706 



Prof. H. E. Armstrong on Low-Temperature Research^- 



James Dewar's investigations and it is obvious that still more re- 

 markable effects would be obtained if the absolute zero were more 

 nearly approached — but this appears to be a very difficult matter. 

 The boihng-point of hydrogen is 20° absolute, that of helium is 

 about 4 -5'. By evaporating liquid helium under reduced pressure, 

 Kammerling Onnes has not been able to go below 3° absolute. Sir 

 James Dewar estimates that, if a gas were available as much more 



A. Liquid hydrogen. B. Liquid air. 



C. Charcoal cooled in liquid air. 



D. Connexion with air pump. 



volatile than hehum as this latter is more volatile than hydrogen^ 

 possibly the temperature might be lowered to within 0*5 of the 

 absolute zero but not to zero." ' - 



It is surprising, however, how low a temperature may be reached 

 even by the evaporation of ice alone, which can be shown to vaporize 

 at as low a temperature as - 78° C. A striking demonstration of 

 Guy Lussac's statement, that it is possible to freeze mercury by the 

 evaporation of ice, was given by Sir James Dewar in his lecture on 

 January 19, 1912 (Proceedings, xx, 581). 



