LOW-TEMPERATURE RESEARCHES 



compressed gas, which, in its turn, is made colder, both 

 before and after expansion, than any that had gone 

 before. This intensification of cooling goes on until 

 the expansion-temperature is far lower than it was at 

 starting; and if the apparatus be well arranged the 

 effect is so powerful that even the smaller amount of 

 cooling due to the free expansion of gas through a 

 throttle-valve, though pronounced by Siemens and 

 Coleman incapable of being utilized, may be made to 

 liquefy air without using other refrigerants." 



So well is this principle carried out in Dr. Hamp- 

 son's apparatus for liquefying air that compressed air 

 passing into the coil at ordinary temperature without 

 other means of refrigeration begins to liquefy in about 

 six minutes a result that seems almost miraculous 

 when it is understood that the essential mechanism by 

 which this is brought about is contained in a cylinder 

 only eighteen inches long and seven inches in diameter. 



As has been said, it was by adopting this principle of 

 self-intensive refrigeration that Professor Dewar was 

 able to liquefy hydrogen. More recently the same 

 result has been attained through use of the same prin- 

 ciple by Professor Ramsay and Dr. Travers at Uni- 

 versity College, London, who are to be credited also 

 with first publishing a detailed account of the various 

 stages of the process. It appears that the use of the 

 self -intensification principle alone is not sufficient with 

 hydrogen as it is with the less volatile gases, including 

 air, for the reason that at all ordinary temperatures 

 hydrogen does not cool in expanding, but actually 

 becomes warmer. It is only after the compressed hy- 



45 



