1907.] oa High Vacua and Helium. 758 



A liquid air calorimeter such as I have described in former lectures* 

 was modified so as to make a direct determination possible of the heat 

 evolved during charcoal absorption at low temperatures. The apparatus 

 will easily be understood from the drawing (Plate III.). The value of 

 hydrogen absorption per molecule found by this method was 1940 

 calories, which is of the same order of magnitude as that which re- 

 sulted from the occlusion pressure observations taken a little below 

 the boihng point of liquid air. 



RADI03IETER STUDIES. 



Xo instrument is more convenient for the demonstration of the 

 high vacuum produced by cooled charcoal than the radiometer of Sir 

 William Crookes. A convenient arrangement of the attached char- 

 coal tube is shown in Plate IV., Fig. 8. In order to wash out the 

 radiometers, it was found that a bulb containing perchlorate of potash 

 was the most reliable source of pure oxygen, and when the gases from 

 minerals have to be examined a side tube must be added. The 

 general arrangement is shown in Plate IV., Fig. 2, where A is the 

 perchlorate bulb and E the side tube. 



HELiuii Radiometer. 



A Crookes radiometer, filled with helium, having a glass tube ending 

 in a bulb containing charcoal, remained inactive to the concentrated 

 beam of the electric arc after the charcoal was cooled in liquid air, but 

 on replacing the Uquid air by liquid hydrogen the radiometer vanes 

 l)egan to spin with great rapidity. 



On further reducing the temperature by exhausting the hydrogen 

 rill it froze, the rotation seemed to be but little altered. This final 

 drop in temperature to 14° absolute without much change in the 

 motion indicated that there was still a considerable gas pressure left, 

 from which we infer, by analogy with other gases, that the freezing 

 point of hydrogen is still very much higher than the boiling-point of 

 lielium. 



HyDROCtEX PtADIOMETER. 



A Crookes radiometer was filled with hydrogen instead of helium. 

 When the charcoal bulb attached to this radiometer was immersed ir 

 liquid air, and the beam from the electric arc was focused on the 

 vanes, rotation took place. This corresponded with what happened 

 when the charcoal bulb of the helium radiometer was immersed in 

 liquid or solid hydrogen ; in both cases the cooling had rarefied the 

 gases sufficiently to permit motion. But, on allowing the vanes of 

 the hydrogen radiometer to come to rest, and immersing its charcoal 



* Roy. Inst. Proc, 1894, vol. xiv. p. 398, and 1904, vol. xvii. p. 581. 



Vol. XVIII. (No. 101) 3 c 



