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.
[Illustration: FIG. 53.—Display of fireworks on the Seine, Paris.]