Water, the substance most familiar to us, is known in the liquid, in
the solid, and in the gaseous state. Everybody knows that by heating
the solid it passes into the liquid state, and that by heating the
liquid it passes into the form of gas or vapor. So also everybody
knows that when the vapor of water is cooled it is liquefied, and that
by cooling liquid water sufficiently it becomes solid or turns to ice.
In the same way many of the substances that are known to us as
liquids, such as alcohol and ether, can be converted into the form of
gas or vapor by heat. In fact, this is true of most liquids. The
temperature at which a solid passes into the liquid state is called
its melting point, and the temperature at which a liquid passes into
the gaseous state is called its boiling point. The boiling point of
water, for example, is 100° C. (212° F.) in the open air. But the
boiling point varies with the pressure exerted upon the surface. The
pressure that we ordinarily have to deal with is that of the
atmosphere. If the pressure is increased the boiling point is raised,
and if the pressure is decreased the boiling point is lowered. In
dealing, then, with the conversion of a gas into a liquid, or that of
a liquid into a gas, both the temperature and the pressure have to be
considered.
Just as water is most familiar to us in the liquid form, so there are
substances that are most familiar to us in the gaseous form. In fact,
the only gaseous substances that can be said to be familiar to
everybody are the gases contained in the air. The principal
constituents of the air are nitrogen and oxygen, which form
respectively about four fifths and one fifth of its bulk. Besides
these gases, however, the air contains water vapor, carbonic-acid gas,
ammonia, argon in small quantities, and many other substances in still
smaller quantities. For the purposes of this article it is only
necessary to have in mind the nitrogen, oxygen, water vapor, and
carbonic acid. Of these, the water vapor is easily converted into
liquid, as, for example, in the formation of rain, while the other
constituents are liquefied with difficulty. The name "liquid air" is
applied to the substance that is obtained by converting the air as a
whole into a liquid; but in this process the water and the carbonic
acid become solid and can be filtered from the liquid so that the
latter consists almost wholly of oxygen and nitrogen. A few years ago
this liquid was obtainable in only very small quantities. To-day,
thanks especially to the efforts of Mr. Charles E. Tripler, of New
York, it can be produced in any desired quantity, and at moderate
cost. In consequence of this, it has come to be talked about in a
familiar way, and many persons have had the privilege of seeing and
feeling it, and of learning something about its wonderful properties.
The object of this article is to explain the method employed in the
production of liquid air, to give an account of some of its
properties, and to indicate some of the uses to which it may possibly
be put.
In the older text-books of physics and of chemistry certain gases were
classed as "permanent," under the impression that these could not be
liquefied, and this impression was based upon the fact that all
efforts to liquefy them had failed. A brief account of these efforts
will be helpful.
Among the so-called permanent gases was chlorine. An English chemist,
Northmore, first succeeded, early in this century, in liquefying
chlorine. His work was, however, lost sight of, and in 1823 Faraday at
the Royal Institution showed independently that this transformation of
gaseous chlorine into the liquid can be effected comparatively easily.
The method used by him is this: When chlorine gas is passed into cold
water it forms with the water a solid product known as chlorine
hydrate. If kept well cooled this hydrate can be dried. If then its
temperature is raised even to the ordinary temperature of the room,
the solid hydrate is decomposed into liquid water and gaseous
chlorine. Faraday put some of the solid hydrate into a stout glass
tube sealed at one end and bent at the middle. The other end of the
tube was then closed. The tube was then suspended so that the two ends
were turned downward. On gently warming the end in which was the solid
hydrate, this was decomposed into chlorine and water. But the gas
given off would under ordinary conditions have occupied a much larger
space than the solid hydrate. Being prevented from expanding by the
tube in which it was inclosed, it was under very considerable
pressure. The end of the tube that was not warmed was cooled, and in
this end, in consequence of the pressure and the comparatively low
temperature, chlorine, which is gaseous under the ordinary pressure of
the air, appeared as a liquid. The general method made use of by
Faraday in this classical experiment is that which is always made use
of for the purpose of liquefying gases, but for some gases pressures
very much higher and temperatures very much lower are required.
Faraday himself succeeded in liquefying all the gases then known
except oxygen, hydrogen, nitrogen, nitric oxide, and marsh gas. He
subjected oxygen to a pressure of about one thousand pounds to the
square inch, or nearly seventy atmospheres, but it showed no signs of
liquefaction. Later experimenters increased the pressure to four
thousand pounds to the square inch, with no better results, so that it
is not surprising that it came to be held that some gases are
permanent.
