By JULES JAMIN, of the Institute of France.
The earlier experiments of MM. Cailletet and Raoul Pictet in the
liquefaction of gases, and the apparatus by means of which they performed
the process, were described in the _Popular Science Monthly_, March and
May, 1878. The experiments have since been continued and improved upon by
MM. Cailletet and Pictet, and others, with more complete results than had
been attained at the time the first reports were published, and with the
elucidation of some novel properties of gases, and the disclosure of
relations, previously not well understood, between the gaseous and the
liquid condition. The experiments of Faraday, in the compression of gases
by the combined agency of pressure and extreme cold, left six gases which
still refused to enter into the liquid state. They were the two elements
of the atmosphere (oxygen and nitrogen), nitric oxide, marsh-gas, carbonic
oxide, and hydrogen. Many new experiments were tried before the principle
that governs the change from the gaseous to the liquid, or from the liquid
to the gaseous form was discovered. Aime sank manometers filled with air
into the sea till the pressure upon them was equal to that of four hundred
atmospheres; Berthelot, by the expansion of mercury in a thermometer tube,
succeeded in exerting a pressure of seven hundred and eighty atmospheres
upon oxygen. Both series of experiments were without result. M. Cailletet,
having fruitlessly subjected air and hydrogen to a pressure of one
thousand atmospheres, came to the conclusion that it was impossible to
liquefy those gases at the ordinary temperature by pressure alone.
Previously it had been thought that the obstacle to condensing gases by
pressure alone lay in the difficulty of obtaining sufficient pressure, or
in that of finding a vessel suitable for manipulation that would be
capable of resisting it. M. Cailletet's thought led to the discovery of
another fundamental property of gases.
The experiments of Despretz and Regnault had shown that the scope of
Mariotte's law (that the volume of gases increases or diminishes inversely
as the pressure upon them) was limited, and that its limits were different
with different substances. Andrews confirmed the observations of these
investigators, and extended them. Compressing carbonic acid at 13° C. (55°
Fahr.), he found that the rate of diminution in volume increased more
rapidly than Mariotte's law demanded, and at a progressive rate. At fifty
atmospheres the gas all at once assumed the liquid form, became very
dense, and fell to the bottom of the vessel, where it remained separated
from its vapor by a clearly defined surface, like that which distinguishes
water in the air. Experimenting in the same way with the gas at a higher
temperature (21° C. or 70° Fahr.), he found that the same result was
produced, but more slowly; and it seemed to be heralded in advance by a
more rapid diminution in volume previous to the beginning of the change,
which continued after the process had been accomplished; as if an
anticipatory preparation for the liquid state were going on previous to
the completion of the change. Performing the experiment again at 32° C.
(90° Fahr.), the anticipatory preparation and the after-continuation of
the contraction were more marked, and, instead of a separate and distinct
liquid, wavy and mobile striæ were perceived on the sides of the vessel as
the only signs of a change of state which had not yet been effected. At
temperatures above 32° C. (90° Fahr.), there were neither striæ nor
liquefaction, but there seemed to be a suggestion of them, for, under a
particular degree of pressure, the density of the gas was augmented, and
its volume diminished at an increasing rate. The temperature of 32° C.
(90° Fahr.) is, then, a limit, marking a division between the temperatures
which permit and those which prevent liquefaction; it is the critical
point, at which is defined the separation, for carbonic acid, between two
very distinct states of matter. Below this point, the particular matter
may assume the aspect of a liquid; above it, the gas cannot change its
appearance, but enters into the opposite constitution from that of a
liquid.
