Thus air has been reduced to a volume a thousand or fifteen hundred
times less than under ordinary conditions. It ceased to be a gas and took
the appearance of water. This astonishing result is only the last in a
long list of experiments which for a long time were fruitless; it is the
finishing touch to a building begun long ago, and on which many workmen
have labored. What has been the work of each of them? It is a long story.
Van Marum, a philosopher and chemist of Harlem, is celebrated as the
constructor of an electric machine, the largest known, but he is more
justly celebrated for having been the first to liquefy a gas. Wishing
to know if ammonia would obey Mariotte’s law, he compressed it. Under a
pressure of six atmospheres it changed quickly to a transparent liquid.
Van Marum did not foresee the consequences of his experiments, and is
honored only as being the first successful performer of the experiment.
But Lavoisier, whose keener mind grasped all that these results implied,
did not hesitate to declare the general law that all substances were
capable of existing in three different states, and he illustrated his
belief most forcibly. “Let us consider for a moment what would happen to
the different substances which form the earth, if the temperature should
be quickly changed. Let us suppose that the earth were suddenly placed in
a region where the temperature would be much above that of boiling water;
soon the air, all liquids which can be vaporized at a temperature near
that of boiling water, and many metallic substances even, would expand,
be transformed into air-like fluids, and form part of the atmosphere.
“On the contrary, if the earth should be suddenly placed in a very cold
temperature, for example, that of Jupiter or Saturn, the water of our
rivers and seas, and, probably, the greatest number of liquids which we
know would become solid.”
“Air,” according to this supposition, or at least a part of the air-like
substances which compose it, “would doubtless cease to exist in its
present form; it would be changed to a liquid state, and this change
would produce new liquids of which we know nothing.”
Lavoisier was mistaken about the temperature of Jupiter and Saturn, but
was right in his supposition that air would become a liquid; however,
as experiment did not prove the theory, the prediction was forgotten
and the question dropped. It slept a long time, for it was not until
1823 that it was revived by Faraday. The first experiments of this great
philosopher were on this subject. He was but twenty-two when he made
his first discovery, the liquefaction of chlorine. The details of this
experiment have been told by Tyndall. It is well known that when chlorine
gas and cold water are united, crystals are formed which contain to every
molecule of chlorine ten molecules of water. Faraday put some of these
into a closed tube and heated them until two separate liquids appeared;
one was water, the other floated on the surface of the water, and a
certain professor of Paris declared that it could be nothing but oil
carelessly left in the vessel. Faraday having opened the tube, found that
this substance began to boil, and then changed with an explosion into a
green gas. It was chlorine. Faraday, who was quick-tempered, immediately
took his revenge on the professor, to whom he wrote: “You will be pleased
to know, sir, that the oil left by carelessness in my apparatus was
nothing less than liquefied chlorine.”
This first success decided the career of the young chemist. He announced
that all gases could be reduced to this state if subjected to a
sufficient pressure, and he undertook a series of experiments, of which
the success was doubtful, but the danger certain. He operated in this
way: He took a thick glass tube in the form of an inverted U; one branch
was left empty, in the other the materials for producing the gas to be
studied were placed and the whole closed. Obliged to gather in the empty
branch, the gas continually increased in pressure, and there were two
possible results to the experiment; either the gas would not change its
state, and the pressure would increase until the vessel broke, or when a
certain limit of pressure was reached, then the liquid would appear and
would continue to accumulate as long as the gas was disengaged. A dozen
gases were reduced in this way; among them were the following, which we
shall need: Ammonia, sulphurous acid, carbonic acid, and protoxide of
nitrogen, which at a temperature of ten degrees required a pressure equal
to sixty atmospheres.
This pressure leaves no doubt about the danger which one runs in carrying
on such researches. If we remember that steam boilers generally support
a pressure of no more than ten atmospheres, if we recall the number and
the horror of their explosions we can hardly understand how a simple
glass tube could resist a pressure five or six times as great. When a gas
reaches the point of liquefaction, then the pressure ceases to increase,
but if it does not change from that condition the pressure increases
until an explosion necessarily occurs, and the debris of the vessel is
scattered as powder scatters the fragments of a shell. In the course of
Faraday’s researches he had thirty explosions. They did not stop him, but
it is easy to see that they did not encourage others.
Happily there is a less dangerous method of reaching the same result,
it is to freeze the gas. In the same way that the vapor of water is
condensed when the temperature is lowered, so gases, which are really
vapors, will yield to sufficient cold. In 1824, Bussy succeeded in
condensing sulphurous acid gas. The gas was introduced into a balloon,
which was plunged into a freezing mixture of ice and salt. The gas was
liquefied and could be preserved indefinitely, if the balloon were
enclosed in an enamel vessel. In heating, it gave off vapors which, by
their pressure kept the remainder of the fluid, providing the glass was
strong enough. Thus, in two ways, by cold and by pressure, and still
better, by both combined, it is possible to liquefy a large number of
gases.