Within comparatively recent years several gases have been liquefied on
the large scale by means of pressure. These are ammonia, carbonic
acid, nitrous oxide, and chlorine. Ammonia is used for producing low
temperatures, as in breweries and in cold-storage plants and in the
manufacture of ice; carbonic acid, for fire extinguishers and for
charging beer with the gas; nitrous oxide, for producing anæsthesia;
and chlorine in connection with several branches of chemical
manufacture. The production of low temperatures by means of liquid
ammonia and of liquid carbonic acid will be more fully dealt with
further on, when the principles involved will be briefly presented. It
is to be borne in mind that these substances are liquefied by means of
pressure alone, at temperatures that are easily reached, so that it
appears that by mechanical pressure it is possible to produce low
temperatures. In 1869 an important fact was discovered by Andrews. It
was that for every gas there is a temperature above which it is
impossible to liquefy it by pressure. Thus, if chlorine is at any
temperature above 146° C. (294° F.) it can not be liquefied. This
temperature is called the "critical temperature" of chlorine. The
pressure to which the gas must be subjected at the "critical
temperature" in order that the gas may be liquefied is called the
"critical pressure." In the case of chlorine this is 93.5 atmospheres.
Now, the critical temperature of the gases that were called permanent
gases are very low--lower than could be reached by the means at the
command of earlier experimenters. The critical temperature of oxygen,
for example, is -118.8° C. (-182° F.), while that of nitrogen is -146°
C. (-230° F.). The critical pressures are 50.8 and 35 atmospheres
respectively. As there is no difficulty in obtaining these pressures,
the problem of liquefying oxygen and nitrogen and air resolves itself
into finding a method of producing temperatures below the critical
temperatures of these gases.
It is well known that a temperature somewhat below the freezing point
of water can be produced artificially by mixing ice and salt. The
ordinary ice-cream freezer is a familiar application of this method of
producing cold. Other freezing mixtures that are sometimes used
consist of calcium chloride and snow, that gives the temperature -48°
C. (-54.4° F.), and solid carbonic acid and ether, that is capable of
lowering the temperature to -100° C. (-148° F.). But even with the
latter mixture it is not possible to reach the critical temperature of
oxygen or that of nitrogen. How, then, is it possible to reach these
extremely low temperatures?
In order to answer this question it will be necessary to take into
consideration certain temperature changes that are observed when
solids are melted and liquids are boiled, as well as when gases are
liquefied and liquids are frozen. When heat is applied to a mass of
ice at its melting point it melts and forms a mass of water having the
same temperature. Heat disappears in the operation. It is stored up in
the water. This disappearance of heat that accompanies the melting of
ice can be shown in a very striking way by mixing a certain weight of
ice with the same weight of water that has been heated to 80° C. (176°
F.). The ice will melt and all the water obtained will be found to
have the temperature of the melting ice--that is, 0° C. (32° F.). The
water of 80° C. is thus cooled down to 0° by the melting of the ice.
Again, when heat is applied to water its temperature rises until the
boiling point is reached. Then it is converted into vapor, but this
vapor has the temperature of the boiling water. During the process of
boiling there is no rise in the temperature of the water or of the
vapor. Heat disappears, therefore, or is used up in the process of
vaporization. Similar phenomena are observed whenever a solid is
melted or a liquid is boiled. When, however, a gas is liquefied it
gives up again the heat that is absorbed by it when it is formed from
a liquid; and so also when a liquid solidifies it gives up the heat it
absorbs when it is formed from a solid.
But it is not necessary that a gas should be converted into a liquid
in order that it should give up heat. Whenever it is compressed it
becomes warmer. Some of the heat stored up in it is, as it were,
squeezed out of it. Conversely, whenever a gas expands, it takes up
heat and, of course, surrounding objects from which the heat is taken
become colder. Now, it is a comparatively simple matter to compress
air. Every wheelman knows that, and he also knows that the process
causes a rise in temperature; at least he knows it if he uses a small
hand pump. With large pumps run by steam any desired pressure can be
reached. This is simply a question of securing the proper engines, and
vessels sufficiently strong to stand the pressure. It has already been
pointed out that several gases are now liquefied on the large scale by
means of pressure. It is to be noted that low temperatures can be
produced by converting certain gases, such as ammonia and carbonic
acid, into liquids, and by compressing certain gases, as, for example,
air. When liquefied gases are used it is only necessary to allow them
to pass rapidly into the gaseous state, when more or less heat is
absorbed. This is the basis for the use of liquid ammonia in the
manufacture of ice. A vessel containing the liquid ammonia is placed
in another containing water. The inner vessel being opened, the liquid
ammonia is rapidly converted into the gas; heat is absorbed from the
water; it freezes. When a vessel containing liquid carbonic acid is
opened so that the gas that is formed escapes through a small valve,
so much heat is absorbed that a part of the liquid carbonic acid is
itself frozen. In this case the substance is present in all three
states of aggregation--the solid, the liquid, and the gaseous. The use
of a mixture of ether and solid carbonic acid as a freezing mixture
has already been referred to. Its value depends, of course,
principally upon the fact that solid carbonic acid is liquefied, and
the liquid then converted into gas, both of which operations involve
absorption of heat.
