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LIQUID GASES
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LIQUID GASES
.' Though See also:Lavoisier remarked that if the See also:earth were removed to very See also:cold regions of space, such as those of See also:Jupiter or See also:Saturn, its See also:atmosphere, or at least a portion of its aeriform constituents, would return to the See also:state of liquid ((Euvres, ii
.
8o5), the See also:history of the liquefaction of gases may be said to begin with the observation made by See also: 189-198) . See also:Early in the following year he published an " See also: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 See also: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 See also:air a portion of it was solidified into a See also:snow-like substance (See also:Ann. chim. phys., 1835, 6o, pp . 427-432) . Four years later J . K . See also:Mitchell in See also:America, by mixing this snow with See also:ether and exhausting it under an air See also: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 See also: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 See also:simple See also:law into the liquid state, Faraday returned to the subject about 1844, in the " See also:hope of seeing See also:nitrogen, oxygen and hydrogen either as liquid or solid bodies, and the latter probably as a See also: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 See also:great, would convert the gas into a liquid." He further surmised that the Cagniard de la Tour See also:condition might have its point of temperature for oxygen, nitrogen, hydrogen, &c., below that belonging to the See also:bath of solid carbonic acid and ether, and he realized that in that See also:case no pressure which any apparatus would be able to See also: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 See also:lower, and at the same See also:time he subjected the gases to pressures up to 50 atmospheres by the use of two pumps working in See also:series . In this way he added six substances, usually gaseous, to the See also:list of those that could be obtained in the liquid state, and reduced seven, including ammonia, nitrous See also:oxide and sulphuretted hydrogen, into the solid See also:form, at the same time effecting a number of valuable determinations of vapour tensions . But he failed to condense oxygen, nitrogen and hydrogen, the See also:original See also:objects of his pursuit, though he found See also: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 See also:change of state in these gases was confirmed by subsequent investigators, such as M . P . E . See also:Berthelot, who in 185o compressed oxygen to 78o atmospheres (Ann. chim. phys., 1850, 30, p .
237), and Natterer, who a few years later subjected the permanent gases to a pressure of 2i90 atmospheres, without result; and in 1869 See also: But subsequent knowledge showed that even this proximate liquefaction could not have taken See also: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 65o atmospheres and cooled by the cascade method (see CONDENSATION OF GASES) to — 140° C., he saw issuing from the orifice an opaque See also: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 its See also:collecting any of the liquid, which—whatever else it may have been, whether See also: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 See also:Cracow . In See also:April 1883 the former announced to the See also:French See also:Academy that he had obtained oxygen in a completely liquid state and (a few days later) that nitrogen at a temperature of – 136C., 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 See also:expanded it quickly from loo atmospheres to one, and obtained the See also:appearance of an instantaneous ebullition
.
Olszewski confirmed this result by expanding from a pressure of 'no atmospheres the gas cooled by liquid oxygen and nitrogen boiling under reduced pressure, and even announced that he saw it See also: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 See also:goal at which they aimed, their work was of great value in elucidating the conditions 21i the problem and in perfecting the details of the
apparatus employed
.
Wroblewski in particular devoted the closing years of his See also:life to a most valuable investigation of the isothermais of hydrogen at low temperatures
.
From the data thus obtained he constructed a See also:van der Waals See also:equation which enabled him to calculate the critical temperature, pressure and See also: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 See also:research
.
