392 Prof. H. E. Armstrong on Loiv-Temperature Research 



at about this temperature. Actually the solidification of liquid 

 nitrogen is easily effected by bubbling hydrogen through it. A 

 similar argument shows that liquefied hydrogen should be solidified 

 by passing the more volatile helium through it, and this was found 

 to take place when mixtures of hydrogen and helium, cooled to the 

 temperature of exhausted liquid air, circulated through regenerator 

 coils. 



Liquid Air and liquid Hydrogen Calorimeters. — An achievement 

 of great importance which comes within the period under review is 

 the use of liquefied air and hydrogen as calori metric substances by 

 the evaporation of which the heat given out on cooling various sub- 

 stances through a known range of temperature — their heat capacities 

 in fact — can be accurately determined. 



In the case of air, the apparatus used is that shown in Fig. 12, in 

 which B is the calorimeter — a small vacuum flask some 25-50 c.c. in 



Fig. 12. 



capacity— placed inside a larger vacuum vessel A, C being a tulje 

 connected with B by the flexible joint D and containing the substance 

 to be dropped intoB. The arrangement at the side represents an 

 alternative method of introducing the substance in single pieces at a 

 time into the calorimeter. By heating or cooling C or C, it is 

 possible to determine the heat capacity of the substance between some 

 particular temperature and that at which the liquid in the calorimeter 

 boils. In practice, A is charged with a couple of litres of old liquid 

 air rich in oxygen and some of the same liquid is introduced into B ; 

 in this way it is possible to maintain a fairly constant temperature 

 throughout an experiment. The instrumeut is standardised by 