We are now prepared to understand the important experiments of
Cailletet and of Pictet, the results of which were published in 1877.
It should be said that they worked independently of each
other--Cailletet in Paris and Pictet in Geneva. Pictet liquefied
carbonic acid and sulphur dioxide by pressure. The liquid carbonic
acid was passed through a tube that was surrounded by liquid sulphur
dioxide boiling in a partial vacuum. The liquid carbonic acid thus
cooled was then boiled under diminished pressure in a jacket
surrounding a tube in which the gas to be liquefied was contained
under high pressure. When this gas was allowed to escape from a small
opening its temperature was so reduced by the expansion that a part of
it was liquefied in the tube and passed off as a liquid. Cailletet
worked in essentially the same way, but on a smaller scale. Neither of
these experimenters liquefied oxygen or nitrogen on the large scale,
but they pointed out the way that must be followed in order that
success may be attained. They destroyed the belief in "permanent"
gases.
Later experimenters in this field are Wroblewski, Olszewski, and
Dewar, who have been interested mainly in the purely scientific side
of the problem, while Linde in Germany, Hampson in England, and
Tripler in the United States have their minds on the practical side.
Notwithstanding the low temperatures involved in the experiments, a
number of heated discussions have been carried on in the scientific
journals touching the question of priority. To the unprejudiced
observer it appears that all of those named above are entitled to
credit. They have all helped the cause along, but just how to
apportion the credit no one knows. In a general way, however, some of
the results obtained by each in turn should be given. Wroblewski and
Olszewski have carried on the work begun by Cailletet and Pictet, and
have produced lower temperatures. In the latest form of apparatus used
by Olszewski, liquid ethylene is used as the cooling agent. Its
boiling point is -102° C. (-151.6° F.). By causing it to boil rapidly
under diminished pressure a temperature below the critical temperature
of oxygen can be reached. As early as 1891 Olszewski obtained as much
as two hundred cubic centimetres of liquid air by this method. Dewar
has also made use of liquid ethylene. This was passed through a spiral
copper tube surrounded by solid carbonic acid and ether. It was then
passed into a cylinder surrounded by another cylinder containing solid
carbonic acid and ether. A spiral copper tube, which runs through the
outer cylinder and also through the inner cylinder in which the
ethylene was boiling under diminished pressure, carried the air. This
was liquefied and then collected in a vacuum vessel below. Later he
found that air can be liquefied by using liquid carbonic acid alone as
the cooling agent. A sectional drawing of his apparatus described in
1896 is given herewith. As he remarks: "With this simple machine, one
hundred cubic centimetres of liquid oxygen can readily be obtained,
the cooling agent being carbon dioxide, at the temperature of -79°. If
liquid air has to be made by this apparatus, then the carbonic acid
must be kept under exhaustion of about one inch of mercury pressure,
so as to begin with a temperature of -115°."
[Illustration:
FIG. 1.--LABORATORY LIQUEFACTION APPARATUS OF DEWAR FOR THE PRODUCTION
OF LIQUID OXYGEN, ETC.
A, air or oxygen inlet; B, carbon-dioxide inlet; C, carbon-dioxide
valve; D, regenerator coils; F, air or oxygen expansion valve; G,
vacuum vessel with liquid oxygen; H, carbon-dioxide and air outlet;
o, air coil; ø, carbon-dioxide coil.]
