THE ATMOSPHERE
~Atmosphere and air.~ The term _atmosphere_ is applied to the gaseous
envelope surrounding the earth. The term _air_ is generally applied to a
limited portion of this envelope, although the two words are often used
interchangeably. Many references have already been made to the
composition and properties of the atmosphere. These statements must now
be collected and discussed somewhat more in detail.
~Air formerly regarded as an element.~ Like water, air was at first
regarded as elementary in character. Near the close of the eighteenth
century Scheele, Priestley, and Lavoisier showed by their experiments
that it is a mixture of at least two gases,--those which we now call
oxygen and nitrogen. By burning substances in an inclosed volume of air
and noting the contraction in volume due to the removal of the oxygen,
they were able to determine with some accuracy the relative volumes of
oxygen and nitrogen present in the air.
~The constituents of the atmosphere.~ The constituents of the atmosphere
may be divided into two general groups: those which are essential to
life and those which are not essential.
1. _Constituents essential to life._ In addition to oxygen and nitrogen
at least two other substances, namely, carbon dioxide and water vapor,
must be present in the atmosphere in order that life may exist. The
former of these is a gaseous compound of carbon and oxygen having the
formula CO_{2}. Its properties will be discussed in detail in the
chapter on the compounds of carbon. Its presence in the air may be shown
by causing the air to bubble through a solution of calcium hydroxide
(Ca(OH)_{2}), commonly called lime water. The carbon dioxide combines
with the calcium hydroxide in accordance with the following equation:
Ca(OH)_{2} + CO_{2} = CaCO_{3} + H_{2}O.
The resulting calcium carbonate (CaCO_{3}) is insoluble in water and
separates in the form of a white powder, which causes the solution to
appear milky.
The presence of water vapor is readily shown by its condensation on cold
objects as well as by the fact that a bit of calcium chloride when
exposed to the air becomes moist, and may even dissolve in the water
absorbed from the air.
2. _Constituents not essential to life._ In addition to the essential
constituents, the air contains small percentages of various other gases,
the presence of which so far as is known is not essential to life. This
list includes the rare elements, argon, helium, neon, krypton, and
xenon; also hydrogen, ammonia, hydrogen dioxide, and probably ozone.
Certain minute forms of life (germs) are also present, the decay of
organic matter being due to their presence.
~Function of each of the essential constituents.~ (1) The oxygen
directly supports life through respiration. (2) The nitrogen,
on account of its inactivity, serves to dilute the oxygen, and
while contrary to the older views, it is possible that life
might continue to exist in the absence of the atmospheric
nitrogen, yet the conditions of life would be entirely changed.
Moreover, nitrogen is an essential constituent of all animal
and plant life. It was formerly supposed that neither animals
nor plants could assimilate the free nitrogen, but it has been
shown recently that the plants of at least one natural order,
the Leguminosæ, to which belong the beans, peas, and clover,
have the power of directly assimilating the free nitrogen from
the atmosphere. This is accomplished through the agency of
groups of bacteria, which form colonies in little tubercles on
the roots of the plants. These bacteria probably assist in the
absorption of nitrogen by changing the free nitrogen into
compounds which can be assimilated by the plant. Fig. 27 shows
the tubercles on the roots of a variety of bean. (3) The
presence of water vapor in the air is necessary to prevent
excessive evaporation from both plants and animals. (4) Carbon
dioxide is an essential plant food.
[Illustration: Fig. 27]
~The quantitative analysis of air.~ A number of different methods have
been devised for the determination of the percentages of the
constituents present in the atmosphere. Among these are the following.
1. _Determination of oxygen._ (1) The oxygen is withdrawn from a
measured volume of air inclosed in a tube, by means of phosphorus.
