46 PHYSICAL SCIENCE 



a closed vessel till the pressure rises to several 

 atmospheres, is let out suddenly, its volume is, of 

 course, greatly increased by the sudden expansion. 

 Room has to be made for the increase of volume, 

 and this process requires the expenditure of work, 

 for the atmosphere is pressing on the gas on all 

 sides, and has to be forced back when the expan- 

 sion occurs. Moreover, if the particles of the gas 

 attract each other, work must be done in the 

 separation necessary for the increase of volume. 

 Thus internal as well as external work may be 

 performed during the expansion. Unless heat is 

 supplied from without, the energy needed to 

 perform all this work must come from the heat 

 supply of the gas itself, which becomes cooled in 

 the process. If the expansion is sudden and 

 therefore rapid, there is no time for heat to enter 

 the gas, and the cooling represents the full effect 

 of the work done. By this means, Pictet finally 

 liquefied his oxygen. The highly compressed gas, 

 which had been cooled in liquid carbonic acid 

 boiling under low pressure, was allowed suddenly 

 to escape into the atmosphere. A large amount 

 of external work was thus done, intense cooling 

 resulted, and liquid oxygen was seen as spray in 

 the issuing jet of gas. It was by a still more 

 sudden expansion that Cailletet liquefied oxygen, 

 using preliminary cooling only to 30"" below the 

 Centigrade zero. 



In modern forms of apparatus for the lique- 

 faction of gases it is found advisable to sacrifice 

 the cooling gained by the performance of external 

 work, and to rely on that due to the internal work 

 alone. By this means it is possible to construct 

 much more powerful and efficient refrigerating 



