EARTH’S FOUR ATMOSPHERES
The earth has four important atmospheres and others of less
importance. The principal ones are oxygen, nitrogen, vapor of water,
and carbon dioxide, each comporting itself as it would do if the
others were not present. There is space between the molecules of
each gas, and therefore it is easily compressed. A doubling of its
pressure reduces its volume one half.
=Composition of Atmospheric Air.= It is difficult for the mind to
form a picture of the infinitely small molecules of the air. Let
us therefore use terms and comparisons that will the more directly
appeal to the human senses. First let us imagine each molecule
enlarged to the size of a small grain of sand. Then with the
molecules from one cubic inch of air transformed into grains of sand
we could build a roadway ten feet deep and one hundred feet wide
extending from New York to San Francisco. May one still further grasp
the idea of the atom, many of which are required to make up the
molecules? If so, the imagination has been stretched to its limits to
enable the human mind to comprehend some of the simplest facts with
regard to the wonderful fluid in which we live.
Sir William Thomson, afterwards Lord Kelvin, in endeavoring to give
relative values that would appeal to the imagination, said that if a
drop of water were enlarged to the size of the earth, the molecules
of which it is composed would be no larger than cricket balls, and
the smallest about the size of small peas.
More than a thousand years before the birth of Christ a great
Phœnician philosopher believed that all matter—solids, liquids, and
gases—was built up from infinitely small aggregations of atoms. The
learned men of Greece enlarged upon his views but this philosophy
passed into oblivion with the destruction of Rome and the coming of
the Dark Ages, and it was not revived until about one hundred and
fifty years ago. The ancients could not prove their theory, while
we to-day can count the atoms and determine their size and motions;
and, exceedingly small though they be, we no longer believe them
to be indivisible in structure. On the contrary, we know that each
atom consists of particles of positive and negative electricity.
The negative electrons arrange themselves about a positive electron
for a nucleus and, rotating about it as if it were a central sun
with planets, constitute an atom. All matter reduced to the ultimate
electron is precisely alike. The difference in matter is determined
by the number of negative electrons that are attracted and held in
place by the positive nucleus that is at the center of each atom of
which a particular kind of matter is composed. Each of the ninety-two
elements which we believe constitute the ninety-two different forms
of simple matter has an atom with its own peculiar type of nucleus,
which nucleus differs from those of the others only in the amount of
positive electricity it contains. Thus hydrogen, the lightest of all
gases, whose weight is taken as unity in measuring the magnitude of
other gases, has a nucleus whose positive charge of electricity is
only sufficient to attract one negative electron. The next element,
helium, has a nucleus with a double positive charge and consequently
holds two electrons or planets to pay it homage. In like manner
the carbon atom contains six electrons; oxygen, eight; aluminum,
thirteen; nitrogen, fourteen; sulphur, sixteen; iron, twenty-six;
copper, twenty-nine; silver, forty-seven; gold, seventy-nine;
mercury, eighty; lead, eighty-two; bismuth, eighty-three; radium,
eighty-eight; thorium, ninety; and uranium, ninety-two. The chemical
union of these elementary forms of matter creates other forms. For
instance, the union of two atoms of hydrogen and one of oxygen
constitutes a molecule of water. But the gases of the atmosphere
are not in chemical union; they exist in the form of a mechanical
mixture, each acting as though the others were not present.
It is important that this mixture of gases that constitutes our air
be maintained in the right proportion. Only a slight difference in
relative amounts might be disastrous to life. An increase in the
oxygen would stimulate mental and physical activities and hold the
human faculties at a higher tension. Man would accomplish more in a
given time, but his span of life would be shortened; and too great an
increase in the proportion of this stimulating element would quickly
terminate life. Conversely an increase in the nitrogen would render
all life more lethargic and man would be slower to act and to think;
and too great an increase would smother every living thing.
In addition to the gases named, the air contains small amounts of
many other substances,—argon, nitric acid, ammonia, ozone, 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.
The proportion of each component of the atmosphere by volume of the
total atmosphere is different from its proportion by weight. The
percentages for the more abundant gases are as follows:
===============+=============+===========
| BY VOLUME | BY WEIGHT
---------------+-------------+-----------
Nitrogen | 78.04 | 75.46
Oxygen | 20.99 | 23.19
Argon | 0.94 | 1.30
Carbon dioxide | 0.03 | 0.05
+-------------+-----------
| 100.00 | 100.00
===============+=============+===========
=Nitrogen.= Its principal functions are to dilute the oxygen and to
furnish food to vegetation. It is inert and does not manifest many
marked chemical affinities. Its lack of activity is shown by the fact
that it will neither support combustion nor burn.
