THE RESOURCES OF THE ATMOSPHERE
In the economic stress of our times much is heard about “natural
resources.” This phrase suggests to most people’s minds the store of
minerals, fuels, and oil locked up in the ground; the waters available
for drinking, washing, irrigation, power production, and navigation;
the forests and other natural growths of useful vegetation; and the
soil in which we raise our crops. A moment’s reflection, however,
will show that this is a one-sided enumeration. The resources of the
_atmosphere_ are as essential to humanity as those of the land and the
waters, if not more so.
The coal that is dug out of the earth consists mainly of carbon,
which, in bygone ages, was extracted by plants from the air. Moreover,
it would be of no use to us if we did not have the oxygen of the air
in which to burn it. Neither could we smelt metallic ores without
oxygen. All our forests and all our crops draw far more of their solid
substance from the air than from the soil. Fuel and water are valuable
sources of power, but so is the moving air that drives sailing ships
and windmills, and the atmospheric pressure that helps to operate
suction pumps. It is the moisture of the air that feeds our streams
and, directly or indirectly, waters all plants that grow upon the
land. Lastly, it is the atmospheric oxygen that we breathe that keeps
us from very speedily becoming incapable of using any of the other
resources of Nature.
Air and water together contain, in their oxygen, nitrogen, hydrogen,
and carbon, all the major constituents of our foods in unlimited
abundance. It is tantalizing to think of the slow and roundabout way
in which these things are wrought into edible shape--and the prices
we have to pay for them. No less tantalizing, when coal is scarce and
costly, is the thought that every vagrant breeze is laden with the
carbon dioxide from which the chemistry of living plants so readily
extracts the chief element of fuels. The total carbon dioxide of the
atmosphere amounts to something like 2,200,000,000,000 tons, equivalent
to 600,000,000,000 tons of carbon.
We have spoken of the utility of the air as a source of power. It is,
perhaps, even more useful as providing an easy means of storing and
transmitting power. The engineer stores up energy in a mass of air
by compressing it. When the air subsequently expands it gives up its
energy, and, in so doing, may be made to perform a variety of useful
tasks. By a somewhat analogous process energy is applied to creating a
vacuum, in order that the ordinary pressure of the atmosphere may be
made available for doing a particular piece of work. The suction pump,
the siphon, and the vacuum cleaner furnish examples of this process;
and so do such familiar operations as sucking beverages through a straw
and filling a medicine dropper.
From crude types of bellows, with which, from remote antiquity, air
was compressed for the purpose of blowing fires, have been developed
a host of wonder-working appliances of the present day, such as the
air brake, the pneumatic tube, the compressed-air locomotive, diving
apparatus, the caisson, certain kinds of refrigerating machinery, and
a long list of pneumatic tools. To cap the climax of ingenuity in this
field, methods involving both the compression and the expansion of air
have been discovered whereby this invisible, elusive substance may
be changed to a visible liquid and a visible solid; a process having
extremely valuable applications, as we shall presently see.
Compressed air, as a means of transmitting power, rivals such
mechanical devices as gearing, belting, and rope drives, when it is
applied near the compressor; or it may be conducted for many miles in
pipes, thus competing with the electric current; or, finally, it may
be transported in tanks to the place where it is to be used, a process
analogous to the use of the electric storage battery. Compressed air
has, moreover, certain advantages over other methods of transmitting
power for a number of special purposes. Thus for use in coal mines it
is safer than electricity because it is free from the danger of sparks.
There are a great many cases in which the air itself is used in the
process to which the power is applied, as in different kinds of air
blast, from the simple bellows to the blowers of blast furnaces; also
in aerating apparatus, oil and fuel burners, spraying, cleansing, etc.
A familiar form of air compressor is the hand pump used for inflating
bicycle tires. This simple device illustrates two important facts;
first, that a considerable amount of energy must be used to overcome
the expansive force of the air, and, second, that part of the energy
applied to the pump produces heat. That the heat thus produced and
dissipated in the surrounding air represents a loss of energy is
apparent; but energy is wasted in another way that is, perhaps, not so
evident. When a gas is heated its expansive force is increased. Hence,
on account of the heating of the air in the tire, the pump has to do
more work to accomplish a given amount of compression than it would
need to do if the air remained cool.
