A HISTORY OF SCIENCE 



temperature, is not sufficient to reduce a gas to the 

 liquid state. In other words, the fact of a so-called 

 "critical temperature," varying for different sub- 

 stances, above which a given substance is always a 

 gas, regardless of pressure, was definitively discovered. 

 It became clear, then, that before the resistant gases 

 would be liquefied means of reaching extremely low 

 temperatures must be discovered. And for this, what 

 was needed was not so much new principles as elab- 

 orate and costly machinery for the application of a 

 principle long familiar the principle, namely, that an 

 evaporating liquid reduces the temperature of its im- 

 mediate surroundings, including its own substance. 



Ingenious means of applying this principle, in con- 

 nection with the means previously employed, were 

 developed independently by Pictet in Geneva and 

 Cailletet in Paris, and a little later by the Cracow pro- 

 fessors Wroblewski and Olzewski, also working inde- 

 pendently. Pictet, working on a commercial scale, em- 

 ployed a series of liquefied gases to gain lower and 

 lower temperatures by successive stages. Evaporating 

 sulphurous acid liquefied carbonic acid, and this in 

 evaporating brought oxygen under pressure to near its 

 liquefaction point; and, the pressure being suddenly 

 released (a method employed in Faraday's earliest ex- 

 periments), the rapid expansion of the compressed 

 oxygen liquefies a portion of its substance. This re- 

 sult was obtained in 1877 by Pictet and Cailletet al- 

 most simultaneously. Cailletet had also liquefied the 

 newly discovered acetylene gas. Five years later 

 Wroblewski liquefied marsh gas, and the following year 

 nitrogen ; while carbonic oxide and nitrous oxide yielded 



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