To make the determination, a graduated tube is filled with
water and inverted in a vessel of water. Air is introduced into
the tube until it is partially filled with the gas. The volume
of the inclosed air is carefully noted and reduced to standard
conditions. A small piece of phosphorus is attached to a wire
and brought within the tube as shown in Fig. 28. After a few
hours the oxygen in the inclosed air will have combined with
the phosphorus, the water rising to take its place. The
phosphorus is removed and the volume is again noted and reduced
to standard conditions. The contraction in the volume of the
air is equal to the volume of oxygen absorbed.
[Illustration: Fig. 28]
(2) The oxygen may also be estimated by passing a measured volume of air
through a tube containing copper heated to a high temperature. The
oxygen in the air combines with the copper to form copper oxide (CuO).
Hence the increase in the weight of the copper equals the weight of the
oxygen in the volume of air taken.
(3) A more accurate method is the following. A eudiometer tube is filled
with mercury and inverted in a vessel of the same liquid. A convenient
amount of air is then introduced into the tube and its volume accurately
noted. There is then introduced more than sufficient hydrogen to combine
with the oxygen present in the inclosed air, and the volume is again
accurately noted. The mixture is then exploded by an electric spark, and
the volume is once more taken. By subtracting this volume from the total
volume of the air and hydrogen there is obtained the contraction in
volume due to the union of the oxygen and hydrogen. The volume occupied
by the water formed by the union of the two gases is so small that it
may be disregarded in the calculation. Since oxygen and hydrogen combine
in the ratio 1: 2 by volume, it is evident that the contraction in
volume due to the combination is equal to the volume occupied by the
oxygen in the air contained in the tube, plus twice this volume of
hydrogen. In other words, one third of the total contraction is equal to
the volume occupied by the oxygen in the inclosed air. The following
example will make this clear:
Volume of air in tube 50.0 cc.
Volume after introducing hydrogen 80.0
Volume after combination of oxygen and hydrogen 48.5
Contraction in volume due to combination (80 cc.-48.5 cc.) 31.5
Volume of oxygen in 50 cc. of air (1/3 of 31.5) 10.5
All these methods agree in showing that 100 volumes of dry air contain
approximately 21 volumes of oxygen.
2. _Determination of nitrogen._ If the gas left after the removal of
oxygen from a portion of air is passed over heated magnesium, the
nitrogen is withdrawn, argon and the other rare elements being left. It
may thus be shown that of the 79 volumes of gas left after the removal
of the oxygen from 100 volumes of air, approximately 78 are nitrogen and
0.93 argon. The other elements are present in such small quantities that
they may be neglected.
3. _Determination of carbon dioxide._ The percentage of carbon dioxide
in any given volume of air may be determined by passing the air over
calcium hydroxide or some other compound which will combine with the
carbon dioxide. The increase in the weight of the hydroxide equals the
weight of the carbon dioxide absorbed. The amount present in the open
normal air is from 3 to 4 parts by volume in 10,000 volumes of air, or
about 0.04%.
4. _Determination of water vapor._ The water vapor present in a given
volume of air may be determined by passing the air over calcium chloride
(or some other compound which has a strong affinity for water), and
noting the increase in the weight of the chloride. The amount present
varies not only with the locality, but there is a wide variation from
day to day in the same locality because of the winds and changes in
temperature.
~Processes affecting the composition of the air.~ The most important of
these processes are the following.
1. _Respiration._ In the process of respiration some of the oxygen in
the inhaled air is absorbed by the blood and carried to all parts of the
body, where it combines with the carbon of the worn-out tissues. The
products of oxidation are carried back to the lungs and exhaled in the
form of carbon dioxide. The amount exhaled by an adult averages about 20
l. per hour. Hence in a poorly ventilated room occupied by a number of
people the amount of carbon dioxide rapidly increases. While this gas is
not poisonous unless present in large amounts, nevertheless air
containing more than 15 parts in 10,000 is not fit for respiration.
2. _Combustion._ All of the ordinary forms of fuel contain large
percentages of carbon. On burning, this carbon combines with oxygen in
the air, forming carbon dioxide. Combustion and respiration, therefore,
tend to diminish the amount of oxygen in the air and to increase the
amount of carbon dioxide.