In order to avoid this loss, the air compressors used for industrial
purposes are provided with some sort of device for keeping the air cool
during compression. This is accomplished by a spray of water inside
the compressor cylinder, or, more commonly, by inclosing the cylinder
in a water jacket. In producing high pressures, the air is compressed
by degrees in two or more cylinders, and cooled between the successive
stages. Lastly, before compressed air is applied to driving tools or
machinery, it is often reheated to increase its pressure. For most
industrial purposes the pressure of compressed air does not exceed 75
pounds to the square inch (5 “atmospheres”). For charging the tanks
of compressed-air locomotives, for liquefying gases, and a few other
purposes, much higher pressures are used. In laboratory experiments air
has been compressed to the enormous pressure of 60,000 pounds to the
square inch, or 4,000 atmospheres. At a pressure of 14,000 pounds to
the square inch compressed air has been successfully used for blasting
in mines in place of ordinary explosives.
The use of pneumatic tools began in the sixties of the last century,
when pneumatic drills were employed with conspicuous success in the
construction of the Mont Cenis and Hoosac tunnels. Such tools are now
indispensable adjuncts not only of tunneling and mining, but also of
nearly every department of metal-working and wood-working, and have
contributed incalculably to the welfare of mankind.
Imagine a workman with an ordinary hammer driving such a tool as
a chisel, punch, or calking iron, and estimate the amount of work
accomplished in the course of a day spent in this wearisome labor.
Then consider how such operations are performed with the help of that
versatile substance, air. The pneumatic hammer consists of a piston
working in a cylinder, to which compressed air is conveyed from a
compressor by means of a flexible hose. The hammer is so designed that
the air causes the piston to work back and forth with great rapidity.
A chisel, rammer, or other percussion tool is loosely fitted in the
nose of the hammer, so that the piston will strike it a blow at each
forward motion. The workman has nothing to do but hold the tools in
place. With a common hammer or mallet a workman will strike from twenty
to a hundred blows a minute, according to the nature of the work. The
speed of the pneumatic hammer ranges from 1,000 to 20,000 blows per
minute, so that its sound is a continuous buzz. Such hammers are used
for calking, chipping, riveting, and a great number of other purposes.
In another large class of pneumatic tools work is done by rotation
instead of percussion. The piston is replaced by a motor, which turns
an auger, drill, or other tool for such operations as boring, screwing,
reaming, etc.
The use of pneumatic tubes for transporting letters, parcels, and
the like, although suggested as early as 1667, has been in practical
operation only since 1854, when a tube 220 yards long was built in
London to convey telegraphic dispatches. The articles to be transported
are placed in a carrier fitting closely inside the tube and propelled
either by introducing air under pressure behind it or by exhausting
the air in front of it. Scores of miles of such tubes laid underground
are now in operation in London, Paris, Berlin, New York, and other
large cities for carrying mail matter. In the United States the
pneumatic cash carrier, used in stores, is the commonest application of
“pneumatic dispatch,” as this system of transportation is called.
The use of compressed air instead of a brush for applying paint,
varnish, and whitewash is a further illustration of the versatile
possibilities of air as a means of transmitting power.
When an inclosed body of air or other gas is subjected to pressure,
its volume is diminished and its density is increased. It is natural
to inquire what will happen if the external pressure be increased
indefinitely. Will the inclosed substance eventually cease to be
gaseous and become a solid or a liquid? The answer to this question,
furnished about half a century ago through the researches of Thomas
Andrews, is that no amount of pressure will liquefy a gas unless
its temperature is below a certain point. This point, known as the
_critical temperature_, is widely different for different substances.
For most of the atmospheric gases it is exceedingly low. Thus oxygen
must be cooled to 118° below zero Centigrade (180° below zero
Fahrenheit) before it will liquefy under any pressure, and the critical
temperature of nitrogen is still lower. Efforts to liquefy the gases
of the atmosphere were unsuccessful for a long time on account of the
difficulty of attaining such low temperatures.
Nowadays the problem is so completely solved that the manufacture
of liquid air is a commonplace commercial enterprise, and millions
of gallons are produced every year. Liquid air is the principal
commercial source of pure oxygen, nitrogen, and other gases found in
the atmosphere. It is also used as a refrigerating substance in various
industrial and scientific processes, and new uses are being found for
it from year to year.
