78 PROTOPLASMIC ACTION AND NERVOUS ACTION 



but not others. It is well known that the interfacial 

 tension between an adsorbing surface and a solution of 

 an adsorbable substance is a direct function of the degree 

 of adsorption of the latter. 



When the adsorbed substances are of low molecular 

 weight — e.g., in homologous series of alcohols, organic 

 acids, or similar compounds — it is usually found that the 

 order of relative adsorption is not altered by altering 

 the adsorbent, although the degree of adsorption may 

 vary widely with the different adsorbents. With more 

 complex molecules, however, relations of an apparently 

 arbitrary or specific kind often enter, and presumably the 

 relations between the molecular structure or configura- 

 tion of the adsorbing surface and that of the adsorbed 

 substance then become important. Specific adsorptions, 

 like specific chemical combinations (between enzyme and 

 substrate, or antigen and anti-body) are thus probably 

 largely dependent on similarities of chemical configura- 

 tion. Hence a good adsorbent for one substance may 

 be a poor one for another.^ Freundlich cites various 

 instances illustrating the differences between the adsorb- 

 ent powers of different materials for the same substance.^ 

 Thus charcoal adsorbs crystal violet 20 times as effec- 

 tively as silk, and 156 times as effectively as cotton wool. 

 He found that different adsorbents usually showed the 

 same order of relative adsorption for the dyes used; 

 with four solid adsorbents the general order of adsorbent 

 action was charcoal>wool> silk > cotton, but the ratios 

 of the adsorption constants varied with different dyes. 



^ Cf. Bayliss, op. cit., p. 60, for instances of specific adsorption; also 

 Bancroft's Applied Colloid Chemistry, p. 3. 



' Freundlich, Kapillarchemie, p. 155. 



