CHANGE OF STATE LIQUID VAPOUR. 19? 



far from true for the halogen derivatives of benzene, but that for many 

 other substances it does not hold. The equation and its deductions 

 are to be regarded not so much as expressing the actual relations for 

 substances in general as an approximation for some substances sufficiently 

 near the truth to suggest lines of experiment, while its divergence from 

 the truth in other cases found by these experiments may ultimately lead 

 to some more correct and more general equation. The modifications 

 hitherto suggested, while more difficult to deal with, are hardly more in 

 accordance with experiment.* 



Liquefaction Of Gases. When Andrews made his celebrated 

 researches, several gases, as oxygen, nitrogen, carbon monoxide, and 

 hydrogen, had never been liquefied, and such gases were often termed 

 permanent gases. But the results which Andrews obtained with carbon 

 dioxide gave weight to the suggestion of Faraday that these gases were 

 far above their critical points at ordinary temperatures, and that they 

 could only be liquefied by cold as well as pressure. Acting on this idea, 

 Cailletet and Pictet, working independently, both succeeded at nearly 

 the same time in 1877 in liquefying oxygen and other gases. The 

 general principle of their experiments, though carried out with very 

 different details, consisted in compressing a quantity of gas under 300 or 

 500 atmospheres, cooling it by surrounding the containing vessel with 

 liquid sulphurous acid or liquid carbonic acid evaporating freely, and 

 then making a sudden expansion of the gas thus compressed and cooled. 

 The work done by the sudden expansion implied a further transfer of 

 energy from the gas, which fell below its boiling point and became 

 liquid. In Cailletet's early experiments the volume of the containing 

 vessel was suddenly increased, and the liquid appeared as a mist. In 

 Pictet's experiments the gas was allowed to issue through a nozzle, the 

 gas behind doing work on that in front, and cooling so far that ulti- 

 mately it issued as a liquid jet, and sometimes even gave signs of solidifi- 

 cation. There were doubtful indications of the liquefaction of hydrogen. f 



Much valuable work on the liquefaction of gases was done by 

 Wroblewski and Olzewski, at first working together and later inde- 

 pendently. They used the evaporation of liquid ethylene as a cooler, 

 and to find the temperature reached they used either a hydrogen ther- 

 mometer or a thermo-electric couple. Olzewski ultimately used a 

 platinum resistance thermometer. In some of his experiments he 

 placed a graduated test tube in the tube containing the compressed 

 gas, but separated from it by a narrow space. On the expansion, liquid 

 collected in and round the test tube. That inside the test tube was 

 protected by an evaporating liquid jacket, and could be preserved for 

 some time. The boiling point could then be found. Collecting the 

 vapour or gas given off from a measured volume of liquid, the liquid 

 density could be determined. 



Dewar was also one of the early workers in the field, and has made 

 very valuable researches, on a much larger scale than any other worker. 



* Further information as to Van der Waals' equation will be found in Nernst's 

 Theoretical Chemistry. 



t A full account of the various methods of liquefying gases, with a valuable 

 history of the subject, will be found in Harden's Liquefaction of Gases and Travers' 

 Study of Gases. We only mentioii a few workers to illustrate methods. 