Still, nothing was added to the See also: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 See also:procedure at the disposal of experimenters in this See also: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 See also: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 -goo° 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 See also: This was in all See also: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 can-not say, for experiments with it were abandoned in favour of other See also:measures, which led finally to a more assured success . Vacuum Vessels.—The problem involved in the liquefaction of hydrogen was in reality a See also: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 See also:external heat, and since the See also:rate at which heat is transferred from one See also: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 See also: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 See also: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 See also:rule very efficient, although some workers, e.g . Dr Kamerlingh Onnes, of See also:Leiden, reported some success with their use . In 1892 it occurred to See also:Dewar that the principle of an arrangement he had used nearly twenty years before for some calorimetric experiments on the See also:physical constants of hydrogenium, which was a natural See also:deduction from the work of See also:Dulong and See also:Petit on See also:radiation, might be employed with See also: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 owingto the vacuum . But in addition these vessels See also:lent themselves to an arrangement by which radiant heat could still further be cut off, since it was found that when the inner See also:wall was coated with a See also:bright See also:deposit of See also:silver, the influx of heat was diminished to one-See also:sixth of the amount existing without the metallic coating . The See also: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 See also: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 See also:main vessel, and connected with a powerful air-pump . A quantity of See also:mercury having been placed in it, it is heated in an oil- or air-bath to about goo° C., so as to volatilize the mercury, the vapour of which is removed by the pump . After the See also:process has gone on for some time, the See also:pipe leading to the pump is sealed off, the vessel immediately removed from the bath, and the small subsidiary part immersed in some cooling See also: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 See also:left in the large one . The final step is to See also:seal off the tube connecting the two . In this way a vacuum may be produced having a vapour pressure of about the See also:hundred-millionth of an atmosphere at o° 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 See also: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 See also:index, which involves great reflecting See also:power, and the See also:bad heat-conducting See also:powers of mercury . With the See also:discovery of the remarkable power of gas absorption possessed by See also: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 See also:pocket within the vacuous space that it is cooled by the liquid in the interior of the vessel . Metal vacuum vessels (fig . I), of a capacity of from 2 to 20 litres, may be formed of See also:brass, See also:copper, See also:nickel or tinned See also: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 See also:carbon, See also:magnesia, or See also:silica, liquid air placed in the interior will See also: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 See also: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 See also:size as possible, were mounted in sets of three on a See also:common See also: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 See also:standard . The time required for a certain quantity of liquid air to evaporate from the interior of this empty bulb being called I, 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 1 See also:Graphite 1.3 ) Lampblack . . 4 Alumina 3 3 See also:Lycopodium 2.5 See also:Calcium carbonate z•5 Calcium fluoride 1.25 See also:Barium carbonate I.3 Mercuric iodide . Calcium phosphate . . 2.7 See also:Phosphorus (amor- See also:Lead oxide 2 phous) I See also:Bismuth oxide . . 6 . 1.5 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 i Three turns silver See also:paper, bright See also:surface inside . Three turns silver paper, bright sur- See also:face outside . . Vacuum space empty i Three turns See also:gold paper, gold outside . 4 Some pieces of gold-See also:leaf put in so as to make contact between walls of vacuum-tube . . 0.3 (b) Vacuum space empty, silvered on inside surfaces . . Silica in silvered vacuum space . . I.I It appears from these experiments that silica, charcoal, See also:lamp-See also:black, and oxide of bismuth all increase the heat insulations to four, five and six times that of the empty vacuum space . As the See also:chief communication of heat through an exhausted space is by molecular See also:bombardment, the fine powders must shorten the See also:free path of the gaseous molecules, and the slow See also:conduction of heat through the porous mass must make the See also:conveyance of heat-See also:energy more difficult than when the gas molecules can impinge upon the relatively hot See also: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 See also:Tempera- tures.-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 See also:direct weighing when immersed in such fluids . If the density of the liquid gas is accurately known, then the loss of See also:weight by fluid displacement gives the specific gravity compared to water . The metals and See also: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 See also:hydraulic pressure into coherent blocks weighing about 40 to 5o 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 See also:ice at the boiling point of air is not more than 0.93, the mean coefficient of expansion being therefore o•00008r . As the value of the same coefficient between o° C. and -27° C. is x•oo0155, it is clear the rate of contraction is diminished to about one-See also: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 o°C., far less 4° C . In other words, the See also:volume of ice at the zero of temperature would not be the minimum volume of the water See also:molecule, though we have every reason to believe it would be so in the case of the See also:majority of known substances . Another substance of See also:special See also: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 s of the co-efficient 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 See also:sulphur, which at 4o° C. has a value of x•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 be tween those temperatures, in the case of a number of hydrated salts and other substances: Density Density Mean at-185° at -{-17° See also:coe ffi cient of expansion C . C . -18between 5° C. and +17° C . See also:Aluminium sulphate (18)1 1.7194 1.6913 o•00008, i See also:Sodium biborate (io) . i•7284 I.6937 0.0001000 Calcium chloride (6) 1.7187 1.6775 0.0001191 See also:Magnesium chloride (6) 1.6039 I'5693 0.0001072 Potash See also:alum (24) . . 