J. Dewar — Thermal Evolution of Gases. 295 



Art. XXXI Y. — The Adsorption and Thermal Evolution of 

 Gases occluded in Charcoal at Low Temperatures ; by 

 Sir James Dewar.** 



[Eead before the Koyal Society of London, June 16, 1904.] 



During the year 1874-5, in association with the late Pro- 

 fessor Tait, a research was undertaken which involved the 

 production of very perfect vacua, and with the object of 

 improving on the then known methods, dense charcoal was 

 employed as an efficient absorbent of traces of any gaseous 

 residuum. An account of these experiments communicated 

 to the Royal Society of Edinburgh appeared in Nature, July 

 15, 1875, under the title of " Charcoal Yacua." 



In Professor Clerk Maxwell's Notes on "Molecular Physics" 

 the following succinct description of the process is given : 

 " Another method employed by Professor Dewar is to place 

 in a compartment of the vessel a piece of freshly heated 

 cocoanut charcoal, and to heat it strongly during the last stages 

 of the exhaustion by the mercury pump. The vessel is then 

 sealed up, and as the charcoal cools it absorbs a very large 

 proportion of the gases remaining in the vessel. 



u The interior of the vessel, after exhaustion, is found to be 

 possessed of very remarkable properties. One of these prop- 

 erties furnishes a convenient test of the completeness of the 

 exhaustion. The vessel is provided with two metallic elec- 

 trodes, the ends of which within the vessel are within a quarter 

 of an inch of each other. When the vessel contains air at the 

 ordinary pressure a considerable electromotive force is required 

 to produce an electric discharge across this interval. As the 

 exhaustion proceeds, the resistance to the discharge diminishes 

 till the pressure is reduced to that of about a millimeter of 

 mercury. When, however, the exhaustion is made very per- 

 fect the discharge cannot be made to take place between the 

 electrodes within the vessel, and the spark actually passes 

 through several inches of air outside the vessel before it will 

 leap the small interval in the empty vessel. A vacuum, there- 

 fore, is a stronger insulator of electricity than any other 

 medium." 



At one of the conferences held in connection with the 

 Special Loan Collection of Scientific Apparatusf in the year 

 1876, I showed that with a vapor like bromine the absorptive 

 power of the charcoal was so effective that a space filled with 



* From an advance proof-sheet by the author. 



f See Science Conferences, Physics and Mechanics, p. 154. 



