ADSORPTION PHENOMENA 201 



the interfacial tension, its concentration in the surface 

 layer will be identical with its bulk concentration in the 

 phase. The first case represents positive adsorption, the 

 second negative adsorption or desorption. The quantity 

 adsorbed, 1 denoted by the symbol |~\ is defined as the mass 

 of the substance in grams reckoned per unit area of the inter- 

 face in excess of the mass which would be there if capillary 

 effects were absent. 



The conclusion referred to above was expressed by Gibbs 

 in a generalised equation involving the chemical potentials of 

 the substances present, the masses adsorbed, the surface 

 tension, and the entropy. This equation has been simplified 

 and applied by Gibbs to the following case : 



" If liquid mercury meet the mixed vapours of water and 

 mercury in a plane surface, and we use fi x and /n 2 to denote the 

 chemical potentials of mercury and water respectively and 

 place the dividing surface so that P ± — o, i.e. so that the total 

 quantity of mercury is the same as if the liquid mercury reached 

 this surface on one side and the mercury vapour on the other 

 without change of density on either side, then p 2 , 1 will repre- 

 sent the amount of water in the vicinity of the surface above 

 that which there would be if the water vapour just reached 

 the surface without change of density, and this quantity 

 which we may call the quantity condensed [i.e. adsorbed] 

 upon the mercury will be determined by the equation : 



Po 1=- — 



12,1 fa 

 where c- is the surface tension. In this equation and the 

 following the temperature is constant and the surface of dis- 

 continuity plane. If the pressures in the mixed vapours con- 

 form to the law of Dalton, we shall have for constant temperature : 



dp 2= cdfi2 

 p 2 denotes the part of the pressure in the vapour due to the 

 water- vapour and c the density of the water-vapour. Hence : 



P2,i = -c-rr . . ." 

 ap2 



This expression may be converted into a more convenient 



1 Gibbs himself does not employ the term " adsorption." He speaks instead of 

 matter condensed upon a surface. 



