Dewar's Method of producing High Vacua, 497 



The retentive force between the tinfoil and the press- 

 spahn increases far more rapidly than the voltage, the contact 

 must therefore be more perfect with the higher voltage ; this 

 accounts in some measure for the changes of apparent dielec- 

 tric-resistance with voltage shown in Tables II. and V. It 

 is possible that a similar effect may account for small dif- 

 ferences of capacity observed with different voltage with some 

 tinfoil condensers, and it may also serve to explain to some 

 extent certain abnormal " electrifications " with cables, where 

 for any reason there is imperfect contact between the con- 

 ductor and the dielectric. 



LVII. Dewar's Method of producing High Vacua. By Lord 

 Blythswood, LL.D., and H. S. Allen, M.A., B.Sc* 



§1. TT has long been known that charcoal, freed from gas 

 JL by heating to redness, is capable of absorbing gases 

 in large quantities. The subject was first investigated by 

 Saussure, and a detailed examination of this property of 

 charcoal was made by Hunter (Phil. Mng. [4] vol. xxv. p. 364, 

 1863, & xxix. p. 116, 1865). Of all the charcoals he examined, 

 that made from the cocoanut had by far the greatest absorb- 

 ing power, one volume of the charcoal absorbing 171*7 volumes 

 of ammonia, 17*9 of oxygen, 15*2 of nitrogen, and 4*4 of 

 hydrogen. In 1875 Professor Dewar employed this property 

 of charcoal to improve the vacuum in a vessel exhausted by 

 a mercury-pump. His recent discovery that when the char- 

 coal is cooled to the low temperatures now available it is 

 capable of absorbing still larger quantities of gas, is of great 

 importance. This result promises to be of the greatest service 

 in scientific research, and it is even possible that it may have 

 important commercial applications. 



§2. Sir James Dewar f has described experiments to test 

 the amount of exhaustion reached by the use of a given weight 

 of cocoanut charcoal. He found that 30 grams, cooled to the 

 temperature of liquid air, absorbed so much air that the pressure 

 in an electric-discharge tube of 1300 c.c. capacity fell from 

 760 to 50 mm. of mercury. Starting with the tube initially 

 at half an atmosphere, the exhaustion reached was now 

 beyond the striae stage. A further experiment, starting with 

 one-fourth of an atmosphere, gave a vacuum through which 

 no discharge passed. 



* Communicated by the Authors. A preliminary account of some of 

 the experiments described below has been given by one of us in a paper 

 read before the Royal Philosophical Society of Glasgow, March 22nd, 

 1905. 



t Proc, Roy. Soc. Ixxiv. pp. 122-131 (1904). 

 Phil. Mag. jS, 6. Vol. 10. No. 58. Oct. 1905. 2 M 