=Oxygen.= Oxygen, unlike nitrogen, is an active element that readily
enters into chemical combination with many other elements, and it
is second in quantity to nitrogen. With hydrogen it constitutes
eight ninths, by weight, of water; combined with other elements it
constitutes forty to fifty per cent. of the crust of the earth. It
burns so readily that were it not greatly diluted by an inert gas
like nitrogen it would be difficult if not impossible to stop a
conflagration when once started. It is the vitalizing principle in
all forms of life. By its chemical union with carbon in the tissues
of plants and animals it develops the energy manifested in their
movements.
In the free air up to about seven miles high there is no variation
in the proportion of oxygen. But variations of marked importance to
health and life occur in places where ventilation is restricted, and
especially where living creatures exist in closed rooms, and where
combustion occurs in confined places. The following variations in
percentages by volume were found in careful analyses by Robert Angus
Smith: On the seashore of Scotland, 20.99; open places in London,
20.95; in a small room where a petroleum lamp had been burning six
hours, 20.83; pit of a theater at 11:30 P.M., 20.74; in a court room,
20.65; in mine pits, 20.14. He took samples from one mine that showed
18.27, the candles going out when the amount had decreased to 18.50.
The absorption of oxygen by putrid matter and by living beings
in the process of breathing, and the giving out of carbon dioxide
by both explain the deficiency of oxygen that is found over large
cities, which is more marked when the air is moving but little and
where the city is located in a depression or near swampy lands.
Both animals and plants inhale oxygen and exhale carbon dioxide with
the unchanged nitrogen. The process automatically proceeds both night
and day. It should not be confused with the opposite action of plants
under the influence of sunlight in taking in and decomposing carbon
dioxide and expelling pure oxygen.
=Carbon Dioxide.= It forms the chief food supply of all green-leaved
plants. It is as necessary to the life of vegetation as is oxygen
in the supporting of animal life. In the ratio of seventy-seven to
one hundred there is less of this gas present in the atmosphere
in the winter than in the summer; there also is a diurnal maximum
and minimum. In the open country the amount averages about 0.035
per cent. by volume. In cities the amount is considerably greater,
frequently rising to 0.07, and at times to 0.10 when the wind
velocity is too low to scatter the excess amount that accumulates
near the ground. Any quantity in excess of 0.06 per cent.,
especially if combined with the organic matter exhaled from the
lungs and from the pores of the skin by animals and man, is injurious
to health. Angus Smith found as much as 0.32 per cent. in crowded
theaters, and 2.50 in mines. The latter amount soon would destroy
animal life.
Vegetation, in addition to the inhalation of oxygen and the
expiration of carbon dioxide at all hours, absorbs the latter during
the day, and under the influence of sunlight the green granular
matter that constitutes the chlorophyll of the cells of the leaves
decomposes it, the plant retaining the carbon and giving out the
oxygen. Because of the absence of sunshine the chemical activities of
the plant are altered at night and the absorption of carbon dioxide
ceases; therefore over the land the maximum amount occurs during the
nighttime. This gas is dissolved in sea water and given off with a
rise in temperature, which causes the maximum amount over oceans to
occur at midday.
Carbon dioxide is 1.50 times as dense as an equal volume of
atmospheric air. Its greater density causes it to collect in mines,
sewers, cellars, and other low places, unless there is forceful
ventilation.
The American cold wave should be welcomed as the mighty scavenger of
the air. Its high velocity and great density cause it to search into
cracks, crevices, sewers, and cellars and expel foul accumulations.
How sweet and clean the air smells and how vigorous physically and
buoyant mentally one feels after a rain and high winds! All nature
smiles and every form of life adds its pæan of joy. Rain washes
out the carbonic acid gas (carbon dioxide) from the air, with dust
and other particles in suspension; and the cold wave enters our
places of habitation and drives out the thieving accumulations of
poisonous gases that would rob us of health and maintain conditions
of morbidity.
It cannot be too forcefully stated that oxygen, the life-sustaining
principle of the air, decreases, and carbon dioxide, a poison,
increases in air that is breathed, or in air in which lamps or gas
jets are burning; and that all places of habitation, especially
sleeping rooms, should have a continuous supply of fresh air.
=Water Vapor.= It is only a little over one half as dense as
atmospheric air. In the arid regions of the west it may form only a
fraction of one per cent. of the air by weight; while in the humid
regions in the eastern part of the United States it may constitute
as much as five per cent. The temperature being the same, the same
amount is required to saturate a given space, whether it be a vacuum
or whether it be filled with air. Air doubles its capacity for water
vapor with each increase of eighteen to twenty degrees. On a hot day
in summer, near large bodies of water, it may constitute as much as
one twentieth by weight of the lower air, while on a cold day in
winter it may form no more than one thousandth part. When the air
contains all the water vapor it can hold, it is said to be saturated;
no more can be added to it until its temperature is raised, and but
a slight lowering of its temperature will precipitate a part of its
water vapor in the form of dew, frost, rain, hail, or snow. This is
the reason it is usually called water vapor instead of a gas. Under
the influence of heat that is below the freezing point, ice and snow
may be changed from the solid to the gaseous form, and water vapor
may be precipitated as frost or snow without passing through the
liquid state.
