Transcriber's notes:
(1) Numbers following letters (without space) like C2 were originally
printed in subscript. Letter subscripts are preceded by an
underscore, like C_n.
(2) Characters following a carat (^) were printed in superscript.
(3) Side-notes were relocated to function as titles of their respective
paragraphs.
(4) Macrons and breves above letters and dots below letters were not
inserted.
(5) [root] stands for the root symbol; [alpha], [beta], etc. for greek
letters.
(6) The following typographical errors have been corrected:
ARTICLE LIQUID GASES: "Another important use of liquid gases is as
analytic agents, and for this purpose liquid air is becoming an
almost essential laboratory reagent." 'as' amended from 'an'.
ARTICLE LIQUOR LAWS: "The proceedings are stricter and more summary
in the case of a new licence; notice of application must be given
to the local authorities; ..." 'authorities' amended from
'authorites'.
ARTICLE LISBON: "... and the royal national theatre (Teatro de Dona
Maria Segunda), erected on the site which the Inquisition buildings
occupied from 1520 to 1836." 'Teatro' amended from 'Theatro'.
ARTICLE LIVERSEDGE: "... Great Northern, and London & North Western
railways. Pop. (1901) 13,980." 'Northern' amended from 'Nothern'.
ARTICLE LOCRI: "... and found in Syracuse a support against
Rhegium: it was thus an active adversary of Athenian aggrandisement
in the west." 'of' amended from 'or'.
ARTICLE LOG: "The Bliss resembles the Rocket log in shape, and is
secured to the taffrail by a rope or sling." 'sling' amended from
'slung'.
ENCYCLOPAEDIA BRITANNICA
A DICTIONARY OF ARTS, SCIENCES, LITERATURE
AND GENERAL INFORMATION
ELEVENTH EDITION
VOLUME XVI, SLICE VII
Liquid Gases to Logar
ARTICLES IN THIS SLICE:
LIQUID GASES LLANWRTYD WELLS
LIQUORICE LLEWELYN
LIQUOR LAWS LLORENTE, JUAN ANTONIO
LIRA LLOYD, EDWARD
LIRI LLOYD, WILLIAM
LIROCONITE LLOYD, WILLIAM WATKISS
LISBON LLOYD GEORGE, DAVID
LISBURN LLOYD'S
LISIEUX LLWYD, EDWARD
LISKEARD LOACH
LISLE, ALICE LOAD; LODE
LISMORE (Scottish island) LOAF
LISMORE (Australian town) LOAM
LISMORE (Irish town) LOAN
LISSA (Austrian island) LOANDA
LISSA (Prussian town) LOANGO
LIST, FRIEDRICH LOBACHEVSKIY, NICOLAS IVANOVICH
LIST LOBANOV-ROSTOVSKI, ALEXIS BORISOVICH
LISTA Y ARAGON, ALBERTO LÖBAU
LISTER, JOSEPH LISTER LOBBY
LISTER, MARTIN LOBBYING
LISTON, JOHN LOBE
LISTON, ROBERT LOBECK, CHRISTIAN AUGUST
LISZT, FRANZ LOBEIRA, JOÃO
LITANY LOBELIA
LITCHFIELD (Connecticut, U.S.A.) LOBENSTEIN
LITCHFIELD (Illinois, U.S.A.) LOBO, FRANCISCO RODRIGUES
LITCHI LOBO, JERONIMO
LITERATURE LOBSTER
LITERNUM LOCAL GOVERNMENT
LITHGOW, WILLIAM LOCAL GOVERNMENT BOARD
LITHGOW LOCARNO
LITHIUM LOCH, HENRY BROUGHAM LOCH
LITHOGRAPHY LOCHABER
LITHOSPHERE LOCHES
LITHUANIANS and LETTS LOCHGELLY
LITMUS LOCHGILPHEAD
LITOPTERNA LOCHMABEN
LITOTES LOCK, MATTHIAS
LITTER LOCK
LITTLE FALLS (Minnesota, U.S.A.) LOCKE, JOHN
LITTLE FALLS (New York, U.S.A.) LOCKE, MATTHEW
LITTLEHAMPTON LOCKERBIE
LITTLE ROCK LOCKER-LAMPSON, FREDERICK
LITTLETON, EDWARD LOCKHART, GEORGE
LITTLETON, SIR THOMAS DE LOCKHART, JOHN GIBSON
LITTRÉ, MAXIMILIEN PAUL ÉMILE LOCKHART, SIR WILLIAM ALEXANDER
LITURGY LOCK HAVEN
LITUUS LOCKPORT (Illinois, U.S.A.)
LIUDPRAND LOCKPORT (New York, U.S.A.)
LIVE OAK LOCKROY, ÉDOUARD
LIVER LOCKWOOD, SIR FRANK
LIVERMORE, MARY ASHTON [RICE] LOCKWOOD, WILTON
LIVERPOOL, EARLS OF LOCKYER, SIR JOSEPH NORMAN
LIVERPOOL LOCLE, LE
LIVERSEDGE LOCMARIAQUER
LIVERY LOCOMOTOR ATAXIA
LIVERY COMPANIES LOCO-WEEDS
LIVIA DRUSILLA LOCRI (people of ancient Greece)
LIVINGSTON, EDWARD LOCRI (Italian city)
LIVINGSTON, ROBERT R. LÖCSE
LIVINGSTON, WILLIAM LOCUS
LIVINGSTONE, DAVID LOCUST
LIVINGSTONE MOUNTAINS LOCUST-TREE
LIVIUS ANDRONICUS LODÈVE
LIVNO LODGE, EDMUND
LIVONIA LODGE, HENRY CABOT
LIVY LODGE, SIR OLIVER JOSEPH
LIZARD LODGE, THOMAS
LIZARD POINT LODGE
LJUNGGREN, GUSTAF HÅKAN JORDAN LODGER AND LODGINGS
LLAMA LODI
LLANBERIS LODZ
LLANDAFF LOESS
LLANDEILO GROUP LOFFT, CAPEL
LLANDILO LOFOTEN AND VESTERAALEN
LLANDOVERY LOFT
LLANDOVERY GROUP LOFTUS, ADAM
LLANDRINDOD LOG
LLANDUDNO LOGAN, JOHN (American Indian chief)
LLANELLY LOGAN, JOHN (Scottish poet)
LLANES LOGAN, JOHN ALEXANDER
LLANGOLLEN LOGAN, SIR WILLIAM EDMOND
LLANQUIHUE LOGAN (Utah, U.S.A.)
