different, it might be expected that on the liquefaction of
atmospheric air the two elements would appear as two separate liquids.
Such, however, is not the case; they come down simultaneously as one
homogeneous liquid. Prepared on a large scale, liquid air may contain
as much as 50% of oxygen when collected in open vacuum-vessels, but
since nitrogen is the more volatile it boils off first, and as the
liquid gradually becomes richer in oxygen the temperature at which it
boils rises from about -192° C. to about -182° C. At the former
temperature it has a density of about 0.910. It is a non-conductor of
electricity. Properly protected from external heat, and subjected to
high exhaustion, liquid air becomes a stiff transparent jelly-like
mass, a magma of solid nitrogen containing liquid oxygen, which may
indeed be extracted from it by means of a magnet, or by rapid rotation
of the vacuum vessel in imitation of a centrifugal machine. The
temperature of this solid under a vacuum of about 14 mm. is -216°. At
the still lower temperatures attainable by the aid of liquid hydrogen
it becomes a white solid, having, like solid oxygen, a faint blue
tint. The refractive index of liquid air is 1.2068.
_Fluorine_, prepared in the free state by Moissan's method of
electrolysing a solution of potassium fluoride in anhydrous
hydrofluoric acid, was liquefied in the laboratories of the Royal
Institution, London, in 1897. Exposed to the temperature of
quietly-boiling liquid oxygen, the gas did not change its state,
though it lost much of its chemical activity, and ceased to attack
glass. But a very small vacuum formed over the oxygen was sufficient
to determine liquefaction, a result which was also obtained by cooling
the gas to the temperature of freshly-made liquid air boiling at
atmospheric pressure. Hence the boiling-point is fixed at about -187°
C. The liquid is of a clear yellow colour, possessing great mobility.
Its density is 1.14, and its capillarity rather less than that of
liquid oxygen. The liquid, when examined in a thickness of 1 cm., does
not show any absorption bands, and it is not attracted by a magnet.
Cooled in liquid hydrogen it is frozen to a white solid, melting at
about 40° abs.
_Hydrogen._--Liquid hydrogen is the lightest liquid known to the
chemist, having a density slightly less than 0.07 as compared with
water, and being six times lighter than liquid marsh-gas, which is
next in order of lightness. One litre weighs only 70 grammes, and 1
gramme occupies a volume of 14-15 cc. In spite of its extreme
lightness, however, it is easily seen, has a well-defined meniscus and
drops well. At its boiling-point the liquid is only 55 times denser
than the vapour it is giving off, whereas liquid oxygen in similar
condition is 258 times denser than its vapour, and nitrogen 177 times.
Its atomic volume is about 14.3, that of liquid oxygen being 13.7,
and that of liquid nitrogen 16.6, at their respective boiling-points.
Its latent heat of vaporization about the boiling-point is about 121
gramme-calories, and the latent heat of fluidity cannot exceed 16
units, but may be less. Hydrogen appears to have the same specific
heat in the liquid as in the gaseous state, about 3.4. Its surface
tension is exceedingly low, about one-fifth that of liquid air at its
boiling-point, or one-thirty-fifth that of water at ordinary
temperatures, and this is the reason that bubbles formed in the liquid
are so small as to give it an opalescent appearance during ebullition.
The liquid is without colour, and gives no absorption spectrum.
Electric sparks taken in the liquid between platinum poles give a
spectrum showing the hydrogen lines C and F bright on a background of
continuous spectrum. Its refractive index at the boiling-point has
theoretically the value 1.11. It was measured by determining the
relative difference of focus for a parallel beam of light sent through
a spherical vacuum vessel filled successively with water, liquid
oxygen and liquid hydrogen; the result obtained was 1.12. Liquid
hydrogen is a non-conductor of electricity. The precise determination
of its boiling-point is a matter of some difficulty. The first results
obtained from the use of a platinum resistance thermometer gave -238°
C., while a similar thermometer made with an alloy of rhodium-platinum
indicated a value 8 degrees lower. Later, a gold thermometer indicated
about -249° C., while with an iron one the result was only -210° C. It
was thus evident that electrical resistance thermometers are not to be
trusted at these low temperatures, since the laws correlating
resistance and temperature are not known for temperatures at and below
the boiling-point of hydrogen, though they are certainly not the same
as those which hold good higher up the thermometric scale. The same
remarks apply to the use of thermo-electric junctions at such
exceptional temperatures. Recourse was therefore had to a
constant-volume hydrogen thermometer, working under reduced pressure,
experiments having shown that such a thermometer, filled with either a
simple or a compound gas (e.g. oxygen or carbonic acid) at an initial
pressure somewhat less than one atmosphere, may be relied upon to
determine temperatures down to the respective boiling-points of the
gases with which they are filled. The result obtained was -252° C.
