128 THE FITNESS OF THE ENVIRONMENT 



magnitude of that quantity in the case of 

 water. 



Finally, surface tension is of great impor- 

 tance, indeed in simple cases is the one effective 

 agent, in the phenomenon called adsorption. 1 

 On the basis of thermodynamical considera- 

 tions first developed by Willard Gibbs, it is 

 easy to show that whenever the dissolution 

 of a substance changes the surface tension 

 of a solvent, the distribution of the dis- 

 solved substance will not be strictly homo- 

 geneous. If the solution has a lower surface 

 tension than the solvent, the surface of the 

 solution will become more concentrated than 

 the interior; or if the surface tension of the 

 solution be greater than that of the solvent, 

 the surface of the solution will become less 

 concentrated than the interior. This result, 

 quite insignificant in simple solutions, becomes 

 a matter of much moment when, as in the 

 case of suspensions of fine particles like ani- 

 mal charcoal, in emulsions, jellies, or any 

 other system of like disperse heterogeneity 

 of physical constitution, there occurs very 

 great increase of surface area. Then it is 



1 A familiar example of adsorption is the use of bone- 

 black to decolorize sirup in the process of sugar refining. 

 The colored matters are almost completely removed from 

 solution, and cling to the surface of the charcoal. 



