142 



KNOWLEDGE, 



[August 1, 1892. 



produced by heating volatile liquids, such as alcohol or ether 

 ill closed tubes. In these experiments the liquid put into 

 the tube was sufficient to till it half- full, and the whole of 

 the tube was heated ; so that the conditions were different 

 from those of Faraday's experiments. De la Tour observed 

 that up to a certain temperature the liquid continued slowly 

 to evaporate, the bulk of liquid diminishing, and the quantity 

 of vapour increasing. This was the ordinary process of 

 evaporation, which the eye can trace by observing that the 

 position of the mcniscux which separates the liquid and the 

 vapour becomes lower and lower as the temperature 

 increases. 



When, however, a certain temperature is reached the 

 meniscu.f suddenly disappears, and there no longer appears 

 to be any hquid in the tube. On cooling the tube, the 

 inverse change takes place with equal suddenness, a mnusciiti 

 (the line of separation between liquid and vapour) suddenly 

 appears, showing that on lowering the temperature a large 

 auiount of liquid is suddenly formed. De la Tour's 

 experiments point to conclusions quite opposite to those of 

 Davy, quoted above; since the "approximation of 

 particles " brought about by pressure was more than 

 counterbalanced by the repellant action of heat. 



In 1845, in a second paper on Liquefaction of Gases, 

 Faraday writes with a mastery of the subject which neither 

 he nor Davy possessed in 1820 He says (Phil. Trans., 

 1845, p. 155) : " My hopes of success beyond that heretofore 

 obtained depending more upon depression of temperature 

 than on the pressure which I could employ in these tubes, 

 I endeavoured to obtain a still greater degree of cold. 

 There are, in fact, some results no pressure may be able to 

 effect. Thus, solidification has not yet been conferred on 

 a fluid (/.(■., a gas) by any degree of pressure. Agaui, that 

 beautiful condition which Caignier de la Tour made 

 known, and which comes on with liquids at a certain heat, 

 may have its point of temperature for some of the bodies to 

 be experimented with, as oxygen, hydrogen, nitrogen, 

 &c., below that belonging to the bath of carbonic acid and 

 ether, and in that case no pressure which any apparatus 

 could bear would be able to bring them into the liquid or 

 the solid state." The " bath of carbonic and ether" here 

 referred to, was a device for obtaining a very low tempera- 

 ture, a land of improved freezing mixture in fact. Thilorier 

 and afterwards Natterer had constructed apparatus in 

 which carbonic acid gas could be liquefied by pressure 

 alone. When liquid carbonic acid is exposed to the air at 

 the ordinary pressure, some of the liquid evaporates very 

 rapidly, thereby chilling the lower layers of liquid to such 

 an extent that they freeze, forming solid carbonic acid. If 

 ether be mixed with this solid carbonic acid, and if the 

 pressure be diminished by means of the air pump, the 

 further cooling due to evaporation of the ether reduces the 

 temperature to - 110° C. This extremely low temperature 

 was employed in experiments by which Faraday endeavoured 

 unsuccessfully to liquefy hydrogen, oxygen and nitrogen. 

 Andrews employed the same means of producing cold, but 

 used more powerful apparatus for compression. Although 

 by combined cold and pressure the above-named gases were 

 reduced to less than -g^j^ of their original volume no lique- 

 faction took place. Andrews also conducted experiments 

 upon the phenomenon observed by De la Tour, and showed 

 that above the " critical temperature " of 31° C. the greatest 

 pressure which he could bring to bear was not sufficient 

 to liquefy carbonic acid, but that if the temperature were 

 lowered the liquefaction took place at once. 



In the later and successful experiments on the lique- 

 faction of the "permanent" gases, such as nitrogen, 

 oxygen, and hydrogen, the skill of the experimenter has 

 been chiefly shown in the means devised for obtaining 



temperatures below the "critical point" of these gases. 

 Pictet, of Geneva, who liquefied oxygen in 1877, relied for 

 the production of a low temperature upon the well-known 

 principle of latent heat of evaporation. The novelty in 

 his application of this principle lay in the fact that he 

 employed tiro evaporating substances — namely, sulphur- 

 dioxide and carbonic acid. By the use of the doubly- 

 acting pumps employed in refrigerating machinery (vide 

 Knowledge, March, 1891, "Artificial Cold") liquid 

 sulphur-dioxide can be obtained having a temperature of 

 — 65° C. In Pictet's apparatus the cold liquid sulphur- 

 dioxide is contained in an annular vessel, which forms a 

 jacket round the tube in which carbonic acid is condensed 

 by the action of another doubly-acting pump. 



The liquid carbonic acid is reduced to a temperature of 

 G5° C. by the cooling action of the " jacket," and, 

 consequently, when the pump is reversed and used as an 

 exhaust pump, the evaporation of the cooled liquid produces 

 an extremely low temperature, and the vapour can again 

 be condensed so as to form a liquid jacket round the oxygen 

 tube, having a temperature of —130° C. This tube of copper 

 was made very strong to withstand pressure. The oxygen 

 gas was passed into the tube direct from a strong iron 

 retort in which it was evolved, and thus the pressure 

 continued to rise as more and more oxygen entered the 

 tube. At a pressure of about 500 atmospheres, the 

 manometer remained stationary, showing that liquefaction 

 had begun. The whole of the tube was at length filled 

 with liquid oxygen, which was examined by opening 

 a stoiJcock, when a jet of a lustrous liquid issued with 

 great force from the tube, to be speedily dissipated by 

 evaporation. 



Cailletet, who worked independently at the same problem, 

 first succeeded in liquefying oxygen on the very same day 

 as Pictet. He employed simpler appliances, and worked 

 on a different principle. He relied for obtaining his 

 frigorific effect upon the chaleur de ile.tcittf, or latent heat of 

 expansion, of the gas with which he was working. The 

 term " latent heat of expansion " is not a very good one, 

 as gas is not cooled by merely expanding to fill a vacuum. 

 When, however, a gas is allowed to expand in such a way 

 as to do mechanical work, the gas loses in heat the 

 thermal equivalent of the mechanical work performed. 

 In Cailletet's experiments, the gas was contained over 

 mercury in the capillary bore of an immensely strong 

 glass tube. The tube was screwed into an hydraulic- 

 press worked by the leverage of a large wheel. The 

 experimental glass tube was surrounded by a freezing 

 mixture of which the temperature for the experiments was 

 not lower than about —30° C. So that in CaiUetct's 

 experiments no attempt was made to surround the gas 

 with a very cold atmosphere. When the pressure attained 

 300 atmospheres the oxygen still remained in the gaseous 

 condition, being much above the critical temperature, but, 

 on suddenly withdrawing the constrahiing force from the 

 piston of the press, the gas as suddenly expands, the 

 elasticity or spring of the gas drives back the liquid and 

 the piston, and the sudden mechanical eflbrt of the gas is 

 accompanied by a sudden chill sufficient to brmg the 

 temperatirre below the critical point. The liquefied oxygen 

 was seen in the tube immediately the pressure was 

 released. Cailletet's method had the advantage, that the 

 process could be watched through the glass walls of the 

 tube. On the other hand, Pictet's arrangement enabled 

 him to prepare a larger quantity of the material, and to 

 observe its behaviour when exposed freely to the atmos- 

 phere. As we have said, the liquefied oxygen was 

 dissipated immediately by evaporation, so that no ex- 

 amination of its properties could be made. A fine mist, or 