Subsequently various other determinations were carried out in
thermometers filled with hydrogen derived from different sources, and
also with helium, the average value given by the experiments being
-252.5° C. (See "The Boiling Point of Liquid Hydrogen determined by
Hydrogen and Helium Gas Thermometers," _Proc. Roy. Soc._, 7th February
1901.) The critical temperature is about 30° absolute (-243° C.), and
the critical pressure about 15 atmospheres. Hydrogen has not only the
lowest critical temperature of all the old permanent gases, but it has
the lowest critical pressure. Given a sufficiently low temperature,
therefore, it is the easiest gas to liquefy so far as pressure is
concerned. Solid hydrogen has a temperature about 4° less. By
exhaustion under reduced pressure a still lower depth of cold may be
attained, and a steady temperature reached less than 16° above the
zero of absolute temperature. By the use of high exhaustion, and the
most stringent precautions to prevent the influx of heat, a
temperature of 13° absolute (-260° C.) may be reached. This is the
lowest steady temperature which can be maintained by the evaporation
of solid hydrogen. At this temperature the solid has a density of
about 0.077. Solid hydrogen presents no metallic characteristics, such
as were predicted for it by Faraday, Dumas, Graham and other chemists
and neither it nor the liquid is magnetic.
_The Approach to the Absolute Zero._--The achievement of Kamerlingh
Onnes has brought about the realization of a temperature removed only 3°
from the absolute zero, and the question naturally suggests itself
whether there is any probability of a still closer approach to that
point. The answer is that if, as is not impossible, there exists a gas,
as yet unisolated, which has an atomic weight one-half that of helium,
that gas, liquefied in turn by the aid of liquid helium, would render
that approach possible, though the experimental difficulties of the
operation would be enormous and perhaps prohibitive. The results of
experiments bearing on this question and of theory based on them are
shown in table II. The third column shows the critical temperature of
the gas which can be liquefied by continuous expansion through a
regenerative cooling apparatus, the operation being started from the
initial temperature shown in the second column, while the fourth column
gives the temperature of the resulting liquid. It will be seen that by
the use of liquid or solid hydrogen as a cooling agent, it should be
possible to liquefy a body having a critical temperature of about 6° to
8° on the absolute scale, and a boiling point of about 4° or 5°, while
with the aid of liquid helium at an initial temperature of 5° we could
liquefy a body having a critical temperature of 2° and a boiling point
of 1°.
TABLE II.
+------------------+-------------+-------------+---------------+
| | Initial | Critical |Boiling Points.|
| Substance. | Temperature.|Temperature. | Abs. Degrees. |
| |Abs. Degrees.|Abs. Degrees.| |
+------------------+-------------+-------------+---------------+
| (Low red heat) | 760 | 304 | 195 (CO2) |
| (52° C.) | 325 | 130 | 86 (Air) |
| Liquid air under | | | |
| exhaustion | 75 | 30 | 20 (H) |
| Liquid hydrogen | 20 | 8 | 5 (He) |
| Solid hydrogen | 15 | 6 | 4 |
| Liquid helium | 5 | 2 | 1 |
+------------------+-------------+-------------+---------------+
It is to be remarked, however, that even so the physicist would not have
attained the absolute zero, and he can scarcely hope ever to do so. It
is true he would only be a very short distance from it, but it must be
remembered that in a thermodynamic sense one degree low down the scale,
say at 10° absolute, is equivalent to 30° at the ordinary temperature,
and as the experimenter gets to lower and lower temperatures, the
difficulties of further advance increase, not in arithmetical but in
geometrical progression. Thus the step between the liquefaction of air
and that of hydrogen is, thermodynamically and practically, greater than
that between the liquefaction of chlorine and that of air, but the
number of degrees of temperature that separates the boiling-points of
the first pair of substances is less than half what it is in the case of
the second pair. But the ratio of the absolute boiling-points in the
first pair of substances is as 1 to 4, whereas in the second pair it is
only 1 to 3, and it is this value that expresses the difficulty of the
transition.
