief foods for plants. And so the rain drops, charged with such ammonia as they have been able to collect, bear it to the rootlets in the soil, as a valuable and important food, and one which has been proved to have a most stimulating influence upon their growth.

There is not opportunity here for a discussion of the offices and the interplay of the other substances existing in atmospheric air; for they are more local in their effects and more difficult to trace and to describe.

The Air is not a Chemical Compound.

The importance of the atmosphere and its great abundance have naturally led to most thorough scientific scrutiny of it. Thus the amounts of its principal constituents have been studied with extreme care. One result has been that the principal constituents—the oxygen and the nitrogen— have been found to exist in air in proportions singularly constant in amount. This fact indeed has suggested to some chemists the impression that air is a true chemical compound. This latter suggestion appears not to be sustained by the most rigid examinations that have been made. In fact they give ample support to the opinion already declared—that the air consists of a mass of merely mingled gases and that these gases are uniformly maintained in their proper proportional amounts by the beautiful interaction of the physical and chemical properties with which they are endowed.

Fitness of Atmospheric Air for its Uses.

The statements already presented must have suggested to the reader that the atmospheric air fulfils its offices in nature much as any contrivance carefully devised by an intelligent framer would accomplish the work for which it was planned. Beside those chemical adaptations which have been the principal grounds upon which this line of thought has been supported here, there are others which may be briefly suggested.

By reason of the _mobility_ of air, as well as its tendencies to expansion by heat, our atmosphere is necessarily in a state of most intricate ebbing and flowing. One prominent effect of the motion thus set up is to cause a transfer of warm air, and so a distribution of heat, from more favored portions of the globe to the others.

This same result is also more completely attained by the influence of the _specific heat_ of air. Atmospheric air has remarkable power, in which it resembles to some extent water: to take up a very large amount of _heat_ with but a slight rise in _temperature_; similarly a slight fall of temperature is associated with a large evolution of heat. By reason of these properties, air, like water, has an exceptional storage power for heat. This contributes largely to the equalization of climates.

The _elasticity_ of the atmosphere permits it to become a useful servant of man in the transmission of sound. Thus human beings—and with less distinctness most of the living creatures of the lower orders—communicate their thoughts by means of spoken words through that line of atmospheric air extending from them to their hearer or hearers.

Again the characteristics of the atmosphere are such that it _diffuses_ sunlight. By this is meant that in air, sunlight does not confine itself to those strictly straight lines which it follows in empty spaces. So then this property of air mitigates the blackness of shadows and, for example, he who walks into a shady lane does not plunge into absolute darkness, as he might if we were deprived of this beneficial diffusing influence of the atmospheric air.

The two considerations last adduced contribute much towards making the earth a cheerful home for human beings; for they aid materially in the distribution of intelligible ideas. Moreover those properties of air by virtue of which its undulating waves makes music possible, and further those which permit the flight of light, and so allow of the existence of the graphic arts, certainly make no mean contributions to the happiness of man; and thus they help to furnish the earth as his place of residence.