Like many other latter-day miracles, compared with which the alleged
feats of necromancy seem tame and puerile, the liquefaction of air is
founded on quite simple principles. The earliest commercial process
was invented, in its main features, by Linde in 1895, and the newer
processes are merely modifications of this one.
Experiments of the English physicists Joule and Thomson showed that
when a gas under pressure is forced through a small orifice, beyond
which it expands, it undergoes a certain amount of cooling. This fall
in temperature, known as the “Joule-Thomson effect,” is generally quite
small, but Linde devised a means of multiplying it in his “regenerative
cooling process.” The air to be liquefied is first compressed to,
say, 100 atmospheres, cooled as much as possible by water, and passed
through a long spiral tube. At the end of the spiral it escapes through
a small nozzle, and is thus somewhat further cooled by the effect above
mentioned. This cooled air then passes back around the spiral tube, and
causes still more cooling of the air in the latter. The escaping air is
again compressed and goes through the same process as before. Thus its
temperature grows constantly lower, until finally the stream issuing
from the nozzle is a liquid instead of a gas. The liquid collects in a
reservoir, from which it can be drawn off when desired.
The liquid air thus obtained has a temperature of about 315° below
zero Fahrenheit. It is generally drawn into a vessel called, from the
name of the inventor, the Dewar flask, which is open at the top, but
otherwise insulated from the temperature of the surrounding air by
having a double wall, with a vacuum between the walls. The familiar
thermos bottle is constructed on the same principle. In such a vessel
liquid air can be kept for hours and even days, and it is thus
available for use in many interesting laboratory experiments.
Liquid air looks much like water, except for its slight bluish color.
It boils--i. e., changes back to ordinary air--at a temperature
only slightly above that at which it is produced, and this boiling,
of course, goes on rapidly at the surface of the liquid, owing to
absorption of heat from the air above. Liquid air is lighter than
water, upon which it consequently will float. A cubic foot of liquid
air is the equivalent of about 800 cubic feet of ordinary air at 60°
Fahrenheit and atmospheric pressure.
The curious effects of liquid air, only a few of which can be mentioned
here, are not irrelevant to the subject of atmospheric resources,
since they aid in various ways in carrying out important scientific
researches. Almost all liquids are solidified and almost all solids are
hardened and stiffened by immersion in liquid air. Alcohol is promptly
frozen in it, and at the same time gives out so much heat that the
liquid air boils violently and the congealing alcohol overflows the
vessel in a little avalanche of snow. India rubber becomes as brittle
as glass. Meats become so hard that when struck by a hammer they
ring like steel. Chemical action is enormously reduced by exposure to
the low temperature of liquid air, and so is the electric resistance
of metals. One might suppose that such a temperature would be fatal
to all forms of life, but this is not the case. A goldfish, frozen
solid in liquid air, revives and swims vigorously a few seconds after
being replaced in water. Bacteria survive hours of exposure to the
temperature of liquid air, while the seeds of higher plants, even after
several days of similar treatment, sprout the same as other seeds.
Most of the atmospheric gases have not only been liquefied, but
also frozen solid. An important exception is helium, which has been
liquefied only at a temperature of 452° below zero Fahrenheit. The
remarkable feat of liquefying helium was accomplished in 1908 by the
Dutch physicist Kamerlingh Onnes, who subsequently, in his attempts
to solidify this substance, attained the unprecedented temperature of
less than 2 (Centigrade) degrees above “absolute zero,” or 456° below
zero Fahrenheit, by the rapid evaporation of the liquid under greatly
reduced pressure.