But though Ultima Thule may continue to mock the physicist's efforts, he
will long find ample scope for his energies in the investigation of the
properties of matter at the temperatures placed at his command by liquid
air and liquid and solid hydrogen. Indeed, great as is the sentimental
interest attached to the liquefaction of these refractory gases, the
importance of the achievement lies rather in the fact that it opens out
new fields of research and enormously widens the horizon of physical
science, enabling the natural philosopher to study the properties and
behaviour of matter under entirely novel conditions. We propose to
indicate briefly the general directions in which such inquiries have so
far been carried on, but before doing so will call attention to the
power of absorbing gases possessed by cooled charcoal, which has on that
account proved itself a most valuable agent in low temperature research.
TABLE III.--_Gas Absorption by Charcoal._
+---------------------------+-------------+-------------+
| | Volume | Volume |
| | absorbed at | absorbed at |
| | 0° Cent. | -185° Cent. |
+---------------------------+-------------+-------------+
| Helium | 2 cc. | 15 cc. |
| Hydrogen | 4 | 135 |
| Electrolytic gas | 12 | 150 |
| Argon | 12 | 175 |
| Nitrogen | 15 | 155 |
| Oxygen | 18 | 230 |
| Carbonic oxide | 21 | 190 |
| Carbonic oxide and oxygen | 30 | 195 |
+---------------------------+-------------+-------------+
_Gas Absorption by Charcoal._--Felix Fontana was apparently the first to
discover that hot charcoal has the power of absorbing gases, and his
observations were confirmed about 1770 by Joseph Priestley, to whom he
had communicated them. A generation later Theodore de Saussure made a
number of experiments on the subject, and noted that at ordinary
temperatures the absorption is accompanied with considerable evolution
of heat. Among subsequent investigators were Thomas Graham and
Stenhouse, Faure and Silberman, and Hunter, the last-named showing that
charcoal made from coco-nut exhibits greater absorptive powers than
other varieties. In 1874 Tait and Dewar for the first time employed
charcoal for the production of high vacua, by using it, heated to a red
heat, to absorb the mercury vapour in a tube exhausted by a mercury
pump; and thirty years afterwards it occurred to the latter investigator
to try how its absorbing powers are affected by cooling it, with the
result that he found them to be greatly enhanced. Some of his earlier
observations are given in table III., but it must be pointed out that
much larger absorptions were obtained subsequently when it was found
that the quality of the charcoal was greatly influenced by the mode in
which it was prepared, the absorptive power being increased by
carbonizing the coco-nut shell slowly at a gradually increasing
temperature. The results in the table were all obtained with the same
specimen of charcoal, and the volumes of the gases absorbed, both at
ordinary and at low temperatures, were measured under standard
conditions--at 0° C., and 760 mm. pressure. It appears that at the lower
temperature there is a remarkable increase of absorption for every gas,
but that the increase is in general smaller as the boiling-points of the
various gases are lower. Helium is conspicuous for the fact that it is
absorbed to a comparatively slight extent at both the higher and the
lower temperature, but in this connexion it must be remembered that,
being the most volatile gas known, it is being treated at a temperature
which is relatively much higher than the other gases. At -185° (= 88°
abs.), while hydrogen is at about 4½ times its boiling-point (20° abs.),
helium is at about 20 times its boiling-point (4.5° abs.), and it might,
therefore, be expected that if it were taken at a temperature
corresponding to that of the hydrogen, i.e. at 4 or 5 times its
boiling-point, or say 20° abs., it would undergo much greater
absorption. This expectation is borne out by the results shown in table
IV., and it may be inferred that charcoal cooled in liquid helium would
absorb helium as freely as charcoal cooled in liquid hydrogen absorbs
hydrogen. It is found that a given specimen of charcoal cooled in liquid
oxygen, nitrogen and hydrogen absorbs about equal volumes of those three
gases (about 260 cc. per gramme); and, as the relation between volume
and temperature is nearly lineal at the lowest portions of either the
hydrogen or the helium absorption, it is a legitimate inference that at
a temperature of 5° to 6° abs. helium would be as freely absorbed by
charcoal as hydrogen is at its boiling-point and that the boiling-point
of helium lies at about 5° abs.