3. _Action of plants._ Plants have the power, when in the sunlight, of
absorbing carbon dioxide from the air, retaining the carbon and
returning at least a portion of the oxygen to the air. It will be
observed that these changes are just the opposite of those brought about
by the processes of respiration and combustion.
~Poisonous effect of exhaled air.~ The differences in the
percentages of oxygen, carbon dioxide, and moisture present in
inhaled air and exhaled air are shown in the following
analyses.
INHALED AIR EXHALED AIR
Oxygen 21.00% 16.00%
Carbon dioxide 0.04 4.38
Moisture variable saturated
The foul odor of respired air is due to the presence of a
certain amount of organic matter. It is possible that this
organic matter rather than the carbon dioxide is responsible
for the injurious effects which follow the respiration of
impure air. The extent of such organic impurities present may
be judged, however, by the amount of carbon dioxide present,
since the two are exhaled together.
~The cycle of carbon in nature.~ Under the influence of sunlight,
the carbon dioxide absorbed from the air by plants reacts with
water and small amounts of other substances absorbed from the
soil to form complex compounds of carbon which constitute the
essential part of the plant tissue. This reaction is attended
by the evolution of oxygen, which is restored to the air. The
compounds resulting from these changes are much richer in their
energy content than are the substances from which they are
formed; hence a certain amount of energy must have been
absorbed in their formation. The source of this energy is the
sun's rays.
If the plant is burned, the changes which took place in the
formation of the compounds present are largely reversed. The
carbon and hydrogen present combine with oxygen taken from the
air to form carbon dioxide and water, while the energy absorbed
from the sun's rays is liberated in the form of energy of heat.
If, on the other hand, the plant is used as food, the compounds
present are used in building up the tissues of the body. When
this tissue breaks down, the changes which it undergoes are
very similar to those which take place when the plant is
burned. The carbon and hydrogen combine with the inhaled oxygen
to form carbon dioxide and water, which are exhaled. The energy
possessed by the complex substances is liberated partly in the
form of energy of heat, which maintains the heat of the body,
and partly in the various forms of muscular energy. The carbon
originally absorbed from the air by the plant in the form of
carbon dioxide is thus restored to the air and is ready to
repeat the cycle of changes.
~The composition of the air is constant.~ Notwithstanding the changes
constantly taking place which tend to alter the composition of the air,
the results of a great many analyses of air collected in the open fields
show that the percentages of oxygen and nitrogen as well as of carbon
dioxide are very nearly constant. Indeed, so constant are the
percentages of oxygen and nitrogen that the question has arisen, whether
these two elements are not combined in the air, forming a definite
chemical compound. That the two are not combined but are simply mixed
together can be shown in a number of ways, among which are the
following.
1. When air dissolves in water it has been found that the ratio of
oxygen to nitrogen in the dissolved air is no longer 21: 78, but more
nearly 35: 65. If it were a chemical compound, the ratio of oxygen to
nitrogen would not be changed by solution in water.
2. A chemical compound in the form of a liquid has a definite boiling
point. Water, for example, boils at 100°. Moreover the steam which is
thus formed has the same composition as the water. The boiling point of
liquid air, on the other hand, gradually rises as the liquid boils, the
nitrogen escaping first followed by the oxygen. If the two were
combined, they would pass off together in the ratio in which they are
found in the air.
~Why the air has a constant composition.~ If air is a mixture and changes
are constantly taking place which tend to modify its composition, how,
then, do we account for the constancy of composition which the analyses
reveal? This is explained by several facts. (1) The changes which are
caused by the processes of combustion and respiration, on the one hand,
and the action of plants, on the other, tend to equalize each other. (2)
The winds keep the air in constant motion and so prevent local changes.
(3) The volume of the air is so vast and the changes which occur are so
small compared with the total amount of air that they cannot be readily
detected. (4) Finally it must be noted that only air collected in the
open fields shows this constancy in composition. The air in a poorly
ventilated room occupied by a number of people rapidly changes in
composition.
