difficult to determine the pressures by the McLeod gauge, and the
radiometer experiments referred to above suggested the possibility of
another means of ascertaining such pressures, by determining the
pressures below which the radiometer would not spin. The following
experiment shows how the limit of pressure can be ascertained by
reference to the pressures of mercury vapour which have been very
accurately determined through a wide range of temperature. To a
radiometer (fig. 6) with attached charcoal bulb B was sealed a tube
ending in a small bulb A containing a globule of mercury. The radiometer
and bulb B were heated, exhausted and repeatedly washed out with pure
oxygen gas, and then the mercury was allowed to distil for some time
into the charcoal cooled in liquid air. On exposure to the electric beam
the vanes began to spin, but soon ceased when the bulb A was cooled in
liquid air. When, however, the mercury was warmed by placing the bulb in
liquid water, the vanes began to move again, and in the particular
radiometer used this was found to happen when the temperature of the
mercury had risen to -23° C. corresponding to a pressure of about one
fifty-millionth of an atmosphere.
For washing out the radiometer with oxygen the arrangement shown in fig.
7 is convenient. Here A is a bulb containing perchlorate of potash,
which when heated gives off pure oxygen; C is again the radiometer and B
the charcoal bulb. The side tube E is for the purpose of examining the
gas given off by minerals like thorianite or the gaseous products of the
transformation of radioactive bodies.
[Illustration: FIG. 7.]
_Analytic Uses._--Another important use of liquid gases is as analytic
agents, and for this purpose liquid air is becoming an almost essential
laboratory reagent. It is one of the most convenient agents for drying
gases and for their purification. If a mixture of gases be subjected to
the temperature of liquid air, it is obvious that all the constituents
that are more condensable than air will be reduced to liquid, while
those that are less condensable will either remain as a gaseous residue
or be dissolved in the liquid obtained. The bodies present in the latter
may be separated by fractional distillation, while the contents of the
gaseous residue may be further differentiated by the air of still lower
temperatures, such as are obtainable by liquid hydrogen. An apparatus
such as the following can be used to separate both the less and the more
volatile gases of the atmosphere, the former being obtained from their
solution in liquid air by fractional distillation at low pressure and
separation of the condensable part of the distillate by cooling in
liquid hydrogen, while the latter are extracted from the residue of
liquid air, after the distillation of the first fraction, by allowing it
to evaporate gradually at a temperature rising only very slowly.
In fig. 8, A represents a vacuum-jacketed vessel, containing liquid
air; this can be made to boil at reduced pressure and therefore be
lowered in temperature by means of an air-pump, which is in
communication with the vessel through the pipe _s_. The liquid boiled
away is replenished when necessary from the reservoir C, _p_ being a
valve, worked by handle _q_, by which the flow along _r_ is regulated.
The vessel B, immersed in the liquid air of A, communicates with the
atmosphere by _a_; hence when the temperature of A falls under
exhaustion below that of liquid air, the contents of B condense, and
if the stop-cock _m_ is kept open, and _n_ shut, air from the outside
is continuously sucked in until B is full of liquid, which contains in
solution the whole of the most volatile gases of the atmosphere which
have passed in through _a_. At this stage of the operation _m_ is
closed and _n_ opened, a passage thus being opened along _b_ from A to
the remainder of the apparatus seen on the left side of the figure.