LLANTRISANT LOGANSPORT
LLANTWIT MAJOR LOGAR
LIQUID GASES.[1] Though Lavoisier remarked that if the earth were
removed to very cold regions of space, such as those of Jupiter or
Saturn, its atmosphere, or at least a portion of its aeriform
constituents, would return to the state of liquid (_Oeuvres_, ii. 805),
the history of the liquefaction of gases may be said to begin with the
observation made by John Dalton in his essay "On the Force of Steam or
Vapour from Water and various other Liquids" (1801): "There can scarcely
be a doubt entertained respecting the reducibility of all elastic fluids
of whatever kind into liquids; and we ought not to despair of effecting
it in low temperatures and by strong pressures exerted on the unmixed
gases." It was not, however, till 1823 that the question was
investigated by systematic experiment. In that year Faraday, at the
suggestion of Sir Humphry Davy, exposed hydrate of chlorine to heat
under pressure in the laboratories of the Royal Institution. He placed
the substance at the end of one arm of a bent glass tube, which was then
hermetically sealed, and decomposing it by heating to 100° F., he saw a
yellow liquid distil to the end of the other arm. This liquid he
surmised to be chlorine separated from the water by the heat and
"condensed into a dry fluid by the mere pressure of its own abundant
vapour," and he verified his surmise by compressing chlorine gas, freed
from water by exposure to sulphuric acid, to a pressure of about four
atmospheres, when the same yellow fluid was produced (_Phil. Trans._,
1823, 113, pp. 160-165). He proceeded to experiment with a number of
other gases subjected in sealed tubes to the pressure caused by their
own continuous production by chemical action, and in the course of a few
weeks liquefied sulphurous acid, sulphuretted hydrogen, carbonic acid,
euchlorine, nitrous acid, cyanogen, ammonia and muriatic acid, the last
of which, however, had previously been obtained by Davy. But he failed
with hydrogen, oxygen, fluoboric, fluosilicic and phosphuretted hydrogen
gases (_Phil. Trans._, ib. pp. 189-198). Early in the following year he
published an "Historical statement respecting the liquefaction of gases"
(_Quart. Journ. Sci._, 1824, 16, pp. 229-240), in which he detailed
several recorded cases in which previous experimenters had reduced
certain gases to their liquid state.
In 1835 Thilorier, by acting on bicarbonate of soda with sulphuric acid
in a closed vessel and evacuating the gas thus obtained under pressure
into a second vessel, was able to accumulate large quantities of liquid
carbonic acid, and found that when the liquid was suddenly ejected into
the air a portion of it was solidified into a snow-like substance (_Ann.
chim. phys._, 1835, 60, pp. 427-432). Four years later J. K. Mitchell in
America, by mixing this snow with ether and exhausting it under an air
pump, attained a minimum temperature of 146° below zero F., by the aid
of which he froze sulphurous acid gas to a solid.
Stimulated by Thilorier's results and by considerations arising out of
the work of J. C. Cagniard de la Tour (_Ann. chim. phys._, 1822, 21, pp.
127 and 178, and 1823, 22, p. 410), which appeared to him to indicate
that gases would pass by some simple law into the liquid state, Faraday
returned to the subject about 1844, in the "hope of seeing nitrogen,
oxygen and hydrogen either as liquid or solid bodies, and the latter
probably as a metal" (_Phil. Trans._, 1845, 135, pp. 155-157). On the
basis of Cagniard de la Tour's observation that at a certain temperature
a liquid under sufficient pressure becomes a vapour or gas having the
same bulk as the liquid, he inferred that "at this temperature or one a
little higher, it is not likely that any increase of pressure, except
perhaps one exceedingly great, would convert the gas into a liquid." He
further surmised that the Cagniard de la Tour condition might have its
point of temperature for oxygen, nitrogen, hydrogen, &c., below that
belonging to the bath of solid carbonic acid and ether, and he realized
that in that case no pressure which any apparatus would be able to bear
would be able to bring those gases into the liquid or solid state, which
would require a still greater degree of cold. To fulfil this condition
he immersed the tubes containing his gases in a bath of solid carbonic
acid and ether, the temperature of which was reduced by exhaustion under
the air pump to -166° F., or a little lower, and at the same time he
subjected the gases to pressures up to 50 atmospheres by the use of two
pumps working in series. In this way he added six substances, usually
gaseous, to the list of those that could be obtained in the liquid
state, and reduced seven, including ammonia, nitrous oxide and
sulphuretted hydrogen, into the solid form, at the same time effecting a
number of valuable determinations of vapour tensions. But he failed to
condense oxygen, nitrogen and hydrogen, the original objects of his
pursuit, though he found reason to think that "further diminution of
temperature and improved apparatus for pressure may very well be
expected to give us these bodies in the liquid or solid state." His
surmise that increased pressure alone would not suffice to bring about
change of state in these gases was confirmed by subsequent
investigators, such as M. P. E. Berthelot, who in 1850 compressed oxygen
to 780 atmospheres (_Ann. chim. phys._, 1850, 30, p. 237), and Natterer,
who a few years later subjected the permanent gases to a pressure of
2790 atmospheres, without result; and in 1869 Thomas Andrews (_Phil.