TABLE IV.--_Gas Absorption by Charcoal at Low Temperatures._
+--------------------------------------------+--------+----------+
| | Helium.| Hydrogen.|
| Temperature. |Vols. of| Vols. of |
| |Carbon. | Carbon. |
+--------------------------------------------+--------+----------+
| -185° C. (boiling-point of liquid air) | 2½ | 137 |
| -210° C. (liquid air under exhaustion) | 5 | 180 |
| -252° C. (boiling-point of liquid hydrogen)| 160 | 258 |
| -258° C. (solid hydrogen) | 195 | .. |
+--------------------------------------------+--------+----------+
The rapidity with which air is absorbed by charcoal at -185° C. and
under small pressures is illustrated by table V., which shows the
reductions of pressure effected in a tube of 2000 cc. capacity by means
of 20 grammes of charcoal cooled in liquid air.
TABLE V.--_Velocity of Absorption._
+-----------+--------+-----------+---------+
| Time of |Pressure| Time of |Pressure |
|Exhaustion.| in mm. |Exhaustion.| in mm. |
+-----------+--------+-----------+---------+
| 0 sec. | 2.190 | 60 sec. | 0.347 |
| 10 " | 1.271 | 2 min. | 0.153 |
| 20 " | 0.869 | 5 " | 0.0274 |
| 30 " | 0.632 | 10 " | 0.00205 |
| 40 " | 0.543 | 19 " | 0.00025 |
| 50 " | 0.435 | .. | .. |
+-----------+--------+-----------+---------+
TABLE VI.
+----------+----------+-----------+
| Volume | Occlusion| Occlusion |
| of Gas | Hydrogen | Nitrogen |
| absorbed.| Pressure.| Pressure. |
+----------+----------+-----------+
| cc. | mm. | mm. |
| 0 | 0.00003 | 0.00005 |
| 5 | 0.0228 | .. |
| 10 | 0.0455 | .. |
| 15 | 0.0645 | .. |
| 20 | 0.0861 | .. |
| 25 | 0.1105 | .. |
| 30 | 0.1339 | 0.00031 |
| 35 | 0.1623 | .. |
| 40 | 0.1870 | .. |
| 130 | .. | 0.00110 |
| 500 | .. | 0.00314 |
| 1000 | .. | 0.01756 |
| 1500 | .. | 0.02920 |
| 2500 | .. | 0.06172 |
+----------+----------+-----------+
_Charcoal Occlusion Pressures._--For measuring the gas concentration,
pressure and temperature, use may be made of an apparatus of the type
shown in fig. 5. A mass of charcoal, E, immersed in liquid air, is
employed for the preliminary exhaustion of the McLeod gauge G and of
the charcoal C, which is to be used in the actual experiments, and is
then sealed off at S. The bulb C is then placed in a large spherical
vacuum vessel containing liquid oxygen which can be made to boil at
any definite temperature under diminished pressure which is measured
by the manometer R. The volume of gas admitted into the charcoal is
determined by the burette D and the pipette P, and the corresponding
occlusion pressure at any concentration and any temperature below 90°
abs. by the gauge G. In presence of charcoal, and for small
concentrations, great variations are shown in the relation between the
pressure and the concentration of different gases, all at the same
temperature. Table VI. gives the comparison between hydrogen and
nitrogen at the temperature of liquid air, 25 grammes of charcoal
being employed. It is seen that 15 cc. of hydrogen produce nearly the
same pressure (0.0645 mm.) as 2500 cc. of nitrogen (0.06172 mm.). This
result shows how enormously greater, at the temperature of liquid air,
is the volatility of hydrogen as compared with that of nitrogen. In
the same way the concentrations, for the same pressure, vary greatly
with temperature, as is exemplified by table VII., even though the
pressures are not quite constant. The temperatures employed were the
boiling-points of hydrogen, oxygen and carbon dioxide.
[Illustration: FIG. 5.]
TABLE VII.
+---------------+---------------+--------+-----------+
| | Concentration |Pressure|Temperature|
| Gas. |in cc. per grm.| in mm. | Absolute. |
| | of Charcoal. | | |
+---------------+---------------+--------+-----------+
| Helium | 97 | 2.2 | 20° |
| Hydrogen | 397 | 2.2 | 20° |
| Hydrogen | 15 | 2.1 | 90° |
| Nitrogen | 250 | 1.6 | 90° |
| Oxygen | 300 | 1.0 | 90° |
| Carbon dioxide| 90 | 3.6 | 195° |
+---------------+---------------+--------+-----------+
TABLE VIII.