Here E is a vacuum vessel containing liquid hydrogen, and _d_ a
three-way cock by which communication can be established either
between _b_ and D, between _b_ and _e_, the tube leading to the
sparking-tube _g_, or between D and _e_. If now _d_ is arranged so
that there is a free passage from _b_ to D, and the stop-cock _n_ also
opened, the gas dissolved in the liquid in B, together with some of
the most volatile part of that liquid, quickly distils over into D,
which is at a much lower temperature than B, and some of it condenses
there in the solid state. When a small fraction of the contents of B
has thus distilled over, _d_ is turned so as to close the passage
between D and _b_ and open that between D and _e_, with the result
that the gas in D is pumped out by the mercury-pump, shown
diagrammatically at _F_, along the tube _e_ (which is immersed in the
liquid hydrogen in order that any more condensable gas carried along
by the current may be frozen out) to the sparking-tube or tubes _g_,
where it can be examined spectroscopically. When the apparatus is used
to separate the least volatile part of the gases in the atmosphere,
the vessel E and its contents are omitted, and the tube _b_ made to
communicate with the pump through a number of sparking-tubes which can
be sealed off successively. The nitrogen and oxygen which make up the
bulk of the liquid in B are allowed to evaporate gradually, the
temperature being kept low so as to check the evaporation of gases
less volatile than oxygen. When most of the oxygen and nitrogen have
thus been removed, the stop-cock _n_ is closed, and the tubes
partially exhausted by the pump; spectroscopic examination is made of
the gases they contain, and repeated from time to time as more gas is
allowed to evaporate from B. The general sequence of spectra, apart
from those of nitrogen, oxygen and carbon compounds, which are never
eliminated by the process of distillation alone, is as follows: The
spectrum of argon first appears, followed by the brightest (green and
yellow) rays of krypton. Then the intensity of the argon spectrum
wanes and it gives way to that of krypton, until, as Runge observed,
when a Leyden jar is in the circuit, the capillary part of the
sparking-tube has a magnificent blue colour, while the wide ends are
bright pale yellow. Without a jar the tube is nearly white in the
middle and yellow about the poles. As distillation proceeds, the
temperature of the vessel containing the residue of liquid air being
allowed to rise slowly, the brightest (green) rays of xenon begin to
appear, and the krypton rays soon die out, being superseded by those
of xenon. At this stage the capillary part of the sparking-tube is,
with a jar in circuit, a brilliant green, and it remains green, though
less brilliant, if the jar is removed.
[Illustration: FIG. 8.--Apparatus for Fractional Distillation.]
[Illustration: FIG. 9.--Apparatus for continuous Spectroscopic
Examination.]
An improved form of apparatus for the fractionation is represented in
fig. 9. The gases to be separated, that is, the least volatile part of
atmospheric air, enter the bulb B from a gasholder by the tube _a_
with stop-cock _c_. B, which is maintained at a low temperature by
being immersed in liquid hydrogen, A, boiling under reduced pressure,
in turn communicates through the tube _b_ and stop-cock _d_ with a
sparking-tube or tubes _f_, and so on through _e_ with a mercurial
pump. To use the apparatus, stop-cock _d_ is closed and _c_ opened,
and gas allowed to pass from the gasholder into B, where it is
condensed in the solid form. Stop-cock _c_ then being closed and _d_
opened, gas passes into the exhausted tube _f_, where it is examined
with the spectroscope. The vessel D contains liquid air, in which the
tube _e_ is immersed in order to condense vapour of mercury which
would otherwise pass from the pump into the sparking-tube. The success
of the operation of separating all the gases which occur in air and
which boil at different temperatures, depends on keeping the
temperature of B as low as possible, as will be understood from the
following consideration:--
The pressure _p_, of a gas G, above the same material in the liquid
state, at temperature T, is given approximately by the formula
B
log p = A - ---,
T
where A and B are constants for the same material. For some other gas
G´ the formula will be
B1
log p1 = A1 - ---,
T
and
p B1 - B
log --- = A - A1 + ------,
p1 T
Now for argon, krypton and xenon respectively the values of A are
6.782, 6.972 and 6.963, and those of B are 339, 496.3 and 669.2; so
that for these substances and many others A - A1 is always a small
quantity, while (B1 - B)/T is considerable and increases as T
diminishes. Hence the ratio of _p_ to _p_1 increases rapidly as T
diminishes, and by evaporating all the gases from the solid state, and
keeping the solid at as low a temperature as possible, the gas that is
taken off by the mercurial pump first consists mainly of the substance
which has the lowest boiling point, in this case nitrogen, and is
succeeded with comparative abruptness by the gas which has the next
higher boiling point. Examination of the spectrum in the sparking-tube
easily reveals the change from one gas to another, and when that is
observed the reservoirs into which the gases are pumped can be changed
and the fractions stored separately. Or several sparking-tubes may be
arranged so as to form parallel communications between _b_ and _e_,
and can be successively sealed off at the desired stages of
fractionation.
[Illustration: FIG. 10.]
Analytical operations can often be performed still more conveniently
with the help of charcoal, taking advantage of the selective character
of its absorption, the general law of which is that the more volatile
the gas the less is it absorbed at a given temperature. The following
are some examples of its employment for this purpose. If it be required
to separate the helium which is often found in the gases given off by a
thermal spring, they are subjected to the action of charcoal cooled with
liquid air. The result is the absorption of the less volatile
constituents, i.e. all except hydrogen and helium. The gaseous residue,
with the addition of oxygen, is then sparked, and the water thus formed
is removed together with the excess of oxygen, when helium alone
remains. Or the separation may be effected by a method of fractionation
as described above. To separate the most volatile constituents of the
atmosphere an apparatus such as that shown in fig. 10 may be employed.