The introduction of the vacuum vessel by Dewar has been of great
service in all the work on liquefied gases. A vacuum vessel is a
double-walled glass vessel, as shown in Fig. 1, G. The space between
the inner and outer walls of the vessel is exhausted by means of an
air pump before it is closed. The vessel is therefore surrounded by a
vacuum. As heat is not conducted by a vacuum, it is possible to keep
specimens of liquefied gases in such vessels for a surprisingly long
time. Heat enough can not pass through the vacuum to vaporize the
liquid rapidly. The most common form of these vessels is that of a
globe. Such a vessel is known as a Dewar globe or bulb.
It has been found that liquid air can be kept very well by putting it
in a tin or galvanized iron vessel, which in turn is placed in a
larger one, and then filling the space between the two with felt.
Under these conditions vaporization takes place quite slowly, and it
is possible to transport the liquid comparatively long distances. It
has, for example, been transported from New York to Baltimore and
Washington. In one case with which the writer is familiar two cans
were taken from Mr. Tripler's laboratory in the morning, delivered at
the Johns Hopkins University in the afternoon, and used to illustrate
a lecture in the evening. After the lecture there was enough left for
certain experiments that were carried on during the rest of the night.
[Illustration: FIG. 2.--DIAGRAM SHOWING WORKING OF COMPRESSION
APPARATUS FOR MAKING LIQUID AIR.]
Tripler, Linde, and Hampson have all succeeded in devising forms of
apparatus by means of which air can be liquefied without the aid of
other cooling agents than the expanding air. In principle the methods
employed by these three workers are essentially the same. It appears
from the published statements that at the present time Tripler's plant
is the most efficient. While a few years ago a half pint or so of
liquid air is said to have cost five hundred dollars, now five gallons
can be made for about twenty dollars, and probably much less. The
general working of Tripler's apparatus can be made clear by the aid of
the accompanying drawing, Fig. 2. A^1, A^2, A^3 represent steam
compression pumps. Air is taken through I from above the roof of the
laboratory. In the first pump it is compressed to sixty-five pounds to
the square inch. It, of course, becomes heated as it is compressed. In
order to cool it down again it is passed through a coil, B^1, which is
surrounded by water of the ordinary temperature. This compressed and
cooled air is then further compressed in the second pump, A^2, to four
hundred pounds to the square inch. Again it is cooled in the same way
as before by means of water which circulates around the coil B^2. Once
more the air is compressed, this time in the cylinder A^3, in which it
is subjected to a pressure of two thousand to twenty-five hundred
pounds to the square inch; and then this compressed air is brought
down to the ordinary temperature in the cooler B^3. The air under this
great pressure is now passed through the purifier C, where it is freed
from particles of dust and to a great extent from moisture. From C the
air passes into the inner bent tube, about thirty feet in length,
until it reaches D. This may be called the critical point of the
apparatus. Here is situated a needle valve from which the air is
allowed to escape. It, of course, expands enormously, and is
correspondingly cooled. This very cold air passes into the space
between the inner and outer tubes, and finally escapes at F. The
result of this is that the compressed air in the inner tube is soon
cooled down so far that a considerable part of the air that escapes at
D appears in the liquid form. This collects in the lower part of the
jacket, and on opening the stopcock at E the liquid escapes in a
stream the size of one's finger.
In Mr. Tripler's laboratory the liquid is collected in the cans
already referred to. Although for the reasons mentioned the
evaporation of the liquid is comparatively slow, it is constantly
going on, and as the gas formed occupies a very much larger volume
under the pressure of the atmosphere than the liquid from which it is
formed, it is necessary to leave the cans loosely covered. Otherwise
the pressure would increase to such an extent as to burst any but the
strongest vessels. One cubic foot of liquid air gives at atmospheric
pressure eight hundred cubic feet of gaseous air.
Liquid air obtained as described is a turbid, colorless liquid. The
turbidity is due to the presence of solid water and solid carbonic
acid. By passing the liquid through a paper filter the solids are
removed, and a transparent liquid is thus obtained. This, as already
stated, consists mostly of nitrogen and oxygen in the proportion of
about four fifths of the former to one fifth of the latter. Though it
should not be forgotten that this liquid contains argon in small
quantity, besides three or four other substances in still smaller
quantities, as has recently been shown by Professor Ramsay, we may
disregard everything except the nitrogen and oxygen. Liquid air is a
_mixture_ of these two substances. They are not chemically combined as
hydrogen and oxygen are, for example, in water. This mixture boils at
-191° C. (-312° F.), which is the temperature of the liquid as it is
in the cans. As the nitrogen boils at a lower temperature (-194° C. or
318° F.) than oxygen (-183° C. or 297° F.), more nitrogen is converted
into gas in a given time than oxygen, and after a time the liquid
that is left is much richer in oxygen than ordinary air. When liquid
air is poured upon water it, being a little lighter than the water,
floats, not quietly, to be sure, but in a very troubled way. Soon,
however, the liquid sinks to the bottom because the nitrogen, which is
the lighter constituent, passes into the gaseous state, and the liquid
oxygen which is left is a little heavier than water. The experiment is
a very beautiful one. A scientific poet could alone do justice to it.