[Illustration]
[Illustration: EXPLORING THE UPPER AIR. Left: Beginning of a
pilot-balloon flight. Right: Sending up a sounding-balloon. Note the
parachute, which wafts the basket of instruments gently to the ground
after the balloon bursts. (_Photographs from U. S. Weather Bureau._)]
Although, when air is liquefied, the oxygen and nitrogen are condensed
simultaneously, the latter has a lower boiling point than the former
and therefore passes off more rapidly when the liquid is allowed to
evaporate. This fact makes it possible to separate the two substances,
by the process known as “fractional distillation,” and hence liquid air
plants have been established for the special purpose of manufacturing
oxygen and nitrogen, for both of which there is a large and growing
commercial demand. Scores of millions of cubic feet of oxygen are used
every year in the wonderfully efficient process of welding metals
with the oxyacetylene blowpipe, the flame of which has a temperature
of about 6,000° Fahrenheit. Most of the supply now comes from liquid
air. An equally large amount is used in a recently introduced method of
cutting metal. The object to be cut is first heated to incandescence,
after which a jet of oxygen is played upon it. The metal actually burns
away in the stream, and a clean cut is made like that of a saw. It is
interesting to reflect, when we fill our lungs with oxygen in order to
keep our bodily machinery in operation, that the same atmospheric gas
is applied to the building of motor cars, bicycles, safes, boilers, and
battleships. Cartridges made of lampblack, dipped for a few moments
in liquid oxygen and then primed with a fulminate cap, constitute an
explosive as powerful as dynamite and much cheaper to produce. A small
percentage of oxygen added to the air supplied to blast furnaces has
been found to effect a great saving of fuel used in the furnace.
[Illustration: METEOROGRAPH FOR USE WITH SOUNDING BALLOON. (Fergusson
pattern. U. S. Weather Bureau, 1919.) The aluminum case, surrounded by
hoops of rattan to protect the apparatus when it falls to the ground at
the end of the flight, contains a set of very light self-registering
meteorological instruments. (_Photograph from U. S. Weather Bureau._)]
[Illustration: KITE METEOROGRAPH. (U. S. Weather Bureau Pattern.)
The four pens record the barometric pressure, temperature, humidity,
and wind-force on a sheet of paper wound around the large cylinder,
which is turned by clockwork. Note the fan wheel inside the tube, for
measuring the force of the wind. The apparatus is made chiefly of
aluminum and is inclosed in an outer case of aluminum when sent aloft
attached to the kite. (_Photograph from U. S. Weather Bureau._)]
The most important industrial demand for nitrogen is for use in
“fixation” processes--i. e., for making nitrogen compounds to be used
as fertilizers, explosives, etc. Before describing these processes,
it may be of interest to mention that some of the “rare” gases of the
atmosphere are now obtained on a commercial scale as by-products of
the manufacture of oxygen and nitrogen from liquid air. Thus neon, on
account of its exceedingly small resistance to the passage of electric
discharges, is a promising substance for filling glow lamps; especially
as means have been found of correcting the glaring red color of the
light which characterized the original neon lamps. Argon is likewise
used for filling electric lamps.
The idea of using the unlimited store of atmospheric nitrogen for the
benefit of agriculture and the manufacturing industries has been very
prominently before the public in recent years, and gained special
notoriety during the late war, when great efforts were being made
to increase the supply of nitrogenous materials suitable for use in
explosives. Nitrogenous matters in the soil are indispensable to
the growth of plants, and as long ago as 1898 Sir William Crookes,
in an address before the British Association for the Advancement of
Science, alarmed the world by pointing out the possibility of a general
famine owing to the prospective exhaustion of Chilean nitrates and
other sources of nitrogenous fertilizers. Nitrogen also enters on an
immense scale into the composition of many industrial products besides
explosives. No wonder popular writers have dwelt upon the fact that
the atmosphere contains far more nitrogen than mankind needs for every
possible purpose--actually something like 20,000,000 tons over every
square mile of the earth’s surface.
A widespread misunderstanding, however, prevails as to the problem
involved in utilizing this supply of nitrogen. Free (i. e., uncombined)
nitrogen is of no use as a fertilizer, and it cannot be readily used in
the arts. The process of extracting it from the atmosphere is an easy
one, thanks to the liquid air industry. The real difficulty is to make
this inert gas enter into chemical combination with other substances,
forming useful compounds such as ammonia and nitrates; in other words,
to “fix” it.