In one experiment with this, when 200 c.c. was supplied from the
graduated gas-holder F to the vessel D, containing 15 grammes of
charcoal cooled in liquid air, the residue which passed on unabsorbed to
the sparking-tube AB, which had a small charcoal bulb C attached, showed
the C and F lines of hydrogen, the yellow and some of the orange lines
of neon and the yellow and green of helium. By using a second charcoal
vessel E, with stop-cocks at H, I, J, K and L to facilitate
manipulation, considerable quantities of the most volatile gases can be
collected. After the charcoal in E has been saturated, the stop-cock K
is closed and I and J are opened for a short time, to allow the less
condensable gas in E to be sucked into the second condenser D along with
some portion of air. The condenser E is then taken out of the liquid
air, heated quickly to 15° C. to expel the occluded air and replaced.
More air is then passed in, and by repeating the operation several times
50 litres of air can be treated in a short time, supplying
sparking-tubes which will show the complete spectra of the volatile
constituents of the air.
The less volatile constituents of the atmosphere, krypton and xenon, may
be obtained by leading a current of air, purified by passage through a
series of tubes cooled in liquid air, through a charcoal condenser also
cooled in liquid air. The condenser is then removed and placed in solid
carbon dioxide at -78° C. The gas that comes off is allowed to escape,
but what remains in the charcoal is got out by heating and exhaustion,
the carbon compounds and oxygen are removed and the residue, consisting
of nitrogen with krypton and xenon, is separated into its constituents
by condensation and fractionation. Another method is to cover a few
hundred grammes of charcoal with old liquid air, which is allowed to
evaporate slowly in a silvered vacuum vessel; the gases remaining in the
charcoal are then treated in the manner described above.
[Illustration: FIG. 11.]
[Illustration: FIG. 12.]
Charcoal enables a mixture containing a high percentage of oxygen to be
extracted from the atmosphere. In one experiment 50 grammes of it, after
being heated and exhausted were allowed to absorb air at -185° C.; some
5 or 6 litres were taken up in ten minutes, and it then presumably
contained air of the composition of the atmosphere, i.e. 20% oxygen and
80% nitrogen, as shown in fig. 11. But when more air was passed over it,
the portion that was not absorbed was found to consist of about 98%
nitrogen, showing that excess of oxygen was being absorbed, and in the
course of a few hours the occluded gas attained a new and apparently
definite composition exhibited in fig. 12. When the charcoal containing
this mixture was transferred to a vacuum vessel and allowed to warm up
slowly, the successive litres of gas when collected and analyzed
separately showed the following composition:--
1st litre 18.5% oxygen
2nd litre 20.6% "
3rd litre 53.0% "
4th litre 72.0% "
5th litre 79.0% "
6th litre 84.0% "
TABLE IX.
+----------------+--------+--------------+-----------+-------------+
| | | Liquid Volume| |Volume of Gas|
| |Boiling | of 1 gram at |Latent Heat|at 0° C. and |
| Liquid Gases. | Point. | Boiling Point| in gram | 760 mm. per |
| | | in c.c. | Calories. | gram Calorie|
| | | | | in c.c. |
+----------------+--------+--------------+-----------+-------------+
| Sulphurous acid| + 10°C.| 0.7 | 97.0 | 3.6 |
| Carbonic acid | - 78.0 | 0.65 (solid)| 142.4 | 3.6 |
| Ethylene | -103.0 | 1.7 | 119.0 | 7.0 |
| Oxygen | -182.5 | 0.9 | 53.0 | 13.2 |
| Nitrogen | -195.6 | 1.3 | 50.0 | 15.9 |
| Hydrogen | -252.5 | 14.3 | 125.0 | 88.9 |
| Helium | -269.0 | 7.0 | 13.0 | 450.0 |
+----------------+--------+--------------+-----------+-------------+
_Calorimetry._--Certain liquid gases lend themselves conveniently to the
construction of a calorimeter, in which the heat in weighed quantities
of any substance with which it is desired to experiment may be measured
by the quantity of liquid gas they are able to evaporate. One advantage
of this method is that a great range of temperature is available when
liquid air, oxygen, nitrogen or hydrogen is employed as the
calorimetric substance. Another is the relatively large quantity of gas
yielded by the evaporation, as may be seen from table IX., which shows
the special physical constants of the various gases that are of
importance in calorimetry. In consequence it is easy to detect 1/50 gram
calorie with liquid air and so little as 1/300 gram calorie with liquid
hydrogen.