The beauty is enhanced by the fact that while liquid air is colorless,
or practically so, liquid oxygen is distinctly blue.
Although liquid air has the temperature -191° C. (-312° F.), one can
without danger pass the hand through it rapidly. The sensation is a
new one, but it is evanescent. Very serious results would follow if
the hand were allowed to remain in the liquid even for a short time.
The tissues would be killed. So also, it is possible to pass the hand
rapidly through molten lead without injury. In the latter case the
moisture on the hand is converted into vapor which forms a protecting
cushion between the hand and the hot liquid; while, in the former
case, the heat of the hand converts the liquid air immediately
surrounding it into gas which prevents the liquid from coming in
contact with the hand.
When the liquid is poured out of a vessel in the air it is rapidly
converted into gas. The great lowering in the temperature causes a
condensation of the moisture of the air in the form of a cloud. The
same thing is seen when the cover is removed from a can containing the
liquid. Of course, this liquid does not wet things as water does.
When, however, as happened in New York, the lecturer deliberately
pours a dipperful of the liquid upon a priceless Worth gown, he may
expect to hear expressions of horror from the owner. This experiment
passed off most successfully. Every trace of the liquid air was
converted into invisible gases before the fleeting agony of the
sympathetic audience had passed away.
The effects of very low temperature upon a number of substances have
been studied, and some of them can easily be shown. Paraffin, resin,
and rubber immersed in liquid air soon become very brittle, and the
color of the resin is completely changed. A beefsteak or an onion also
becomes brittle, and can be broken into small fragments by the blow of
a hammer. A similar effect is produced in the case of some metals. Tin
and iron, for example, become brittle, and the tenacity of the iron is
greatly increased. A copper wire, however, retains its flexibility. At
low temperatures the electric conductivity of all metals is increased.
In general, the lower the temperature the greater the conductivity. If
a copper wire could by any means be kept cold enough, electrical
energy could be transmitted by it with but little loss--perhaps none.
Mercury is easily frozen by surrounding it with liquid air, and the
solid thus formed is very hard, though if it is cooled down
sufficiently it becomes brittle.
Alcohol can be frozen without difficulty by means of liquid air. By
the aid of the lowest temperatures hitherto attainable it has only
been possible to convert alcohol into a pasty mass. The frozen alcohol
is as hard as ice. When alcohol is dropped into liquid air the drops
retain the globular form. When taken out on a platinum loop the flame
of a Bunsen burner does not set fire to it.
Phosphorescence is greatly increased by cooling substances down to the
temperature of liquid air. This has been shown by means of water,
milk, paper, eggs, and feathers. An egg and a feather could be
distinctly seen in a dark room.
Scarlet iodide of mercury is converted into the yellow variety when it
is subjected to the temperature of liquid air. Some other colors are
changed under the same circumstances, but not enough is known of this
subject to warrant a general statement.
Attention has already been called to the fact that liquid air loses
its nitrogen more rapidly than it does its oxygen, and that, after a
time, the residue contains a large proportion of oxygen. As combustion
is combination with oxygen, combustion or burning takes place more
readily in contact with this liquid oxygen than it does in the air. If
a lighted match is attached to the end of a steel watch spring, and
this then plunged beneath the surface of liquid air, the spring will
soon take fire and burn brilliantly, the sparks flying off for some
distance in beautiful coruscations. Hair felt, which does not burn in
the air, burns in a flash when soaked with liquid air. Finally, when
liquid air is confined in any vessel not capable of sustaining an
enormous pressure, say about ten thousand pounds to the square inch,
the vaporization goes on until the vessel bursts or the stopper is
forced out. It might therefore be used as an explosive without any
addition, but its manipulation is not altogether simple.
