74 THE ROYAL SOCIETY OF CANADA 



(3) Surface Condensation.- — The third theory is that the gas is 

 condensed upon the surface of the charcoal. Early experiments 

 showed that under similar conditions those gases that are more 

 easily liquefied were adsorbed to a greater extent. Thus the investi- 

 gators were led to consider the adsorbed gas as having been condensed 

 by the charcoal. This theory accounts for the large amount of gas 

 adsorbed in a small quantity of charcoal better than the two given 

 above. 



There are two other theories which have been put forward which 

 are only modifications of the above theories. The first was proposed 

 by McBain^ in 1909. His experiments on the adsorption of hydrogen 

 by charcoal showed that two processes take place. The gas is first 

 condensed on the surface of the charcoal. This takes a very short 

 time for its completion; at the most, two or three minutes. He 

 termed this process "adsorption." At the same time the gas is 

 diffusing into the interior of the charcoal forming a solid solution. 

 This requires a much longer time for its completion, about twenty- 

 four hours or more. He called this process "absorption," and 

 proposed the term "sorption" to be used when considering the two 

 processes together. 



A recent theory suggested by Langmuir is in many respects an 

 elaboration of two of the theories given above. According to Lang- 

 muir, a molecule of gas approaching the surface of a solid comes into 

 the field of force of those atoms near it, and, in general, these gas 

 molecules condense on the surface, no matter what the tempr-rature 

 may be. At the same time evaporation comes into play, the amount 

 of evaporation being dependent on the temperature. 



Adsorption is the direct consequence of the time lag between the 

 condensation and the subsequent evaporation of the molecules. The 

 adsorbed layer, according to Langmuir, does not exceed one molecule 

 in thickness, or at the most, a few molecules. In order to account for 

 the large amount of gas adsorbed by charcoal and other solids he 

 assumes that such porous bodies present a much larger surface to the 

 gas than is apparent. 



Heat of Absorptiox 



Saussure, in 1814, was the first to observe that heat is evolved 

 during the process of adsorption of gases by charcoal. Quantitative 

 measurements have been made by Chappuis, DeAvar and Titofï. 



^McBain: Phil. Mag. IS, 1909, p. 916. 