When water is heated, it remains immovable up to 100 degrees Centigrade,
but then it is changed into vapor, or boils. This boiling is
characterized by a peculiar feature, the temperature remains fixed at
100 degrees. It must be concluded, therefore, that the heat produced by
the furnace and absorbed by the liquid is simply used in transforming
the water into vapor. This fact was first discovered by the English
philosopher, Black, who, not being able to explain the phenomenon, was
content to demonstrate it and to speak of the heat as _latent_. He saw
that it took five and a half times as long to change water into vapor
as to heat it from zero to 100 degrees, and that consequently it must
require five and a half times as much heat to work the change. Such is
the law of boiling in the air, but let us see what it is in a vacuum.
It is clear that the pressure of the atmosphere on water is a hindrance
to its expansion into vapor, and that this hindrance increases or
diminishes with the pressure. In a vacuum, of course, the liquid is
free from the pressure, so that boiling ought to take place at a lower
temperature.
And experiment teaches that this is the case; water boils at a
temperature of 82° or 65°, as the pressure is reduced to one half or a
quarter of an atmosphere, it boils at zero, and even below, in a vacuum.
And we reach this remarkable result, that the boiling and freezing points
unite, and that ice is formed while vapor is set free. But, although the
boiling is advanced, although it takes place at zero instead of at 100
degrees, although the vapor is cold instead of hot, and the change takes
place in a vacuum instead of in the air, it is a general law that a large
quantity of heat is used, becomes latent, and enters into the formation
of vapor.
Supposing that we fill a bronze vessel of very thick sides with water,
close it with a lid and fit into it a valve loaded with lead. Place this
in a furnace whose temperature has been raised to, say, 230 degrees. The
water will reach this temperature, and vapor will accumulate until it
reaches a pressure equal to more than twenty-seven atmospheres.
Let us now open the valve, the vapor will escape, and as it carries with
it the heat necessary for its expansion, the temperature of the water
will gradually fall until it reaches 100 degrees, after which the boiling
will continue slowly and regularly; thus the water has been cooled and is
kept below the temperature of its surrounding wall because it must absorb
the extra heat which is required to change it to vapor. This apparatus is
called Papin’s digester.
There is a similar experiment, but performed in a vacuum at the ordinary
temperature. Put some water into a closed decanter which is connected by
a tube with an air pump. As soon as a vacuum is produced the water begins
to boil and to freeze, for the vapor can only be formed by borrowing
heat, and there is nothing to take it from but the water itself, which
soon reaches zero and is frozen. This apparatus makes a very simple ice
house, as useful as convenient, and it proves, first, that boiling takes
place at the lowest temperatures providing the pressure is sufficiently
diminished; secondly, that it is always accompanied by a loss of heat;
and thirdly, that it lowers the temperature of the liquid below that of
the surrounding envelope, and the more as the vacuum is more complete.
Just as opening the valve lets the vapor accumulated above the water in
Papin’s digester escape, and causes a fall in the temperature, so, by
opening the reservoirs in which one has confined a liquefied gas, one
sees it fall back to the boiling point. For example, take the liquid
obtained from the compression of sulphurous acid gas. As soon as the
reservoir containing it is opened the liquid begins to boil, and a
vapor is formed, it is the gas which re-forms. It absorbs the latent
heat necessary, taking it from exterior objects by radiation from the
liquid itself, from the vessel which holds it, and from the materials
into which it has been placed. It cools these until the point at which
sulphurous acid gas boils is reached, twelve degrees below zero; then the
liquid remains balanced between the radiation which tends to heat it and
vaporization, which cools it. The final result is that the temperature is
lowered and remains fixed at twelve degrees below zero. This is not all:
just as the boiling point of water is lowered below zero in a vacuum,
in the same way that of sulphurous acid gas falls below twelve degrees.
Bussy brought it down to sixty-eight, where it remained; not only water,
but mercury may be frozen by this means.