+---------------+-------------+------------+------------+
| |Concentration| Molecular | Mean |
| Gas. | cc. per grm.|Latent Heat.|Temperature.|
| | | | Absolute. |
+---------------+-------------+------------+------------+
| Helium | 97 | 483.0 | 18° |
| Hydrogen | 390 | 524.4 | 18° |
| Hydrogen | 20 | 2005.6 | 78° |
| Nitrogen | 250 | 3059.0 | 82° |
| Oxygen | 300 | 3146.4 | 82° |
| Carbon dioxide| 90 | 6099.6 | 180° |
+---------------+-------------+------------+------------+
_Heat of Occlusion._--In every case when gases are condensed to the
liquid state there is evolution of heat, and during the absorption of
a gas in charcoal or any other occluding body, as hydrogen in
palladium, the amount of heat evolved exceeds that of direct
liquefaction. From the relation between occlusion-pressure and
temperature at the same concentration, the reaction being reversible,
it is possible to calculate this heat evolution. Table VIII. gives the
mean molecular latent heats of occlusion resulting from Dewar's
experiments for a number of gases, having concentrations in the
charcoal as shown. The concentrations were so regulated as to start
with an initial pressure not exceeding 3 mm. at the respective
boiling-points of hydrogen, nitrogen, oxygen and carbon dioxide.
_Production of High Vacua._--Exceedingly high vacua can be obtained by
the aid of liquid gases, with or without charcoal. If a vessel
containing liquid hydrogen be freely exposed to the atmosphere, a rain
of snow (solid air) at once begins to fall upon the surface of the
liquid; similarly, if one end of a sealed tube containing ordinary air
be immersed in the liquid, the same thing happens, but since there is
now no new supply to take the place of the air that has been solidified
and has accumulated in the cooled portion of the tube, the pressure is
quickly reduced to something like one-millionth of an atmosphere, and a
vacuum is formed of such tenuity that the electric discharge can be made
to pass only with difficulty. Liquid air can be employed in the same
manner if the tube, before sealing, is filled with some less volatile
gas or vapour, such as sulphurous acid, benzol or water vapour. But if a
charcoal condenser be used in conjunction with the liquid air it becomes
possible to obtain a high vacuum when the tube contains air initially.
For instance, in one experiment, with a bulb having a capacity of 300
cc. and filled with air at a pressure of about 1.7 mm. and at a
temperature of 15° C., when an attached condenser with 5 grammes of
charcoal was cooled in liquid air, the pressure was reduced to 0.0545
mm. of mercury in five minutes, to 0.01032 mm. in ten minutes, to
0.000139 mm. in thirty minutes, and to 0.000047 mm. in sixty minutes.
The condenser then being cooled in liquid hydrogen the pressure fell to
0.0000154 mm. in ten minutes, and to 0.0000058 mm. in a further ten
minutes when solid hydrogen was employed as the cooling agent, and no
doubt, had it not been for the presence of hydrogen and helium in the
air, an even greater reduction could have been effected. Another
illustration of the power of cooled charcoal to produce high vacua is
afforded by a Crookes radiometer. If the instrument be filled with
helium at atmospheric pressure and a charcoal bulb attached to it be
cooled in liquid air, the vanes remain motionless even when exposed to
the concentrated beam of an electric arc lamp; but if liquid hydrogen be
substituted for the liquid air rapid rotation at once sets in. When a
similar radiometer was filled with hydrogen and the attached charcoal
bulb was cooled in liquid air rotation took place, because sufficient of
the gas was absorbed to permit motion. But when the charcoal was cooled
in liquid hydrogen instead of in liquid air, the absorption increased
and consequently the rarefaction became so high that there was no motion
when the light from the arc was directed on the vanes. These experiments
again permit of an inference as to the boiling-point of helium. A fall
of 75% in the temperature of the charcoal bulb, from the boiling-point
of air to the boiling-point of hydrogen, reduced the vanes to rest in
the case of the radiometer filled with hydrogen; hence it might be
inferred that a fall of like amount from the boiling-point of hydrogen
would reduce the vanes of the helium radiometer to rest, and
consequently that the boiling-point of helium would be about 5° abs.
[Illustration: FIG. 6.]