=The Dew Point= is the temperature of saturation,—the temperature to
which a body of air must be reduced before condensation can occur and
some of its water vapor return to the liquid or solid state.
=The Relative Humidity= is expressed in percentages of the amount
necessary to saturate. At a temperature of 32° air may continue to
increase its vapor of water until it contains 2.11 grains per cubic
foot, when it will be saturated and its relative humidity be one
hundred per cent. If this same air be suddenly raised in temperature
to 51° its capacity per cubic foot will be increased to twice what
it was at 32°, the 2.11 grains will be equal to only one half the
number necessary to saturate, and the relative humidity be expressed
by fifty per cent. instead of one hundred per cent. In this way does
the capacity of air for water vapor increase. Thus it is seen that
the relative humidity of the air may increase during the cooling of
nighttime without the addition of any vapor of water, and, in fact,
with a decrease. The increase of relative humidity after nightfall
is greater in the country than in the city, where the presence of
pavements and brick buildings retards the loss of heat.
=The Absolute Humidity= is expressed in grains the cubic foot. The
hygrometer is employed to measure the amount of water vapor.
=Hydrogen= is the lightest of all known gases. Its density in
comparison with ordinary air is only .0692. It is combustible, and
when five volumes of atmospheric air are mixed with two volumes of
hydrogen the mixture explodes when ignited. It is supplied to the
air by active volcanoes and in other ways, but the speed of its
molecules is such that it readily escapes from the earth’s attraction
and passes outward into space.
=Ozone= (Greek, ozo, I smell) is highly electrified oxygen, in which
the molecules are broken up and reformed so as to contain additional
atoms. It is formed by the disruptive discharge of lightning and by
the great amount of electricity present in the high levels of the
atmosphere, and possibly in minute quantities by the evaporation of
fog and water near the earth. It is always found in the presence of
waterfalls and spraying fountains. It is a powerful sanitary agent,
readily entering into union with decaying matter. This fact accounts
for the total absence of ozone from the air of large cities.
Ozone, in the minute quantities found in nature, is healthful, but
when breathed in a condensed form it has a highly irritating effect
on the mucous surfaces of the respiratory passages, and the quantity
is not large that will cause death. The healthfulness of mountain air
may be due largely to the increase with elevation in the quantity of
ozone and electricity in the air, as well as to the less number of
disease germs and dust motes. The invigorating effects of the crisp
air of the frosty morning and of the cold wave in winter may be
increased by the activities of ozone.
Ozone has two daily maxima, the principal one occurring between 4 and
9 A.M. The minima occur between 10 A.M. and 1 P.M., and between 10
P.M. and midnight. The winter furnishes an amount greatly in excess
of the summer, due not only to the less amount of decaying matter to
take up the ozone in winter, but to the higher and more persistent
winds mixing the lower and upper air. The amount is greater over the
sea than over the land, probably due to the absence of oxidizable
matter, which allows the ozone to accumulate over the water. It is
more abundant with westerly than with easterly winds, due to the fact
that westerly winds have a downward component of motion; but if the
westerly winds be weak and the easterly winds come from over a large
body of water the conditions may be reversed.
=Microbes of the Air.= The air transports vast armies of unseen
workers. Some are enemies; others are benefactors of the human
family. The useful varieties are energetic in clearing away the
refuse of animal and vegetable life, in fixing fertilizing gases in
the soil, in giving flavor to fruits and proper growth to leguminous
crops, in transforming the crudest must into the best claret,
and the poorest tobacco leaf into the fragrant Havana; in curing
cheese and butter and fermenting beer, and in a multitude of other
useful employments. The malevolent varieties, if they gain lodgment
in suitable human tissues before sunlight weakens their virility,
disseminate certain forms of disease.
In picking a permanent place of abode, remember that there are many
less disease microbes in the air of the open country than in that of
the city, and that few are found in the air of mountains, or in that
of the ocean. The average number of bacteria in a cubic meter of air
in the city of Paris has been found to be 4790, while ten miles away
in the country the number was only 345.