1.6414 1.6144 0.0000813 Chrome alum (24) . . I.7842 I.7669 0.0000478 Sodium carbonate (Io) . 1.4926 P4460 0'0001563 Sodium phosphate(12) . P5446 1.5200 0.0000787 Sodium thiosulphate (5) 1.7635 1.7290 0.0000969 See also:Potassium ferrocyanide (3) 1.8988 1.8533 0'0001195 Potassium ferricyanide 1.8944 1.8109 0.0002244 Sodium nitro-prusside (4) 1.7196 I.6803 0.0001138 Ammonium chloride P5757 I.5188 0.0o01820 Oxalic acid (2) 1.7024 1.6145 0.0002643 Methyl oxalate 1.5278 1.4260 0.0003482 See also:Paraffin 0.9770 0.9103 0.0003567 See also:Naphthalene 1.2355 I.1589 0.0003200 See also:Chloral hydrate P9744 1.9151 0.0001482 See also:Urea I.3617 I.3190 0.0001579 lodoform 4'4459 4'1955 0.0002930 See also:Iodine 4'8943 4'6631 0.0002510 Sulphur 2.0989 2.0522 0.0001152 Mercury 14.382 .. o 0000881 2 Sodium . 1.0056 0.972 0.0001810 Graphite (See also:Cumberland) . 2-1302 2.0990 0.0000733 I The figures within parentheses refer to the number of molecules of water of See also:crystallization . 2-189° 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 . See also: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 . See also:Soc . (1902) . Density of Gases at Low Temperatures.-The ordinary mode of determining the density of gases may be followed, provided that the glass See also:flask, with its carefully ground stop-See also:cock sealed on, can stand an See also: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 See also:balance-See also: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° See also:absolute is 4.420 grammes, and therefore the specific volume is 226.25 cc . According to the ordinary gaseous See also: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 See also:device discovered for warding off heat in such vacuum vessels, it remained to arrange some practi- cally efficient method for reducing hydrogen to a temperature sufficiently low for liquefaction . To gain that end, the See also:idea naturally occurred of using adiabatic expansion, not inter- mittently, 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 See also:constant stream of cooled gas passing over it . But further See also:consideration of this See also:plan showed that although the gas jet would be cooled near the point of expansion owing to the See also: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 t6 4 4 i- Vacuum space empty Three turns black paper, black outside Three turns black paper, black inside . . . Vacuum space empty . Three turns, not See also:touch- See also:ing, of See also:sheet lead . . Three turns, not touch- ing, of sheet alumi- nium Empty silvered vacuum Charcoal in silvered vacuum . . . . 1.25 4 4 immediately by the destruction of this See also:mechanical energy through See also:friction and its consequent reconversion into heat . Thus the See also: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 See also:Thomson and See also: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 See also: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 See also: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 See also:industrial plant for the production of liquid air based on the application of this principle . 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 B 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 See also: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 ;° C. per atmosphere difference of pressure at ordinary tem- peratures . 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 . See also:Siemens in 1857, See also: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 See also: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 See also:Germany, but also by Tripler in America and by Hampson and Dewar in See also:England . The last-named experimenter exhibited in See also:December 1895 a laboratory See also:machine of this kind (fig . 2), which when supplied with oxygen initially cooled to -790 C., and at a pressure of 1oo-15o atmospheres, began to yield liquid in about a See also:quarter of an See also:hour after starting . The initial cooling is not necessary, but it has the advantage of Fin . 3.–Hydrogen Jet Apparatus . A, See also: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, See also:Valve . G, See also:Pin-hole nozzle . 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 See also:economy in the production of liquid air and hydrogen will be greatly increased . 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–1qo° C . At this temperature it is still 22 See also:tithes above its critical temperature, and therefore its liquefaction in these circumstances would be comparable to that of air, taken at + 6o° C., in an apparatus like that just described . Dewar showed in 1896 that hydrogen cooled in this way and ex- panded 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 See also:representation of which is shown in fig . 3, it was now found possible at the time to collect the 16 A l J 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 . p, Air coil . 0, Carbon dioxide coil . liquid in an open vessel, owing to its low specific gravity and the rapidity of the gas-current, still the general type of the arrange- ment 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 See also:provision of a device to surmount the difficulty of with- See also:drawing the liquefied hydrogen after it has been made . The desideratum is really a means of forming an See also: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 See also:elasticity is obtained to of Vacuum Vessel. 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 . With the liquefying plant above referred to liquid hydrogen was for the first time collected in an open vessel on the loth of May 1898 . The gas at a pressure of 18o atmospheres was cooled to -20s' 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 See also:con-gelation of the air which was See also:present to a minute extent as an impurity in the hydrogen . This See also: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 See also:foreign matters, such as air or oxygen, which are more condensible than hydrogen, is sufficient to cause See also: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 See also:June 19o1 five litres of it were successfully conveyed through the streets of See also: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 success-fully 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 3o 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 See also:Helium.—The subjection of hydrogen completed the experimental See also: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 See also:decade of the 19th See also:century
.
But a year or so before hydrogen was obtained in the liquid form, a substance known to exist in the See also:sun from spectroscopic re-searches carried out by Sir See also:Edward See also:Frankland and Sir J
.
See also:Norman See also:Lockyer was shown by Sir See also: 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 loth of See also:July 1908 . Having prepared 200 litres of the gas (r6o litres in reserve) from See also:monazite See also:sand,l 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 See also:absence of less volatile gases that might freeze and See also:block the tubes of the apparatus by including in the helium See also: |