1905.] on New Low Temperature Phenomena. 179 



as regards selective action. The following table is a selection made 

 from some of his observations : — 



CocoA-NuT Charcoal Absorption of Vapours at Boiling Point 

 OF Liquid (Hunter). 



Carbon tetrachloride 4 



Chloroform 30 



Ethyl iodide 36 



Alcohol 141 



Benzol 59 



Carbon bisulphide 117 



Ether 87 



Ethylamine 127 



Water 55 



In a note read before the Royal Society of Edinburgh in March 

 1874, connected with a research undertaken in association with the 

 late Professor Tait, the absorptive power of charcoal was employed for 

 the first time in the production of high vacua. A piece of cocoa-nut 

 charcoal was placed in a glass tube, into which were sealed two 

 platinum terminals for electric sparking. The tube was exhausted by 

 the mercury pump, while the charcoal was at the same time heated to 

 a red heat. On sealing off the tube, and allowing the charcoal to 

 cool, the vacuum was so perfect that no spark would pass between 

 the terminals, from a coil giving quarter-of-an-inch sparks in air. 

 Similar experiments were repeated when investigating the theory of the 

 motion of the Crookes Radiometer, and are detailed in " Nature " in 1875. 



One advantage of charcoal vacua in the study of electric dis- 

 charges is that on heating the charcoal slowly, and connecting the 

 platinum terminals to the induction coil, the strice may be reproduced and 

 maintained at any degree of rarefaction desired, and as often as we please. 



At the Conference on May 24, 1876, in connection with the Loan 

 Collection of Scientific Apparatus at South Kensington, I showed a 

 further simplification and advance in the production of high vacua. 

 A little fluid bromine was placed in a tube, and the tube was put in a 

 water bath, so that the bromine boiled off. Meanwhile the charcoal 

 in another part of the tube was heated in the usual way, and when 

 all the bromine had been vaporised the tube was sealed off. On the 

 charcoal cooling it absorbed the bromine so thoroughly that no trace 

 of colour was visible. In this way a vacuum is produced without the 

 use of any exhausting pump. 



Although all charcoals exhibit absoi'ptive powers of a high order, 

 nevertheless differences are noticeable. Among charcoals got from 

 wood, the denser woods seem to produce the more absorptive char- 

 coals. Thus box-wood charcoal is more absorptive than fir-charcoal. 

 Saussure found the following relative absorptive powers from 

 different wood charcoals : — 



Sp. gr. Absorption. 



Cork 0*1 .. Imperceptible. 



Fir 0'4 .. 4J its vol. of air. 



Boxwood 0-6 .. 7| „ 



Russiberg coal (vegetable origin) 1-3 . . 10^ „ 



N 2 