Accurate analyses of the air of crowded tenements always have shown
large numbers of bacteria, but the number was found to be small in
well-ventilated city houses that let in an abundance of sunshine to
their interiors. It is better to have color in the cheeks of the
occupants than in the furnishings of a house. Curtains and heavy
drapery not only furnish a refuge for the microbes of disease, but
they may be so hung as to exclude the purifying sunshine. The amount
of sunshine is nearly as important as the quantity of air, for
most of the microbes of disease quickly die, or are rendered less
virulent, under its influence.
Bacteria exist in small numbers, if at all, at altitudes where snow
forms, but snow gathers them as it falls through the lower air. Ice
contains bacteria, but not in any such quantity as the water from
which it freezes. Ice forms in the open at the surface of the water,
or about numerous small particles of matter in suspension, which rise
at once to the top as soon as the ice congeals about them in the form
of a buoyant covering; meanwhile sediment is continually settling
to the bottom, carrying bacteria with it. Ice forms more readily in
quiet water, where sedimentation has been most rapid, and where,
therefore, there are the fewest bacteria in position to be included.
More disease germs exist in river water in winter than in summer,
which may be due to the greater disinfecting power of the sun’s rays
during summer.
=Dust Motes of the Air.= As the earth pursues its course about the
sun, dust rains into its atmosphere from outer space. Meteors that
are burned through the heat generated by striking into our air
contribute to the supply, as do volcanoes, combustion, spray from the
ocean, and matter lifted up by the action of the wind.
Dust from the eruption of Krakatoa was wafted entirely around the
earth, falling upon the decks of ships in all the seas of the world.
It affected the colors of the sky for two or three years after the
explosion.
As in the case of microbes, the number of dust particles is far
greater in cities than in the country, being least on high mountain
tops and over the oceans. The air in large cities invariably shows
hundreds of thousands of dust motes to the cubic centimeter, that of
the village thousands, and that of the open country some hundreds.
Dust-free air is also germ-free. Many experiments have shown that
air freed of dust motes has at the same time been cleared of the
microörganisms that cause disease, putrefaction, and fermentation;
and that germ-free flesh or liquids may be indefinitely exposed in
such air without fermentation or decay.
=How Dust Motes Are Counted.= Many of the particles are too small
to be seen by the highest powers of the microscope, yet Aitken, by
a most ingenious method of making them centers of condensation—that
is, making them the nuclei of small raindrops—was able to count the
number in a given volume of air. When ordinary air is saturated
and then cooled the cloud formed is so dense that it is impossible
to count the tiny droplets that form the cloud. But we can make
the number of dust particles (and therefore the number of visible
points of condensation) in a given volume of air as small as we
wish by mixing a little dusty air with a large amount of dustless
air, and we can allow the particles to fall on a bright surface and
can count them by means of a lens or microscope. By simply allowing
for the proportion of the dustless to the dusty air, and making a
corresponding allowance for the dilution, we calculate the number of
particles.
=Dust Motes and Illumination of the Atmosphere.= One of the most
important functions of dust motes is the diffusion or scattering of
sunlight. What a different world this would be without these tiny
inanimate friends of man! If there were no dust in suspension in the
air, nothing would be visible except what received direct light, or
light reflected from some illuminated surface, and the air occupying
space between illuminated objects would be practically dark. If the
observer be in a room with a powerful electric light he would see the
walls and the objects in the room, but if the air were free of dust
motes, he would find that the space between him and the walls and
between the various objects would be as inky black as is the space
between the twinkling stars on a clear night.
[Illustration: FIG. 2.—Showing light from lamp _a_ passing into
dust-free air at _b_, and passing out at _c_ without illuminating the
interior.]
Figure 2 is a cubical box, with a glass front. If a glutinous
substance be spread over the bottom and the box allowed to remain
quiescent for from five to seven days the dust motes will slowly
settle down and attach themselves to the bottom. The air then will
be what is called “optically pure.” Now, if it be taken into a dark
room and an inclosed lamp at _a_ be allowed to send a beam of light
into the window at _b_ and out at _c_, it will be found that the
interior remains dark no matter how powerful the light from the lamp.
The light is seen to enter and to leave but where it encounters the
dust-free air there is nothing to scatter the light rays and they
remain invisible to the eye.
=Dust Motes Prolong Twilight.= The bending or refraction of light
as the sun’s rays pass obliquely through the air at sunrise and at
sunset displaces the apparent position of the sun, elevating it by
an amount about equal to its own apparent diameter, so that one may
see it and receive its light when geometrically it is entirely below
the horizon. A little later in the evening and its rays fall upon the
upper air too obliquely to be bent down to the earth by refraction;
but darkness does not yet ensue, for the rays are scattered by the
dust motes and possibly by the molecules of the gases and sent
downward from particle to particle, resulting in a soft shimmering
light that almost imperceptibly fades away, and which in higher
latitudes may last for hours.