THE LIQUEFACTION OF GASES 45 



through 100°. If, instead of water, we had taken 

 some liquid of low boiling-point, such as liquefied 

 sulphur dioxide, or, better still, liquefied carbonic 

 acid, the same process of cooling under exhaustion 

 would have taken place ; but the final temperature 

 reached would have been much lower. 



Starting then with some substance like sulphur 

 dioxide, which is easily liquefied by pressure alone 

 at ordinary temperatures, we can boil it away 

 under exhaustion, and so produce a low tempera- 

 ture. By making a more refractory gas, such as 

 carbonic acid, circulate through a tube surrounded 

 with the cold sulphur dioxide, this new agent is 

 cooled below its critical point, and liquefied. In its 

 turn the liquid carbonic acid is boiled away under 

 low pressure, and used as a refrigerating agent to 

 cool the gas — oxygen, let us say — which we are 

 attempting to conquer. This, sometimes called 

 the cascade method of cooling, was the plan 

 adopted by the Swiss physicist, Pictet of Geneva, 

 in the experiments which, simultaneously with 

 those of his French contemporary Cailletet, first 

 liquefied oxygen. With one of those curious 

 coincidences which the broad wave of advancing 

 knowledge sometimes produces, both these results 

 were announced at a memorable meeting of the 

 French Academy, held on the 24th of December 



1877. 



Even when the gas was thus cooled, however, 

 Pictet's process was not entirely effective. In 

 order to pass the last few degrees and reach the 

 critical point, a second method of cooling had to 

 be brought into play. To explain this second 

 method other principles must be taken into 

 account. When a certain mass of gas, forced into 