Generally, a liquid has considerably greater density than its vapor. But,
if a vessel containing both is heated, the liquid experiences a dilatation
which is gradually augmented till it equals and even exceeds that of the
gas; whence, of course, an equal volume of the liquid will weigh less and
less. On the other hand, a constantly larger quantity of vapor is formed,
which accumulates above the liquid and becomes heavier and heavier. Now if
the density of the vapor increases, and that of the liquid diminishes,
they will reach a point, under a suitable temperature, when they will be
the same. There will then be no reason for the liquid to sink or the vapor
to rise, or for the existence of any line of separation between them, and
they will be mixed and confounded. They will no longer be distinguishable
by their heat of constitution. It is true that, in passing into the state
of a vapor, a liquid absorbs a great deal of latent heat, but that is
employed in scattering the molecules and keeping them at a distance; and
there will be none of it if the distance does not increase. We are then,
at this stage of our experiments, in the presence of a critical point, at
which we do not know whether the matter is liquid or gaseous; for, in
either condition, it has the same density, the same heat of constitution,
and the same properties. It is a new state, the gaso-liquid state. An
experiment of Cagniard-Latour re-enforced this explanation of the
phenomena. Heating ether in closed vessels to high temperatures, he
brought it to a point where the liquid could be made wholly to disappear,
or to be suddenly reformed on the slightest elevation or the slightest
depression of temperature accordingly as it was raised just above or
cooled to just below the critical point. The discovery of these properties
suggested an explanation of the failure of previous attempts to liquefy
air. Air at ordinary low temperatures is in the gaso-liquid condition, and
its liquefaction is not possible except when a difference exists between
the density of the vapor and that of the liquid greater than it is
possible to produce under any conditions that can exist then. It was
necessary to reduce the temperature to below the critical point; and it
was by adopting this course that MM. Cailletet and Raoul Pictet achieved
their success. The rapid escape of the compressed gas itself from a
condition of great condensation at an extremely low temperature was
employed as the agent for producing a greater degree of cold than it had
been possible before to obtain. M. Cailletet used oxygen escaping at -29°
C. from a pressure of three hundred atmospheres; M. Raoul Pictet, the same
gas escaping at -140° from a pressure of three hundred and twenty
atmospheres; and both obtained oxygen and nitrogen, and M. Pictet
hydrogen, in what they thought was a liquid, and possibly even in a solid
form.
Still, it could not be asserted that hydrogen and the elements of the air
had been completely liquefied. These gases had not yet been seen collected
in the static condition at the bottom of a tube and separated from their
vapors by the clearly defined concave surface which is called a
_meniscus._ The experiments had, however, proved that liquefaction is
possible at a temperature of below -120° C. (-184° Fahr.). To make the
process practicable, it was only necessary to find sufficiently powerful
refrigerants; and these were looked for among gases that had proved more
refractory than carbonic acid and protoxide of nitrogen. M. Cailletet
selected ethylene, a hydrocarbon of the same composition as illuminating
gas, which, when liquefied by the aid of carbonic acid and a pressure of
thirty-six atmospheres, boils at -103° C. (-153° Fahr.). M. Wroblewski, of
Cracow, who had witnessed some of M. Cailletet's experiments, and obtained
his apparatus, and M. Olzewski, in association with him, also experimented
with ethylene, and had the pleasure of recording their first complete
success early in April, 1883. Causing liquid ethylene to boil in an
air-pump vacuum at -103° C., they were able to produce a temperature of
-150° C. (-238° Fahr.), the lowest that had ever been observed. Oxygen,
having been previously compressed in a glass tube, became a permanent
liquid, with a clearly defined meniscus. It presented itself, like the
other liquefied gases, under the form of a transparent and colorless
substance, resembling water, but a little less dense. Its critical point
was marked at -113° C. (-171° Fahr.), below which the liquid could be
formed, but never above it; while it boiled rapidly at -186° C. (-303°
Fahr.). A few days afterward, the Polish professors obtained the
liquefaction of nitrogen, a more refractory gas, under a pressure of
thirty-six atmospheres, at -146° C. (-231° Fahr.). Long, difficult, and
expensive operations were required to produce this result, for the extreme
degree of cold it demanded had to be produced by boiling large quantities
of ethylene in a vacuum. M. Cailletet devised a cheaper process, by
employing another hydrocarbon that rises from the mud of marshes, and is
called _formene_. It is less easily liquefied than ethylene, but for that
very reason can be boiled in the air at a lower temperature, or at -160°C.
(-256° Fahr.); and at this temperature nitrogen and oxygen can be
liquefied in a bath of formene as readily as sulphurous acid in the common
freezing mixture.
MM. Cailletet, Wroblewski, and Olzewski have continued their experiments
in liquefaction, and acquired increased facility in the handling of liquid
ethylene, formene, atmospheric air, oxygen, and nitrogen. M. Olzewski was
able to report to the French Academy of Sciences, on the 21st of July,
1884, that by placing liquefied nitrogen in a vacuum he had succeeded in
producing a temperature of -213°C. (-351° Fahr.), under which hydrogen was
liquefied. Contrary to the suppositions founded on the metallic behavior
of this element, that it would present the appearance of a molten metal,
like mercury, the liquid had the mobile behavior and the transparency of
the hydrocarbons.
* * * * *