Trans._, 11) by his researches on carbonic acid finally established the
conception of the "critical temperature" as that temperature, differing
for different bodies, above which no gas can be made to assume the
liquid state, no matter what pressure it be subjected to (see
CONDENSATION OF GASES).
About 1877 the problem of liquefying the permanent gases was taken up by
L. P. Cailletet and R. P. Pictet, working almost simultaneously though
independently. The former relied on the cold produced by the sudden
expansion of the gases at high compression. By means of a specially
designed pump he compressed about 100 cc. of oxygen in a narrow glass
tube to about 200 atmospheres, at the same time cooling it to about -29°
C., and on suddenly releasing the pressure he saw momentarily in the
interior of the tube a mist (_brouillard_), from which he inferred the
presence of a vapour very near its point of liquefaction. A few days
later he repeated the experiment with hydrogen, using a pressure of
nearly 300 atmospheres, and observed in his tube an exceedingly fine and
subtle fog which vanished almost instantaneously. At the time when these
experiments were carried out it was generally accepted that the mist or
fog consisted of minute drops of the liquefied gases. Even had this been
the case, the problem would not have been completely solved, for
Cailletet was unable to collect the drops in the form of a true stable
liquid, and at the best obtained a "dynamic" not a "static" liquid, the
gas being reduced to a form that bears the same relation to a true
liquid that the partially condensed steam issuing from the funnel of a
locomotive bears to water standing in a tumbler. But subsequent
knowledge showed that even this proximate liquefaction could not have
taken place, and that the fog could not have consisted of drops of
liquid hydrogen, because the cooling produced by the adiabatic expansion
would give a temperature of only 44° abs., which is certainly above the
critical temperature of hydrogen. Pictet again announced that on opening
the tap of a vessel containing hydrogen at a pressure of 650 atmospheres
and cooled by the cascade method (see CONDENSATION OF GASES) to -140°
C., he saw issuing from the orifice an opaque jet which he assumed to
consist of hydrogen in the liquid form or in the liquid and solid forms
mixed. But he was no more successful than Cailletet in collecting any of
the liquid, which--whatever else it may have been, whether ordinary air
or impurities associated with the hydrogen--cannot have been hydrogen
because the means he employed were insufficient to reduce the gas to
what has subsequently been ascertained to be its critical point, below
which of course liquefaction is impossible. It need scarcely be added
that if the liquefaction of hydrogen be rejected a fortiori Pictet's
claim to have effected its solidification falls to the ground.
After Cailletet and Pictet, the next important names in the history of
the liquefaction of gases are those of Z. F. Wroblewski and K. S.
Olszewski, who for some years worked together at Cracow. In April 1883
the former announced to the French Academy that he had obtained oxygen
in a completely liquid state and (a few days later) that nitrogen at a
temperature of -136° C., reduced suddenly from a pressure of 150
atmospheres to one of 50, had been seen as a liquid which showed a true
meniscus, but disappeared in a few seconds. But with hydrogen treated in
the same way he failed to obtain even the mist reported by Cailletet. At
the beginning of 1884 he performed a more satisfactory experiment.
Cooling hydrogen in a capillary glass tube to the temperature of liquid
oxygen, he expanded it quickly from 100 atmospheres to one, and
obtained the appearance of an instantaneous ebullition. Olszewski
confirmed this result by expanding from a pressure of 190 atmospheres
the gas cooled by liquid oxygen and nitrogen boiling under reduced
pressure, and even announced that he saw it running down the walls of
the tube as a colourless liquid.
Wroblewski, however, was unable to observe this phenomenon, and
Olszewski himself, when seven years later he repeated the experiment in
the more favourable conditions afforded by a larger apparatus, was
unable to produce again the colourless drops he had previously reported:
the phenomenon of the appearance of sudden ebullition indeed lasted
longer, but he failed to perceive any meniscus such as would have been a
certain indication of the presence of a true liquid. Still, though
neither of these investigators succeeded in reaching the goal at which
they aimed, their work was of great value in elucidating the conditions
of the problem and in perfecting the details of the apparatus employed.
Wroblewski in particular devoted the closing years of his life to a most
valuable investigation of the isothermals of hydrogen at low
temperatures. From the data thus obtained he constructed a van der Waals
equation which enabled him to calculate the critical temperature,
pressure and density of hydrogen with very much greater certainty than
had previously been possible. Liquid oxygen, liquid nitrogen and liquid
air--the last was first made by Wroblewski in 1885--became something
more than mere curiosities of the laboratory, and by the year 1891 were
produced in such quantities as to be available for the purposes of
scientific research. Still, nothing was added to the general principles
upon which the work of Cailletet and Pictet was based, and the "cascade"
method, together with adiabatic expansion from high compression (see
CONDENSATION OF GASES), remained the only means of procedure at the
disposal of experimenters in this branch of physics.
In some quarters a certain amount of doubt appears to have arisen as to
the sufficiency of these methods for the liquefaction of hydrogen.
