'56 



High Vacua and Helium. 



[June 7, 1907. 



radiometer (Plate lY., Fig. 1), with its attached charcoal bulb B, had 

 sealed on to it a tube ending in a small bulb A containino^ a srlobule 

 of mercury. The radiometer and charcoal bulb had previously been 

 heated, exhausted, and repeatedly washed out with pure oxygen gas, 

 and the mercury allowed to distil for some time into the charcoal 

 cooled in liquid air. On exposing the radiometer to the electric 

 beam the vanes began to spin. On cooling the mercury bulb in 

 liquid air, the radiometer soon became inactive ; but on replacing 

 the liquid air by ordinary water, as the temperature rose, the 

 mercury began to evaporate and the radiometer resumed its activity. 

 It was found that the particular radiometer used became active when 

 the temperature of the mercury had risen to -23^0,, which cor- 

 responded to a pressure of about a fifty-millionth of an atmosphere. 

 As an example of the limits to which charcoal exhaustion extends, 

 the following table gives the pressures obtained by the use of 

 5 grammes of charcoal attached to a bulb of 300 c.c. capacity 

 containing air at an initial pressure of about 1 • 7 mm. and at the 

 temperature of 15° C. 



Thus from an initial pressure of ^i^ of an atmosphere, the 

 pressure could be reduced to one 130-millionth of an atmosphere. 



[J.D.] 