Now for the inevitable question: Of what use is liquid air likely to
be? This is a perfectly proper question, and yet, if scientific
workers always stopped to ask it, and would not work unless they could
find a favorable answer, progress would, to say the least, be much
slower than it is. Most great practical discoveries have necessarily
passed through the plaything stage. Some of the most important
discoveries have not even furnished playthings, and have found no
practical applications as this expression is commonly understood. But
the production of liquid air, while furnishing mankind with a
beautiful and instructive plaything, seems likely to find practical
applications. We may look for these in four directions, to each of
which a short paragraph may be devoted:
First, as a cooling agent. Low temperature is marketable. To be sure,
the demand for the extremely low temperature that can be produced by
liquid air does not exist to-day, but this concentrated low
temperature can be diluted to suit conditions. The only question to be
answered in this connection is, then, What is the cost of cold
produced by liquid air? It is impossible for any one to answer this
question at all satisfactorily at present. It can only be said that
this is what experimenters are trying to find out. It appears,
however, that they are on the way to cheap liquid air, and that as the
processes are improved the price will become lower and lower.
Second, for the construction of motors. There is no doubt that liquid
air with its enormous power of expansion can be used as a source of
motive power just as compressed air is. In the case of steam it is
necessary to heat the water in order to convert it into steam, and to
heat the steam to give it the power of expansion. The cost is, in the
first instance, that of the fuel. Given a certain amount of heat, and
a certain amount of work is obtained. If liquid air is used, the
problem is much the same. Engines must be run in order to compress the
air which is to be liquefied. Every gallon of liquid air has been
produced at the expense of work of some kind. Now, the question arises
at once, What proportion of the work that was put in that gallon of
liquid air in the course of its production can be got out of it again?
It is certain that all of it can not be got out unless all that we
have ever learned about such matters goes for nothing. In dealing with
the problem of the application of liquid air as a source of motive
power we are therefore doubly handicapped. In the first place, we do
not know the cost of the liquid when produced on the large scale; and,
in the second place, _we_ do not know the probable efficiency of a
liquid-air motor. I say "we do not know." Perhaps Mr. Tripler and the
others engaged in the experiments on this subject do know
approximately. We certainly can not blame them for not telling us all
they know at this stage of the work. It is unfortunate, however, that
such a statement as was recently published in a popular magazine
should be allowed to gain currency--apparently with the sanction of
Mr. Tripler. The statement referred to is to the effect that ten
gallons of liquid air have been made by the use of three gallons of
liquid air in the engine. If that means that the ten gallons of liquid
air are made from air at the ordinary pressure, the statement is in
direct conflict with well-established principles. If it means that the
ten gallons of liquid air are made from air that has already been
partly compressed, we must know how much work has been done before the
liquid-air engine began. Leaving out of consideration the question of
cost, it may be pointed out that liquid-air engines would have the
advantage of compactness, though they would necessarily be heavy, as
they would have to be strong enough to stand the great pressure to
which they would be subjected.
The third application of liquid air that has been suggested is in the
preparation of an explosive. In fact, an explosive has been made and
used for some time in which liquid air is one of the constituents.
When the liquid from which a part of the nitrogen has boiled off is
mixed with powdered charcoal, the mixture burns with great rapidity
and great explosive force. "To make this explosive, Dr. Linde pours
the liquid containing about forty or fifty per cent of oxygen on
fragments of wood charcoal, two or four cubic millimetres in size.
These are kept from scattering under the ebullition of the liquid by
mixing them into a sort of sponge with about one third of their weight
of cotton wool." Of course, this explosive must be made at or near the
place where it is used. It has been in use in the way of a practical
test in a coal mine at Pensberg, near Munich. It is claimed that the
results were satisfactory. The chief advantage of the explosive is its
cheapness, and the fact that it soon loses its power of exploding.
Finally, the fourth application of liquid air is for the purpose of
getting oxygen from the air. This can be accomplished by chemical
means, but the chemical method is somewhat expensive. Oxygen has
commercial value, and cheap oxygen would be a decided advantage in a
number of branches of industry. It will be observed that it is the
liquid oxygen that makes possible the preparation of the explosive
described in the last paragraph. Oxygen as such in the form of gas is
of value in Deacon's process for the manufacture of chlorine. In this
process air and hydrochloric acid are caused to act upon each other so
as to form water and chlorine. The nitrogen takes no part in the act,
and it would be an advantage if it could be left out. It is only the
oxygen that is wanted. There are many other possible uses for oxygen
either in the liquid or in the gaseous form, but these need no mention
here.
In conclusion it may safely be said that it is highly probable that
liquid air will be found to be a useful substance, but it is
impossible at present to speak with any confidence of the particular
uses that will be made of it. As work with it is being carried on
energetically in at least three countries, we may confidently expect
important developments in the near future.