Olszewski, for example, in 1895 pointed out that the succession of less
and less condensible gases necessary for the cascade method breaks down
between nitrogen and hydrogen, and he gave as a reason for hydrogen not
having been reduced to the condition of a static liquid the
non-existence of a gas intermediate in volatility between those two. By
1894 attempts had been made in the Royal Institution laboratories to
manufacture an artificial gas of this nature by adding a small
proportion of air to the hydrogen, so as to get a mixture with a
critical point of about -200° C. When such a mixture was cooled to that
temperature and expanded from a high degree of compression into a vacuum
vessel, the result was a white mass of solid air together with a clear
liquid of very low density. This was in all probability hydrogen in the
true liquid state, but it was not found possible to collect it owing to
its extreme volatility. Whether this artificial gas might ultimately
have enabled liquid hydrogen to be collected in open vessels we cannot
say, for experiments with it were abandoned in favour of other measures,
which led finally to a more assured success.
_Vacuum Vessels._--The problem involved in the liquefaction of hydrogen
was in reality a double one. In the first place, the gas had to be
cooled to such a temperature that the change to the liquid state was
rendered possible. In the second, means had to be discovered for
protecting it, when so cooled, from the influx of external heat, and
since the rate at which heat is transferred from one body to another
increases very rapidly with the difference between their temperatures,
the question of efficient heat insulation became at once more difficult
and more urgent in proportion to the degree of cold attained. The second
part of the problem was in fact solved first. Of course packing with
non-conducting materials was an obvious expedient when it was not
necessary that the contents of the apparatus should be visible to the
eye, but in the numerous instances when this was not the case such
measures were out of the question. Attempts were made to secure the
desired end by surrounding the vessel that contained the cooled or
liquid gas with a succession of other vessels, through which was
conducted the vapour given off from the interior one. Such devices
involved awkward complications in the arrangement of the apparatus, and
besides were not as a rule very efficient, although some workers, e.g.
Dr Kamerlingh Onnes, of Leiden, reported some success with their use. In
1892 it occurred to Dewar that the principle of an arrangement he had
used nearly twenty years before for some calorimetric experiments on the
physical constants of hydrogenium, which was a natural deduction from
the work of Dulong and Petit on radiation, might be employed with
advantage as well to protect cold substances from heat as hot ones from
cold. He therefore tried the effect of surrounding his liquefied gas
with a highly exhausted space. The result was entirely successful.
Experiment showed that liquid air contained in a glass vessel with two
walls, the space between which was a high vacuum, evaporated at only one
fifth the rate it did when in an ordinary vessel surrounded with air at
atmospheric pressure, the convective transference of heat by means of
the gas particles being enormously reduced owing to the vacuum. But in
addition these vessels lent themselves to an arrangement by which
radiant heat could still further be cut off, since it was found that
when the inner wall was coated with a bright deposit of silver, the
influx of heat was diminished to one-sixth of the amount existing
without the metallic coating. The total effect, therefore, of the high
vacuum and silvering is to reduce the in-going heat to one-thirtieth
part. In making such vessels a mercurial vacuum has been found very
satisfactory. The vessel in which the vacuum is to be produced is
provided with a small subsidiary vessel joined by a narrow tube with the
main vessel, and connected with a powerful air-pump. A quantity of
mercury having been placed in it, it is heated in an oil- or air-bath to
about 200° C., so as to volatilize the mercury, the vapour of which is
removed by the pump. After the process has gone on for some time, the
pipe leading to the pump is sealed off, the vessel immediately removed
from the bath, and the small subsidiary part immersed in some cooling
agent such as solid carbonic acid or liquid air, whereby the mercury
vapour is condensed in the small vessel and a vacuum of enormous tenuity
left in the large one. The final step is to seal off the tube connecting
the two. In this way a vacuum may be produced having a vapour pressure
of about the hundred-millionth of an atmosphere at 0° C. If, however,
some liquid mercury be left in the space in which the vacuum is
produced, and the containing part of the vessel be filled with liquid
air, the bright mirror of mercury which is deposited on the inside wall
of the bulb is still more effective than silver in protecting the
chamber from the influx of heat, owing to the high refractive index,
which involves great reflecting power, and the bad heat-conducting
powers of mercury.
[Illustration: FIG. 1.--Metallic Vacuum Vessel.]
With the discovery of the remarkable power of gas absorption possessed
by charcoal cooled to a low temperature (see below), it became possible
to make these vessels of metal. Previously this could not be done with
success, because gas occluded in the metal gradually escaped and
vitiated the vacuum; but now any stray gas may be absorbed by means of
charcoal so placed in a pocket within the vacuous space that it is
cooled by the liquid in the interior of the vessel. Metal vacuum
vessels (fig. 1), of a capacity of from 2 to 20 litres, may be formed of
brass, copper, nickel or tinned iron, with necks of some alloy that is a
bad conductor of heat, silvered glass vacuum cylinders being fitted as
stoppers. Such flasks, when properly constructed, have an efficiency
equal to that of the chemically-silvered glass vacuum vessels now
commonly used in low temperature investigations, and they are obviously
better adapted for transport. The principle of the Dewar vessel is
utilized in the Thermos flasks which are now extensively manufactured
and employed for keeping liquids warm in hospitals, &c.
_Thermal Transparency at Low Temperatures._--The proposition, once
enunciated by Pictet, that at low temperatures all substances have
practically the same thermal transparency, and are equally ineffective
as non-conductors of heat, is based on erroneous observations. It is
true that if the space between the two walls of a double-walled vessel
is packed with substances like carbon, magnesia, or silica, liquid air
placed in the interior will boil off even more quickly than it will
when the space merely contains air at atmospheric pressure; but in
such cases it is not so much the carbon, &c., that bring about the
transference of heat, as the air contained in their interstices. If
this air be pumped out such substances are seen to exert a very
considerable influence in stopping the influx of heat, and a vacuum
vessel which has the space between its two walls filled with a
non-conducting material of this kind preserves a liquid gas even
better than one in which that space is simply exhausted of air. In
experiments on this point double-walled glass tubes, as nearly
identical in shape and size as possible, were mounted in sets of three
on a common stem which communicated with an air-pump, so that the
degree of exhaustion in each was equal. In two of each three the space
between the double walls was filled with the powdered material it was
desired to test, the third being left empty and used as the standard.
