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



Perfection of Vacua obtained by means of Highly-cooled 

 Charcoal. — Separation of Gases. 



The perfection of the vacuum obtained by means of charcoal 

 at low temperatures is readily demonstrated with the aid of a 

 sparking tube CAB, such as is shown in the left-hand side of Fig. 6, 

 C beiog a bulb containing cocoanut charcoal. When such a tube 

 of 1300 c.c. capacity, sealed to a bulb containing 30 grammes 

 of charcoal, was filled with air at atmospheric pressure and then 

 cooled by immersing the charcoal bulb in liquid air, the pressure fell 

 to 50 mm. of mercury : when, however, the tube was filled at the pres- 



Fig. 6. 



sure of only half an atmosphere, on cooling the charcoal to - 185°, the 

 exhaustion reached beyond the stride stage ; and no spark would pass 

 when the initial charge was only at the pressure of a quarter of an 

 atmosphere. Using a tube containing only 1 gramme of charcoal 

 and charging it with air at a pressure of 3 mm. of mercury, the 

 vacuum produced on cooling the charcoal just reached the beginning 

 of the phosphorescent stage. Starting with an air pressure of between 

 1 and 2 mm. a volume of 300 c.c. connected with a tube containing 

 5 grammes of charcoal cooled in liquid air can reach a vacuum of 

 0*00005 mm. in one hour. 



When the tube was charged with hydrogen, to raise the vacuum 



