Selective Permeability. 



379 



3. Relation between the Selective Adsorption of the Various Solutions and their 



Surface Tensions. 



An inspection of Table V shows that the solutes which are most strongly 

 adsorbed by the seeds of Hordeum, such as aniline, phenol, and the organic 

 acids, are those which give solutions having very low surface tensions. On 

 the other hand, those solutes which are ,not adsorbed, such as the sugars and 

 the polyhydric alcohols, give solutions which have relatively high surface 

 tensions. In fact, the order giving the extent to which the solutes are 

 adsorbed is almost the same as that for the surface tensions. Evidently, 

 Gibbs' well-known rule connecting adsorption and surface tension is 

 applicable with a fair degree of accuracy to adsorption by the colloidal 

 contents of barley seeds.* 



Most important of all, the fact that the solutes giving high surface tensions 

 do not, as a rule, penetrate the membrane, suggests that the selective action 

 of the latter is due to selective adsorption. This hypothesis accounting for 

 the selective properties of membranes of the copper ferrocyanide type has 

 already been advanced by one of us on other grounds.! The barley mem- 

 brane, like almost all other membranes, is colloidal in nature, i.e. it is 

 composed of aggregations of colloidal particles enclosing minute capillaries. 

 Hence any liquid which enters its capillaries is adsorbed liquid ; and that 

 selective adsorption of solvent or solute into the capillaries will take place 

 is practically certain.^ The results of the present investigation indicate 

 that these selective adsorption effects are both varied and considerable in 

 magnitude ; and great enough to account for the selective action. The 

 kind and degree of the selective condensation at the surface of the membrane 

 may not be exactly the same as at the surface of the starch granules, and 

 both may differ from the selective effects at a solution-air interface. But 

 the general agreement of each with Gibbs' relationship shows that a great 

 similarity exists between them. 



* The investigators have found this rule to be more or less applicable to many other 

 boundary surfaces. Vide W. C. McLewis " On the Adsorption of Various Solutes at a 

 Hydrocarbon Oil-Water Interface," 'Phil. Mag.' (6), vol. 15, p. 449 (1908); ibid. (6), 

 vol. 17, p. 466 (1908). 



t Tinker, 'Eoy. Soc. Proc.,' A, vol. 92, p. 357 (1916). 



I Loc. cit., the paper just mentioned. 