The time required for a certain quantity of liquid air to evaporate
from the interior of this empty bulb being called 1, in each of the
eight sets of triple tubes, the times required for the same quantity
to boil off from the other pairs of tubes were as follows:--
/ Charcoal 5 / Lampblack 5
\ Magnesia 2 \ Silica 4
/ Graphite 1.3 / Lampblack 4
\ Alumina 3.3 \ Lycopodium 2.5
/ Calcium carbonate 2.5 / Barium carbonate 1.3
\ Calcium fluoride 1.25 \ Calcium phosphate 2.7
/ Phosphorus (amorphous) 1 / Lead oxide 2
\ Mercuric iodide 1.5 \ Bismuth oxide 6
Other experiments of the same kind made--(a) with similar vacuum
vessels, but with the powders replaced by metallic and other septa;
and (b) with vacuum vessels having their walls silvered, yielded the
following results:--
/(a) Vacuum space empty 1 / Vacuum space empty 1
| Three turns silver | Three turns black paper,
| paper, bright < black outside 3
< surface inside 4 | Three turns black paper,
| Three turns silver \ black inside 3
| paper, bright
\ surfaceoutside 4
/ Vacuum space empty 1 / Vacuum space empty 1
| Three turns gold paper, | Three turns, not touching,
| gold outside 4 < of sheet lead 4
< Some pieces of goldleaf | Three turns, not touching,
| put in so as \ of sheet aluminium 4
| to make contact
| between walls of
\ vacuum-tube 0.3
/(b) Vacuum space empty, / Empty silvered vacuum 1
| silvered on inside < Charcoal in silvered
< surfaces 1 \ vacuum 1.25
| Silica in silvered
\ vacuum space 1.1
It appears from these experiments that silica, charcoal, lampblack,
and oxide of bismuth all increase the heat insulations to four, five
and six times that of the empty vacuum space. As the chief
communication of heat through an exhausted space is by molecular
bombardment, the fine powders must shorten the free path of the
gaseous molecules, and the slow conduction of heat through the porous
mass must make the conveyance of heat-energy more difficult than when
the gas molecules can impinge upon the relatively hot outer glass
surface, and then directly on the cold one without interruption. (See
_Proc. Roy. Inst._ xv. 821-826.)
_Density of Solids and Coefficients of Expansion at Low
Temperatures._--The facility with which liquid gases, like oxygen or
nitrogen, can be guarded from evaporation by the proper use of vacuum
vessels (now called Dewar vessels), naturally suggests that the
specific gravities of solid bodies can be got by direct weighing when
immersed in such fluids. If the density of the liquid gas is
accurately known, then the loss of weight by fluid displacement gives
the specific gravity compared to water. The metals and alloys, or
substances that can be got in large crystals, are the easiest to
manipulate. If the body is only to be had in small crystals, then it
must be compressed under strong hydraulic pressure into coherent
blocks weighing about 40 to 50 grammes. Such an amount of material
gives a very accurate density of the body about the boiling point of
air, and a similar density taken in a suitable liquid at the ordinary
temperature enables the mean coefficient of expansion between +15° C.
and -185° C. to be determined. One of the most interesting results is
that the density of ice at the boiling point of air is not more than
0.93, the mean coefficient of expansion being therefore 0.000081. As
the value of the same coefficient between 0° C. and -27° C. is
0.000155, it is clear the rate of contraction is diminished to about
one-half of what it was above the melting point of the ice. This
suggests that by no possible cooling at our command is it likely we
could ever make ice as dense as water at 0° C., far less 4° C. In
other words, the volume of ice at the zero of temperature would not be
the minimum volume of the water molecule, though we have every reason
to believe it would be so in the case of the majority of known
substances. Another substance of special interest is solid carbonic
acid. This body has a density of 1.53 at -78° C. and 1.633 at -185°
C., thus giving a mean coefficient of expansion between these
temperatures of 0.00057. This value is only about 1/6 of the
coefficient of expansion of the liquid carbonic acid gas just above
its melting point, but it is still much greater at the low temperature
than that of highly expansive solids like sulphur, which at 40° C. has
a value of 0.00019. The following table gives the densities at the
temperature of boiling liquid air (-185° C.) and at ordinary
temperatures (17° C.), together with the mean coefficient of expansion
between those temperatures, in the case of a number of hydrated salts
and other substances:
TABLE I.
