1906.] on Studies on Charcoal and Liquid Air. 437 



the question arises how much more could be got in by the use of 

 higher pressure. The amounts absorbed by 6*7 grammes of charcoal, 

 at the temperature - 185° C. for various pressures are given in the 

 following table : — 



Pressure in Volume in 

 Atmospheres, Cubic Centimetres. 



1 620 



5 925 



10 1,050 



15 1,000 



20 975 



25 925 



The amount absorbed is seen to increase with the pressure to 

 10 atmospheres, after which the absorption in the pores of the 

 charcoal seems to be independent of the pressure. At the tempera- 

 ture of liquid air this sample of charcoal would not absorb more than 

 one litre of hydrogen, even when the pressure was raised from 10 to 

 25 atmospheres or the absorption had come to a limit. Increased 

 absorption, which does not reach more than twice that at the ordinary 

 atmospheric pressure, is the sole result of the use of higher pressures. 



Vacua produced hy Charcoal under different circumstances. 



The absorptive power of charcoal for air is very remarkable. On 

 stopping the passage of a stream of air into a charcoal bulb immersed 

 in liquid air, to which a small manometer was atUxched, hardly 

 any gas pressure was shown. This was further exemplified by the 

 action of a Rontgen tube, partially exhausted, connected to a char- 

 coal bulb. On immersing the bulb in liquid air the characteristic 

 phosphorescent glow of the Rontgen tube was soon reached, but 

 disappeared when the liquid air was removed. When hydrogen was 

 used instead of liquid air, the pressure was so much reduced that 

 the glow was entirely stopped. A Crookes's radiometer, filled with 

 hydrogen at atmospheric pressure, had a charcoal bulb attached. 

 On immersing the charcoal bulb in liquid air no rotation took place, 

 even when the beam of an electric lamp was thrown upon it ; but 

 when liquid hydrogen replaced the liquid air the motion became 

 exceedingly rapid. This shows that high vacua can be attained by 

 the use of charcoal in an atmosphere of pure hydrogen, provided it is 

 cooled in a liquid hydrogen bath instead of liquid air. A similar 

 radiometer, filled with helium at atmospheric pressure, remains 

 inactive even when liquid hydrogen is the cooling agent, thus 

 proving that the absorption of the helium by charcoal at 20° abs. 

 is relatively inefficient for the production of vacua as compared with 

 hydrogen. This is only another mode of proving the much greater 

 volatihty of hehum over hydrogen. 