[Illustration: FIG. 13.--Calorimetric Apparatus.]
The apparatus (fig. 13) consists of a large vacuum vessel A, of 2 or 3
litres' capacity, containing liquid air, in which is inserted a
smaller vacuum vessel B, of 25-30 c.c. capacity, having sealed to it a
long narrow tube G that projects above the mouth of A and is held in
place by some loosely packed cotton wool. To the top of this tube the
test tube C, containing the material under investigation, is connected
by a piece of flexible rubber tubing D; this enables C to be tilted so
as to throw a piece or pieces of the contained material into the
calorimeter. An improved form of this receptacle, attached to B by a
flexible tube at D´, is shown at C´. In this P is a wire movable
through a cork Q and having at its end a hook by which a piece of the
substance under examination can be pulled up and dropped into B. In
the absence of other arrangements the substance is at the temperature
of the room, but when lower initial temperatures are desired a vacuum
vessel H containing solid carbonic acid, liquid ethylene, air or other
gas, can be placed to envelop C or C´, or higher temperatures may be
obtained by filling the surrounding vessel with vapour of water or
other liquids. The gas volatilized in B is conveyed by a side tube E
to be collected in a graduated receiver F over water, oil or other
liquid. If liquid hydrogen is to be used as the calorimetric substance
the instrument must be so modified as to prevent the ordinary
atmosphere from entering G, and to that end a current of hydrogen
supplied from a Kipp apparatus is arranged to flow continuously
through D and E until the moment of making the experiment, when it is
cut off by a suitable stop-cock. In this case the outer vessel must
contain liquid hydrogen instead of liquid air.
TABLE X.
+-----------+---------+----------+----------+
| | 18° to | -78° to | -188° to |
| Substance.| -78° C.,| -188° C.,| -252° C.,|
| | or, at | or, at | or, at |
| | -30° C. | -133° C. | -220° C. |
+-----------+---------+----------+----------+
| Diamond | 0.0794 | 0.0190 | 0.0043 |
| Graphite | 0.1341 | 0.0599 | 0.0133 |
| Ice | 0.463* | 0.285 | 0.146 |
+-----------+---------+----------+----------+
* This is from -18° to -78° in the ice experiment.
Dewar used pure metallic lead for the purpose of conveying definite
amounts of heat to liquid gas calorimeters of this kind, that metal
being selected on the ground of the small variation in its specific heat
at low temperatures. He was thus able to determine the latent heats of
evaporation of liquid oxygen, nitrogen and hydrogen directly at their
boiling points, and he also ascertained the specific heats of a large
number of inorganic and organic bodies, and of some gases in the solid
state, such as carbon dioxide, sulphurous acid and ammonia. Perhaps his
most interesting results were those which showed the variation in the
specific heats of diamond, graphite and ice as typical bodies (table
X.). With Professor Curie he used both the liquid oxygen and the liquid
hydrogen calorimeter for preliminary measurements of the rate at which
radium bromide gives out energy at low temperatures. The quantity of the
salt available was 0.42 gram, and the thermal evolutions were as
follows:--
Gas evolved Calories
per minute. per hour.
Liquid oxygen 5.5 cc. 22.8 \
Liquid hydrogen 51.0 " 31.6 > Crystals.
Melting ice .. 24.1 /
Liquid oxygen 2.0 " 8.3 After fusion.
Liquid oxygen 2.5 " 10.3 Emanation condensed.
The apparent increase of heat evolution at the temperature of liquid
hydrogen was probably due to the calorimeter being too small; hydrogen
spray was thus carried away with the gas, making the volume of gas too
great and inferentially also the heat evolved.
Liquid air and liquid hydrogen calorimeters open up an almost unlimited
field of research in the determination of specific heats and other
thermal constants, and are certain to become common laboratory
instruments for such purposes.