~The properties of the air.~ Inasmuch as air is composed principally of a
mixture of oxygen and nitrogen, which elements have already been
discussed, its properties may be inferred largely from those of the two
gases. One liter weighs 1.2923 g. It is thus 14.38 times as heavy as
hydrogen. At the sea level it exerts an average pressure sufficient to
sustain a column of mercury 760 mm. in height. This is taken as the
standard pressure in determining the volumes of gases as well as the
boiling points of liquids. Water may be made to boil at any temperature
between 0° and considerably above 100° by simply varying the pressure.
It is only when the pressure upon it is equal to the normal pressure of
the atmosphere at the sea level, as indicated by a barometric reading of
760 mm., that it boils at 100°.
~Preparation of liquid air.~ Attention has been called to the fact that
both oxygen and nitrogen can be obtained in the liquid state by strongly
cooling the gases and applying great pressure to them. Since air is
largely a mixture of these two gases, it can be liquefied by the same
methods.
The methods for liquefying air have been simplified greatly in
that the low temperature required is obtained by allowing a
portion of the compressed air to expand. The expansion of a gas
is always attended by the absorption of heat. In liquefying air
the apparatus is so constructed that the heat absorbed is
withdrawn from air already under great pressure. This process
is continued until the temperature is lowered to the point of
liquefaction.
[Illustration: Fig. 29]
~The Dewar bulb.~ It is not possible to preserve air in the liquid state
in a closed vessel, on account of the enormous pressure exerted by it in
its tendency to pass into the gaseous state. It may however be preserved
for some hours or even days before it will completely evaporate, by
simply placing it in an open vessel surrounded by a nonconducting
material. The most efficient vessel for this purpose is the _Dewar bulb_
shown in Fig. 29. The air is withdrawn from the space between the two
walls, thus making it nonconducting.
~Properties and uses of liquid air.~ When first prepared, liquid air is
cloudy because of the presence of particles of solid carbon dioxide.
These may be filtered off, leaving a liquid of slightly bluish color. It
begins to boil at about -190°, the nitrogen passing off first, gradually
followed by the oxygen, the last portions being nearly pure oxygen. To a
certain extent oxygen is now prepared in this way for commercial
purposes.
The extremely low temperature of liquid air may be inferred from the
fact that mercury when cooled by it is frozen to a mass so hard that it
may be used for driving nails.
Liquid air is used in the preparation of oxygen and as a cooling agent
in the study of the properties of matter at low temperatures. It has
thus been found that elements at extremely low temperatures largely lose
their chemical activity.
EXERCISES
1. When oxygen and nitrogen are mixed in the proportion in which they
exist in the atmosphere, heat is neither evolved nor absorbed by the
process. What important point does this suggest?
2. What essential constituent of the air is found in larger amount in
manufacturing districts than in the open country?
3. Can you suggest any reason why the growth of clover in a field
improves the soil?
4. Why are the inner walls of a Dewar bulb sometimes coated with a film
of silver?
5. To what is the blue color of liquid air due? Does this color increase
in intensity on standing?
6. When ice is placed in a vessel containing liquid air, the latter
boils violently. Explain.
7. Taking the volumes of the oxygen and nitrogen in 100 volumes of air
as 21 and 78 respectively, calculate the percentages of these elements
present by weight.
8. Would combustion be more intense in liquid air than in the gaseous
substance?
9. A tube containing calcium chloride was found to weigh 30.1293 g. A
volume of air which weighed 15.2134 g. was passed through, after which
the weight of the tube was found to be 30.3405 g. What was the
percentage amount of moisture present in the air?
10. 10 l. of air measured at 20° and 740 mm. passed through lime water
caused the precipitation of 0.0102 g. of CaCO_{3}. Find the number of
volumes of carbon dioxide in 10,000 volumes of the air.