ADSORPTION PHENOMENA 209 



bubble travelled, carrying with it the adsorbed layer of solute. 

 The apparatus is shown in the accompanying figure (fig. 2). 

 The total adsorbing surface exposed was obtained by knowing 

 the total amount of air which had come from the reservoir 

 and the total number of bubbles passed through the column. 

 The degree of accuracy is considerably greater than in the 

 experiments at the liquid/liquid interface. Donnan and Barker 

 estimate that the change in concentration of the solute pro- 

 duced by the passage of the bubbles is accurate to about 6 per 



cent. We now turn to the calculated adsorption, viz. — ^~ -j-. 



In evaluating this expression, the numerical value attached to 

 R must be that for one gram of the solute, and this is obtained 

 by dividing the molar value of R by the molecular weight. If 

 we use the formula weight, we assume that nonylic acid is 

 completely undissociated ; but at the great dilution employed 

 it may actually be nearer complete dissociation. In other 

 words, van 't Hoff's factor i changes from 1 to 2. The results 

 have been calculated on both assumptions, and the following 

 table indicates the agreement between calculated and observed 

 values : 



Table 3 



T x 107 calculated 

 for »" = 1 for i — 2 



o'55 0*26 



1-14 0-57 



1 "26 0*63 



[1-63 o-8i] 



In view of the difficulties involved in measuring p directly, 

 the agreement is very good, and the results may be regarded 

 as a quantitative confirmation of Gibbs' expression. The 

 same authors investigated the adsorption of saponine at the 

 air surface and found approximate agreement between calcu- 

 lated and observed values, though, owing to the peculiar 

 behaviour of saponine in other circumstances, the conclusion 

 is less certain than it is in the case of nonylic acid. 



Preferential or Selective Capillary Adsorption 



This phenomenon is met with in the case of the simultaneous 

 adsorption of two or more substances. If two substances are 

 competing for a surface, it is evident that the resultant effect 