+---------------------------+---------+--------+------------+
| | | | Mean |
| | | | coefficient|
| | Density | Density|of expansion|
| | at -185°| at +17°| between |
| | C. | C. |-185° C. and|
| | | | +17° C. |
+---------------------------+---------+--------+------------+
| Aluminium sulphate (18)* | 1.7194 | 1.6913 | 0.0000811 |
| Sodium biborate (10) | 1.7284 | 1.6937 | 0.0001000 |
| Calcium chloride (6) | 1.7187 | 1.6775 | 0.0001191 |
| Magnesium chloride (6) | 1.6039 | 1.5693 | 0.0001072 |
| Potash alum (24) | 1.6414 | 1.6144 | 0.0000813 |
| Chrome alum (24) | 1.7842 | 1.7669 | 0.0000478 |
| Sodium carbonate (10) | 1.4926 | 1.4460 | 0.0001563 |
| Sodium phosphate (12) | 1.5446 | 1.5200 | 0.0000787 |
| Sodium thiosulphate (5) | 1.7635 | 1.7290 | 0.0000969 |
| Potassium ferrocyanide (3)| 1.8988 | 1.8533 | 0.0001195 |
| Potassium ferricyanide | 1.8944 | 1.8109 | 0.0002244 |
| Sodium nitro-prusside (4) | 1.7196 | 1.6803 | 0.0001138 |
| Ammonium chloride | 1.5757 | 1.5188 | 0.0001820 |
| Oxalic acid (2) | 1.7024 | 1.6145 | 0.0002643 |
| Methyl oxalate | 1.5278 | 1.4260 | 0.0003482 |
| Paraffin | 0.9770 | 0.9103 | 0.0003567 |
| Naphthalene | 1.2355 | 1.1589 | 0.0003200 |
| Chloral hydrate | 1.9744 | 1.9151 | 0.0001482 |
| Urea | 1.3617 | 1.3190 | 0.0001579 |
| Iodoform | 4.4459 | 4.1955 | 0.0002930 |
| Iodine | 4.8943 | 4.6631 | 0.0002510 |
| Sulphur | 2.0989 | 2.0522 | 0.0001152 |
| Mercury |14.382 | .. | 0.0000881**|
| Sodium | 1.0056 | 0.972 | 0.0001810 |
| Graphite (Cumberland) | 2.1302 | 2.0990 | 0.0000733 |
+---------------------------+---------+--------+------------+
* The figures within parentheses refer to the number of molecules of
water of crystallization.
** -189° to -38.85° C.
It will be seen from this table that, with the exception of carbonate
of soda and chrome alum, the hydrated salts have a coefficient of
expansion that does not differ greatly from that of ice at low
temperatures. Iodoform is a highly expansive body like iodine, and
oxalate of methyl has nearly as great a coefficient as paraffin, which
is a very expansive solid, as are naphthalene and oxalic acid. The
coefficient of solid mercury is about half that of the liquid metal,
while that of sodium is about the value of mercury at ordinary
temperatures. Further details on the subject can be found in the
_Proc. Roy. Inst._ (1895), and _Proc. Roy. Soc._ (1902).
_Density of Gases at Low Temperatures._--The ordinary mode of
determining the density of gases may be followed, provided that the
glass flask, with its carefully ground stop-cock sealed on, can stand
an internal pressure of about five atmospheres, and that all the
necessary corrections for change of volume are made. All that is
necessary is to immerse the exhausted flask in boiling oxygen, and
then to allow the second gas to enter from a gasometer by opening the
stop-cock until the pressure is equalized. The stop-cock being closed,
the flask is now taken out of the liquid oxygen and left in the
balance-room until its temperature is equalized. It is then weighed
against a similar flask used as a counterpoise. Following such a
method, it has been found that the weight of 1 litre of oxygen vapour
at its boiling point of 90.5° absolute is 4.420 grammes, and therefore
the specific volume is 226.25 cc. According to the ordinary gaseous
laws, the litre ought to weigh 4.313 grammes, and the specific volume
should be 231.82 cc. In other words, the product of pressure and
volume at the boiling point is diminished by 2.46%. In a similar way
the weight of a litre of nitrogen vapour at the boiling point of
oxygen was found to be 3.90, and the inferred value for 78° absolute,
or its own boiling point, would be 4.51, giving a specific volume of
221.3.
_Regenerative Cooling._--One part of the problem being thus solved and a
satisfactory device discovered for warding off heat in such vacuum
vessels, it remained to arrange some practically efficient method for
reducing hydrogen to a temperature sufficiently low for liquefaction. To
gain that end, the idea naturally occurred of using adiabatic expansion,
not intermittently, as when gas is allowed to expand suddenly from a
high compression, but in a continuous process, and an obvious way of
attempting to carry out this condition was to enclose the orifice at
which expansion takes place in a tube, so as to obtain a constant stream
of cooled gas passing over it. But further consideration of this plan
showed that although the gas jet would be cooled near the point of
expansion owing to the conversion of a portion of its sensible heat into
dynamical energy of the moving gas, yet the heat it thus lost would be
restored to it almost immediately by the destruction of this mechanical
energy through friction and its consequent reconversion into heat. Thus
the net result would be _nil_ so far as change of temperature through
the performance of external work was concerned. But the conditions in
such an arrangement resemble that in the experiments of Thomson and
Joule on the thermal changes which occur in a gas when it is forced
under pressure through a porous plug or narrow orifice, and those
experimenters found, as the former of them had predicted, that a change
of temperature does take place, owing to internal work being done by the
attraction of the gas molecules. Hence the effective result obtainable
in practice by such an attempt at continuous adiabatic expansion as that
suggested above is to be measured by the amount of the "Thomson-Joule
effect," which depends entirely on the internal, not the external, work
done by the gas. To Linde belongs the credit of having first seen the
essential importance of this effect in connexion with the liquefaction
of gases by adiabatic expansion, and he was, further, the first to
construct an industrial plant for the production of liquid air based on
the application of this principle.
[Illustration: FIG. 2.--Laboratory Liquid Air Machine.
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 air or oxygen.
H, Carbon dioxide and air outlet.
O, Air coil.
O, Carbon dioxide coil.]
