230 Boiling Point of Liquid Hydrogen under Reduced Pressure. 



exhaustion takes place through C. In this way the cold vapours are 

 drawn over the outside of the hydrogen vacuum vessel, and this helps 

 to isolate the liquid from the connective currents of gas. To effect 

 proper isolation the whole apparatus ought to have been immersed in 

 liquid air under exhaustion. Arrangements of this kind add to the 

 complication, so in the first instance the liquid was used as described. 

 The liquid hydrogen evaporated quietly and steadily under a pressure 

 of about 25 mm. of mercury, without the least appearance of solidifica- 

 tion or loss of mobility ; still remaining clear and colourless to the eye. 

 Naturally the liquid does not last long, so the resistance has to be taken 

 quickly. Just before the reduction of pressure began, the resistance 

 of the thermometer was O131 ohm. This result compares favourably 

 with the former observation on the boiling point, which gave a resist- 

 ance of O129 ohm. On reducing the pressure, the resistance diminished 

 to 0'114 ohm, and kept steady for some time. The lowest reading of 

 resistance was 0*112 ohm. This value corresponds to -239-lC., or 

 only one degree lower than the boiling point at atmospheric pressure, 

 whereas the temperature ought to have been reduced some 10 C. 

 or in any case 5 under the assumed exhaustion. The position 

 of the observation on the curve of the relation of temperature 

 .and resistance for No. 7 thermometer is shown on the accompanying 

 diagram. The question arises then as to what is the explanation of 



