122 Sir James Dewar. The absorption, etc., of Gases [June 15, 



"The Absorption and Thermal Evolution of Gases occluded in 

 Charcoal at Low Temperatures." By Sir JAMES DEWAR, 

 M.A., D.Sc., LL.D., F.R.S., Jacksonian Professor, University 

 of Cambridge, and Fullerian Professor, Ptoyal Institution, 

 London. Received June 15, Read June 16, 1904. 



During the year 18745, in association with the late Professor 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 Vacua." 



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. 



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

 of very remarkable properties. 



"One of these properties furnishes a convenient test of the 

 completeness of the exhaustion. The vessel is provided with two 

 metallic electrodes, 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 millimetre of mercury. When, however, 

 the exhaustion is made very perfect 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 

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

 therefore, is a stronger insulator of electricity than any other medium." 



At one of the conferences held in connection with the Special Loan 



ction of Scientific Apparatus* in the year 1876, I showed that 



a vapour like bromine the absorptive power of the charcoal was 



rtive that a space filled with the vapour even at atmospheric 



ure could be made into a fairly high vacuum showing very wide 



striae. 



* See ' Science Conferences,' Physics and Mechanics," p. 154. 