_Chemical Action._--By extreme cold chemical action is enormously
reduced, though it may not in all cases be entirely abolished even at
the lowest temperatures yet attained; one reason for this diminution of
activity may doubtless be sought in the fact that in such conditions
most substances are solid, that is, in the state least favourable to
chemical combination. Thus an electric pile of sodium and carbon ceases
to yield a current when immersed in liquid oxygen. Sulphur, iron and
other substances can be made to burn under the surface of liquid oxygen
if the combustion is properly established before the sample is immersed,
and the same is true of a fragment of diamond. Nitric oxide in the
gaseous condition combines instantly with free oxygen, producing the
highly-coloured gas, nitric peroxide, but in the solid condition it may
be placed in contact with liquid oxygen without showing any signs of
chemical action. If the combination of a portion of the mixture is
started by elevation of temperature, then detonation may take place
throughout the cooled mass. The stability of endothermic bodies like
nitric oxide and ozone at low temperatures requires further
investigation. The behaviour of fluorine, which may be regarded as the
most active of the elements, is instructive in this respect. As a gas,
cooled to -180° C. it loses the power of attacking glass; similarly
silicon, borax, carbon, sulphur and phosphorus at the same temperature
do not become incandescent in an atmosphere of the gas. Passed into
liquid oxygen, the gas dissolves and imparts a yellowish tint to the
liquid; if the oxygen has been exposed to the air for some hours, the
fluorine produces a white flocculent precipitate, which if separated by
filtering deflagrates with violence as the temperature rises. It appears
to be a hydrate of fluorine. As a liquid at -210° fluorine attacks
turpentine also cooled to that temperature with explosive force and the
evolution of light, while the direction of a jet of hydrogen upon its
surface is immediately followed by combination and a flash of flame.
Even when the point of a tube containing solid fluorine is broken off
under liquid hydrogen, a violent explosion ensues.
_Photographic Action._--The action of light on photographic plates,
though greatly diminished at -180°, is far from being in abeyance; an
Eastman film, for instance, remains fairly sensitive at -210°. At the
still lower temperature of liquid hydrogen the photographic activity is
reduced to about half what it is at that of liquid air; in other words,
about 10% of the original sensitivity remains. Experiments carried out
with an incandescent lamp, a Röntgen bulb and the ultra-violet spark
from magnesium and cadmium, to discover at what distances from the
source of light the plates must be placed in order to receive an equal
photographic impression, yielded the results shown in table XI.
TABLE XI.
+--------------------+---------+----------+--------------+
| | Cooled | Uncooled | Ratio of |
| Source of Light. | Plate. | Plate. | Intensities |
| | | | at Balance. |
+--------------------+---------+----------+--------------+
| 16 C.P. lamp | 20 in. | 50 in. | 1 to 6 |
| Röntgen bulb | 10 in. | 24¾ in. | 1 to 6 |
| Ultra-violet spark | 22½ in. | 90 in. | 1 to 16 |
+--------------------+---------+----------+--------------+
It appears that the photographic action of both the incandescent lamp
and the Röntgen rays is reduced by the temperature of liquid air to 17%
of that exerted at ordinary temperatures, while ultra-violet radiation
retains only 6%. It is possible that the greater dissipation of the
latter by the photographic film at low temperatures than at ordinary
ones is due to its absorption and subsequent emission as a
phosphorescent glow, and that if the plate could be developed at a low
temperature it would show no effect, the photographic action taking
place subsequently through an internal phosphorescence in the film
during the time it is heating up. With regard to the transparency of
bodies to the Röntgen radiation at low temperatures, small tubes of the
same bore, filled with liquid argon and chlorine, potassium, phosphorus,
aluminium, silicon and sulphur, were exposed at the temperature of
liquid air (in order to keep the argon and chlorine solid), in front of
a photographic plate shielded with a sheet of aluminium, to an X-ray
bulb. The sequence of the elements as mentioned represents the order of
increasing opacity observed in the shadows. Sodium and liquid oxygen and
air, nitrous and nitric oxides, proved much more transparent than
chlorine. Tubes of potassium, argon and liquid chlorine showed no very
marked difference of density on the photographic plates. It appears that
argon is relatively more opaque to the Röntgen radiation than either
oxygen, nitrogen or sodium, and is on a level with potassium, chlorine,
phosphorus, aluminium and sulphur. This fact may be regarded as
supporting the view that the atomic weight of argon is twice its density
relative to hydrogen, since in general the opacity of elements in the
solid state increases with the atomic weight.