Finally, the boiling of liquefied gases will freeze all neighboring
substances, and the greatest cold which one could obtain is produced
by their boiling in a vacuum. This property of sulphurous acid was
discovered in a still greater degree in protoxide of nitrogen, which was
changed into a liquid at a temperature of 0 degrees, and under a pressure
of thirty atmospheres. If allowed to boil in a vacuum, a temperature of
one hundred and ten degrees below zero was obtained. When science has
sown trade reaps the harvest; since by allowing liquefied gases to boil,
a temperature of one hundred and ten degrees below zero can be obtained,
and since the vapors which they give off carry away an enormous amount
of heat from the surrounding bodies, it is possible by means of this
cold produced to freeze water, make cold drinks, solidify mercury, cool
cellars, prevent food from decay, and to do many other things of similar
nature. A new art became possible, that of making cold. To-day it is
at the height of success. It is founded on this general principle: to
liquefy the gas by means of pressure, taking care that it does not become
heated, to introduce it into a freezer, where it is allowed to boil,
and from which it absorbs the heat, to carry off the gas and introduce
it again into the vessel, where it will by pressure be liquefied. The
action is constant, the same gas acts indefinitely, and there is no other
expense than that which is caused by running the pumps. In spite of these
fine results and the extraordinary efforts put forth, the end was not
attained. To be sure, some gases had yielded, but still there was a large
number which resisted every effort. Was it necessary to give up the idea
that the law of liquefaction of gases was general, or was it true that
the exceptions were only the results of insufficient means? Faraday had
never varied in his belief. One easily returns to the affections of his
youth, and he believed that the time had come for making fresh efforts
to prove his theory. After a rest of twenty-two years he determined to
again take up the liquefaction of the rebellious gases. Means were not
wanting. Thilorier had taught him how to solidify easily large masses
of carbonic acid, and by mixing this solid with ether make a powerful
freezing mixture; protoxide of nitrogen could be prepared with the same
ease and abundance, and would boil regularly in a vacuum at a temperature
of one hundred and twenty degrees below zero. Thus he was able to secure
a degree of cold before unknown. For compression, he had a pump formed
of two parts; one took the gas at its generation, and accumulated it in
a reservoir under a pressure of fifteen atmospheres; the second part
then received it; here it was subjected to a much greater pressure in a
strong glass vessel which was plunged into carbonic acid or protoxide of
nitrogen. Cold and pressure were thus combined. At that time nothing more
could be done; fortunately this was enough to subdue most gases. Faraday
had the satisfaction of liquefying nearly all gases, and of extending the
law which he had announced, but still six, only six, refused to give up;
among them were marsh gas, oxygen, nitrogen, and hydrogen. Science is a
battle which must be continually renewed; the more the gases resisted,
the greater the efforts made to conquer them. At first, new and energetic
means of pressure were invented. Aimé, a professor in Algiers, secured a
pressure of four hundred atmospheres, without result. M. Cailletet used a
hydraulic press which exerted a force equal to seven hundred atmospheres,
and afterward increased this to one thousand atmospheres, but still the
gas resisted. At last it was found that pressure alone, however enormous
it might be, could not liquefy the gases.
An English philosopher, called Andrews, put a new face on matters.
He took carbonic acid gas at a temperature of about thirteen degrees
and compressed it. The gas began to diminish in volume, and under a
pressure of fifty atmospheres was suddenly liquefied, taking quickly
a very great density, and falling to the bottom of the vessel, where
it remained separated from its vapor by a surface as plainly marked
as that which marks water and air. Andrews afterward tried the same
experiment at a higher temperature, about twenty-one degrees. The same
results were produced with but one difference: the liquefaction was less
sudden. At a temperature of thirty-two degrees, instead of a separate
and distinct liquid, undulating striæ appeared as the only signs of a
change in condition which was not completed. Finally, at a temperature
of above thirty-two degrees there was neither striæ nor liquefaction,
but still it seemed as if a trace was preserved, for under certain
pressure the density increased more quickly, and the volume diminished
more rapidly. Thirty-two degrees is then the limit, a point between the
degrees which permit and which prevent liquefaction. It is the _critical
point_ which marks the separation between two very different conditions
of a substance; below, we have a liquid; above, there is no change in
appearance, but there enters a new condition, whose characteristics I
will describe.
Generally a liquid is more dense than its vapor; for this reason it
falls to the bottom, and the two are separated by a level surface.
But supposing that we heat the vessel which contains them. The liquid
expands little by little, until it equals, or even surpasses, the
expansion of the gas, so that an equal volume weighs less and less. On
the other hand, a continually increasing quantity of vapor is formed,
accumulates at the top of the vessel, and becomes constantly heavier.
Now, if the density of the vapor increases, or if that of the liquid
diminishes under the right temperature, the two densities become equal.