As we have stated on another page, lightning discharges cause nitrogen
and oxygen to combine in the atmosphere, and perhaps also combine
nitrogen and hydrogen to form ammonia. There is one other natural
process by which atmospheric nitrogen is fixed. Certain species of
bacteria are able to extract this gas from the atmosphere and combine
it with other materials. Some of these bacteria are independent
organisms, while others form colonies of parasites growing on the roots
of higher plants, chiefly members of the pea family. In the latter case
the bacteria use the nitrogen of the air and carbohydrates drawn from
the roots on which they grow to form nitrogenous compounds, which are,
in part, transmitted to the host plant.
Unfortunately these natural processes do not suffice to maintain
agricultural soils in a high state of fertility. Mineral deposits of
combined nitrogen are practically limited to the nitrate fields of
Chile, from which more than two million tons of nitrate of soda are
exported annually; but this supply cannot last more than a few decades.
Combined nitrogen in the form of ammonia is supplied on a large and
rapidly growing scale from by-product coke ovens, and another perennial
source of nitrogenous matter is found in animal and vegetable refuse of
all kinds, including fish scrap and slaughter-house refuse, garbage,
sewage, manure, etc. Since, however, the demands of agriculture and
the manufacturing industries greatly exceed the total amount of
combined nitrogen obtainable from all these sources, the ingenuity of
inventors has been spurred to the task of fixing atmospheric nitrogen
by artificial methods, and several such methods have now been put in
operation commercially. Their combined product at present constitutes
nearly one-third of the total nitrogen supply of the world.
It is not proposed here to describe these methods in detail, but
it may be mentioned that one of them, known as the “arc process,”
imitates the action of lightning in combining the nitrogen and oxygen
that occur naturally in the air, while the others utilize nitrogen
that has been previously separated from the air by the liquid air
process. The arc process requires, for commercial success, a large
supply of cheap electrical power, and it is at present almost confined
to Norway and Sweden, where electricity is obtained from waterfalls.
In this process air is blown through a huge electric flame, spread
out by a powerful electromagnet. The air yields nitric oxide, which
is combined with water to form nitric and nitrous acids, and these
substances are combined with others to form marketable products. The
most widely used fixation process, and the one which the United States
Government proposed to employ in the large plants that were in course
of construction in this country at the close of the war, is known as
the “cyanamide process.” This process requires, as a part of its raw
materials, large supplies of limestone and coke, from which calcium
carbide is made in an electrical furnace. The calcium carbide, at red
heat, absorbs nitrogen, forming an intermediate product from which, by
further processes, are made ammonia and nitric acid. A third method of
fixing atmospheric nitrogen, which has been applied on a vast scale
in Germany and is now coming into use in other countries, is commonly
called the “Haber process.” In this process nitrogen is combined with
hydrogen, obtained from water, to form ammonia, the combination being
facilitated by the presence of what chemists call a “catalyzer,” i. e.,
a substance that enables other substances to combine without itself
undergoing any change. Several different catalyzers have been used in
the Haber process.
Two or three other methods of nitrogen fixation are beginning to assume
commercial importance.
While the power of the wind holds an important place among the
resources of the atmosphere, it cannot be said that the utilization
of this resource has undergone developments in modern times at all
comparable with the striking inventions and discoveries we have just
been recording, if we except the use of the wind in aeronautics.
Atmospheric resources used by aeronauts will be discussed in subsequent
chapters.
The chief use made of the wind to-day, as in ages past, is to propel
sailing ships, and its use for this purpose is, of course, of less
importance, in a relative sense, than it was before the introduction of
steam. The importance of windmills has also greatly declined. This fact
was strikingly brought out some years ago when the United States Bureau
of Statistics collected, through American consuls abroad, detailed
information concerning the use of the windmills in foreign countries.
In most parts of Europe windmills are rapidly disappearing. In Holland,
for example, the traditional home of the windmill, the perpetual task
of draining the polders is now performed by steam pumps, and the total
number of windmills is estimated to be only about one-tenth what it was
centuries ago. Our own country is probably the only one in which the
use of windmills is increasing. The modern American windmill, with its
disklike assemblage of numerous light sails, and ingenious contrivances
for veering, reefing, etc., is a much more efficient contrivance
than the old-fashioned windmill; but its utility, like that of other
windmills, is limited by the irregular force of the winds.
For years the hope has been entertained that the windmill would
eventually become a common means of generating electricity, but this
hope has not yet been realized, though isolated installations of this
character are in successful use.