The change of temperature due to the Thomson-Joule effect varies in
amount with different gases, or rather with the temperature at which the
operation is conducted. At ordinary temperatures oxygen and carbonic
acid are cooled, while hydrogen is slightly heated. But hydrogen also is
cooled if before being passed through the nozzle or plug it is brought
into a thermal condition comparable to that of other gases at ordinary
temperatures--that is to say, when it is initially cooled to a
temperature having the same ratio to its critical point as their
temperatures have to their critical points--and similarly the more
condensible gases would be heated, and not cooled, by passing through a
nozzle or plug if they were employed at a temperature sufficiently above
their critical points. Each gas has therefore a point of inversion of
the Thomson-Joule effect, and this temperature is, according to the
theory of van der Waals, about 6.75 times the critical temperature of
the body. Olszewski has determined the inversion-point in the case of
hydrogen, and finds it to be 192.5° absolute, the theoretical critical
point being thus about 28.5° absolute. The cooling effect obtained is
small, being for air about ¼° C. per atmosphere difference of pressure
at ordinary temperatures. But the decrement of temperature is
proportional to the difference of pressure and inversely as the absolute
temperature, so that the Thomson-Joule effect increases rapidly by the
combined use of a lower temperature and greater difference of gas
pressure. By means of the "regenerative" method of working, which was
described by C. W. Siemens in 1857, developed and extended by Ernest
Solvay in 1885, and subsequently utilized by numerous experimenters in
the construction of low temperature apparatus, a practicable liquid air
plant was constructed by Linde. The gas which has passed the orifice and
is therefore cooled is made to flow backwards round the tube that leads
to the nozzle; hence that portion of the gas that is just about to pass
through the nozzle has some of its heat abstracted, and in consequence
on expansion is cooled to a lower temperature than the first portion. In
its turn it cools a third portion in the same way, and so the reduction
of temperature goes on progressively until ultimately a portion of the
gas is liquefied. Apparatus based on this principle has been employed
not only by Linde in Germany, but also by Tripler in America and by
Hampson and Dewar in England. The last-named experimenter exhibited in
December 1895 a laboratory machine of this kind (fig. 2), which when
supplied with oxygen initially cooled to -79° C., and at a pressure of
100-150 atmospheres, began to yield liquid in about a quarter of an hour
after starting. The initial cooling is not necessary, but it has the
advantage of reducing the time required for the operation. The
efficiency of the Linde process is small, but it is easily conducted and
only requires plenty of cheap power. When we can work turbines or other
engines at low temperatures, so as to effect cooling through the
performance of external work, then the economy in the production of
liquid air and hydrogen will be greatly increased.
[Illustration: FIG. 3.--Hydrogen Jet Apparatus. A, Cylinder containing
compressed hydrogen. B and C, Vacuum vessels containing carbonic acid
under exhaustion and liquid air respectively. D, Regenerating coil in
vacuum vessel. F, Valve. G, Pin-hole nozzle.]
This treatment was next extended to hydrogen. For the reason already
explained, it would have been futile to experiment with this substance
at ordinary temperatures, and therefore as a preliminary it was cooled
to the temperature of boiling liquid air, about -190° C. At this
temperature it is still 2½ times above its critical temperature, and
therefore its liquefaction in these circumstances would be comparable to
that of air, taken at +60° C., in an apparatus like that just described.
Dewar showed in 1896 that hydrogen cooled in this way and expanded in a
regenerative coil from a pressure of 200 atmospheres was rapidly reduced
in temperature to such an extent that after the apparatus had been
working a few minutes the issuing jet was seen to contain liquid, which
was sufficiently proved to be liquid hydrogen by the fact that it was so
cold as to freeze liquid air and oxygen into hard white solids. Though
with this apparatus, a diagrammatic representation of which is shown in
fig. 3, it was now found possible at the time to collect the liquid in
an open vessel, owing to its low specific gravity and the rapidity of
the gas-current, still the general type of the arrangement seemed so
promising that in the next two years there was laid down in the
laboratories of the Royal Institution a large plant--it weighs 2 tons
and contains 3000 ft. of pipe--which is designed on precisely the same
principles, although its construction is far more elaborate. The one
important novelty, without which it is practically impossible to
succeed, is the provision of a device to surmount the difficulty of
withdrawing the liquefied hydrogen after it has been made. The
desideratum is really a means of forming an aperture in the bottom of a
vacuum vessel by which the contained liquid may be run out. For this
purpose the lower part of the vacuum vessel (D in fig. 3) containing the
jet is modified as shown in fig. 4; the inner vessel is prolonged in a
fine tube, coiled spirally, which passes through the outer wall of the
vacuum vessel, and thus sufficient elasticity is obtained to enable the
tube to withstand without fracture the great contraction consequent on
the extreme cold to which it is subjected. Such peculiarly shaped vacuum
vessels were made by Dewar's directions in Germany, and have
subsequently been supplied to and employed by other experimenters.
[Illustration: FIG. 4.--Bottom of Vacuum Vessel.]
With the liquefying plant above referred to liquid hydrogen was for the
first time collected in an open vessel on the 10th of May 1898. The gas
at a pressure of 180 atmospheres was cooled to -205° C. by means of
liquid air boiling _in vacuo_, and was then passed through the nozzle of
the regenerative coil, which was enclosed in vacuum vessels in such a
way as to exclude external heat as perfectly as possible. In this way
some 20 cc. of the liquid had been collected when the experiment came to
a premature end, owing to the nozzle of the apparatus becoming blocked
by a dense solid--air-ice resulting from the congelation of the air
which was present to a minute extent as an impurity in the hydrogen.
This accident exemplifies what is a serious trouble encountered in the
production of liquid hydrogen, the extreme difficulty of obtaining the
gas in a state of sufficient purity, for the presence of 1% of foreign
matters, such as air or oxygen, which are more condensible than
hydrogen, is sufficient to cause complete stoppage, unless the nozzle
valve and jet arrangement is of special construction. In subsequent
experiments the liquid was obtained in larger quantities--on the 13th of
June 1901 five litres of it were successfully conveyed through the
streets of London from the laboratory of the Royal Institution to the
rooms of the Royal Society--and it may be said that it is now possible
to produce it in any desired amount, subject only to the limitations
entailed by expense. Finally, the reduction of hydrogen to a solid state
was successfully undertaken in 1899. A portion of the liquid carefully
isolated in vacuum-jacketed vessels was suddenly transformed into a
white mass resembling frozen foam, when evaporated under an air-pump at
a pressure of 30 or 40 mm., and subsequently hydrogen was obtained as a
clear transparent ice by immersing a tube containing the liquid in this
solid foam.