Then there is no longer a reason for the liquid falling, the vapor
rising, or for a surface of separation. The two are mingled. Neither are
they any longer distinguished by their different degrees of heat. When
this critical point is reached, it is impossible to tell whether it is
liquid or gas, since in either state it has the same density, the same
heat, the same appearance, the same properties. This is a new state, a
gaseous liquid state. The discovery of these properties showed why all
the attempts to liquefy air had been useless. At an ordinary temperature
the gas is in a gaseous liquid condition. Liquefaction can take place
only when the liquid is separated from the vapor by its own greater
density. The next step was therefore to lower the temperature below that
of the critical point. This was understood and carried out about the
same time by MM. Cailletet and Raoul Pictet. On the 2nd of December,
1877, M. Cailletet subjected oxygen in a glass tube to a pressure of
three hundred atmospheres, and reduced its temperature to twenty-nine
degrees below zero. The gas did not change in appearance, and was in all
probability in the gaseous liquid condition. Nothing but more cold was
wanting to liquefy it. The valve was turned, the gas escaped, and the
temperature fell two hundred degrees, and the characteristic whitish
mist was seen. Oxygen had been liquefied, perhaps solidified. The same
result was reached with nitrogen, but nothing was done with hydrogen.
While M. Cailletet performed this decisive experiment at Paris, M. Raoul
Pictet achieved the same at Geneva. He had at his command all necessary
materials, so that he subjected the oxygen to a pressure of three hundred
and twenty atmospheres, and to a temperature of one hundred and forty
degrees below zero. In this condition the gas was probably below the
critical point, and when the reservoir was opened suddenly it began
to boil and was thrown in every direction. M. Pictet believed that he
liquefied, and even more, had solidified hydrogen, but he was doubtless
mistaken. These results, however, were not satisfactory. M. Cailletet was
preparing a new experiment when the Academy received the two telegrams
given at the beginning of this article.
Wroblewski and his colleague, Olszewski, had boiled ethylene, a gas
similar to that used for heating purposes, in a vacuum. The temperature
fell to one hundred and fifty degrees below zero. It was the greatest
degree of cold yet obtained, and was sufficient. The success was
complete. The oxygen, previously compressed in a glass tube, became
a fixed liquid. It was like the others, in the form of a colorless
and transparent liquid, like water, but of a little less density. Its
critical point was at one hundred and thirteen degrees below zero,
forming itself below, never above, this temperature, and boiling rapidly
at a temperature of one hundred and eighty-six degrees below zero. A few
days after this the two Polish professors succeeded, in the same way, in
liquefying nitrogen.
But if the question was settled for air was it also for nitrogen? M.
Pictet, in his experiment, had used a weight of three hundred and twenty
atmospheres, and cold of one hundred and forty degrees below zero. When
he opened the reservoir a jet of gas, mingled with mist of steel gray
color, burst forth. At the beginning of the experiment, solid fragments
accompanied the jet; these fell to the floor with a sound like that
of grains of lead. Naturally, M. Pictet thought that he had not only
liquefied, but even solidified hydrogen, but unfortunately the experiment
was not wholly satisfactory. For perfect success still more acute cold
was needed, and here was oxygen and nitrogen to get it from. Nitrogen,
the most refractory, was taken, and a degree of cold undreamed of before,
attained; in the open air it reached one hundred and ninety-four degrees
below zero, and in a vacuum two hundred and thirteen degrees below. These
temperatures were so low that it was necessary to invent new methods
for measuring them. A mercury thermometer was useless, because it froze
at forty degrees, and alcohol because it became a solid at one hundred
and thirty degrees. No liquid is able to resist such temperatures, so
electric, or hydrogen thermometers, were employed.
Wroblewski and Olszewski have but lately achieved success. Having
compressed the hydrogen in the above named manner, they froze it by
means of nitrogen boiling in a vacuum. Still it did not liquefy. It was
yet in a gaseous liquid state, but when the tube was opened then there
appeared a transparent and colorless liquid. At last the question of
the liquefaction of gases, which has been discussed so long, has been
settled. When we think of the simplicity of these final experiments, it
seems strange that the problem was so difficult to solve. The trouble lay
in the fact that at the start there was everything to find out; there
was the critical point and the means of freezing to discover. It was
necessary to proceed by steps, using each gas for the reduction of the
one more stubborn than itself. Really, as Biot says, nothing is so easy
as what was discovered yesterday, nothing so difficult as what must be
discovered to-morrow. It might be asked whether the result is worth the
trouble necessary to collect these liquids. The answer must be left to
the future. The chemist will take up this new law of gases, and art will
adapt it to its purposes. For the present, all that it amounts to is
that the natural philosopher has proven that all kinds of materials may
exist in three conditions, and obey the same common laws.—_Abridged and
Translated from “Révue des Deux Mondes” for “The Chautauquan.”_