_Liquefaction of Helium._--The subjection of hydrogen completed the
experimental proof that all gases can be reduced to the liquid and solid
states by the aid of pressure and low temperature, at least so far as
regards those in the hands of the chemist at the beginning of the last
decade of the 19th century. But a year or so before hydrogen was
obtained in the liquid form, a substance known to exist in the sun from
spectroscopic researches carried out by Sir Edward Frankland and Sir J.
Norman Lockyer was shown by Sir William Ramsay to exist on the earth in
small quantities. Helium (q.v.), as this substance was named, was found
by experiment to be a gas much less condensable than hydrogen. Dewar in
1901 expanded it from a pressure of 80-100 atmospheres at the
temperature of solid hydrogen without perceiving the least indication of
liquefaction. Olszewski repeated the experiment in 1905, using the still
higher initial compression of 180 atmospheres, but he equally failed to
find any evidence of liquefaction, and in consequence was inclined to
doubt whether the gas was liquefiable at all, whether in fact it was not
a truly "permanent" gas. Other investigators, however, took a different
and more hopeful view of the matter. Dewar, for instance (_Pres. Address
Brit. Assoc._, 1902), basing his deductions on the laws established by
van der Waals and others from the study of phenomena at much higher
temperatures, anticipated that the boiling-point of the substance would
be about 5° absolute, so that the liquid would be about four times more
volatile than liquid hydrogen, just as liquid hydrogen is four times
more volatile than liquid air; and he expressed the opinion that the gas
would succumb on being subjected to the process that had succeeded with
hydrogen, except that liquid hydrogen, instead of liquid air,
evaporating under exhaustion must be employed as the primary cooling
agent, and must also be used to surround the vacuum vessel in which the
liquid was collected.
Various circumstances combined to prevent Dewar from actually carrying
out the operation thus foreshadowed, but his anticipations were
justified and the sufficiency of the method he indicated practically
proved by Dr H. Kamerlingh Onnes, who, working with the splendid
resources of the Leiden cryogenic laboratory, succeeded in obtaining
helium in the liquid state on the 10th of July 1908. Having prepared 200
litres of the gas (160 litres in reserve) from monazite sand,[2] he
cooled it with exhausted liquid hydrogen to a temperature of 15 or 16°
abs., and expanded it through a regenerative coil under a pressure of 50
to 100 atmospheres, making use of the most elaborate precautions to
prevent influx of heat and securing the absence of less volatile gases
that might freeze and block the tubes of the apparatus by including in
the helium circuit charcoal cooled to the temperature of liquid air.
Operations began at 5.45 in the morning with the preparation of the
necessary liquid hydrogen, of which 20 litres were ready by 1.30. The
circulation of the helium was started at 4.30 in the afternoon and was
continued until the gas had been pumped round the circuit twenty times;
but it was not till 7.30, when the last bottle of liquid hydrogen had
been brought into requisition, that the surface of the liquid was seen,
by reflection of light from below, standing out sharply like the edge of
a knife against the glass wall of the vacuum vessel. Its boiling-point
has been determined as being 4° abs., its critical temperature 5°, and
its critical pressure not more than three atmospheres. The density of
the liquid is found to be 0.015 or about twice that of liquid hydrogen.
It could not be solidified even when exhausted under a pressure of 2
mm., which in all probability corresponds to a temperature of 2° abs.
(see _Communications from the physical laboratory at the University of
Leiden, 1908-1909_).
The following are brief details respecting some of the more important
liquid gases that have become available for study within recent years.
(For argon, neon, krypton, &c., see ARGON.)
_Oxygen._--Liquid oxygen is a mobile transparent-liquid, possessing a
faint blue colour. At atmospheric pressure it boils at -181.5° C.;
under a reduced pressure of 1 cm. of mercury its temperature falls to
-210° C. At the boiling point it has a density of 1.124 according to
Olszewski, or of 1.168 according to Wroblewski; Dewar obtained the
value 1.1375 as the mean of twenty observations by weighing a number
of solid substances in liquid oxygen, noting the apparent relative
density of the liquid, and thence calculating its real density,
Fizeau's values for the coefficients of expansion of the solids being
employed. The capillarity of liquid oxygen is about one-sixth that of
water; it is a non-conductor of electricity, and is strongly magnetic.
By its own evaporation it cannot be reduced to the solid state, but
exposed to the temperature of liquid hydrogen it is frozen into a
solid mass, having a pale bluish tint, showing by reflection all the
absorption bands of the liquid. It is remarkable that the same
absorption bands occur in the compressed gas. Dewar gives the
melting-point as 38° absolute, and the density at the boiling-point of
hydrogen as 1.4526. The refractive index of the liquid for the D
sodium ray is 1.2236.
_Ozone._--This gas is easily liquefied by the use of liquid air. The
liquid obtained is intensely blue, and on allowing the temperature to
rise, boils and explodes about -120° C. About this temperature it may
be dissolved in bisulphide of carbon to a faint blue solution. The
liquid ozone seems to be more magnetic than liquid oxygen.
_Nitrogen_ forms a transparent colourless liquid, having a density of
0.8042 at its boiling-point, which is -195.5° C. The refractive index
for the D line is 1.2053. Evaporated under diminished pressure the
liquid becomes solid at a temperature of -215° C., melting under a
pressure of 90 mm. The density of the solid at the boiling-point of
hydrogen is 1.0265.