230 HYDROGEN ION CONCENTRATION 



the close relationship of phase boundary potentials and membrane 

 potentials was suggested. A layer of an oil interposed between two 

 aqueous phases behaves, under certain conditions, hke a membrane 

 impermeable to certain ions, and hence it should be possible to regard 

 the ionic distribution at equiHbrium in both ways. It seems, how- 

 ever, that the interpretation in terms of phase boundary potentials 

 is the more comprehensive of the two. Donnan's interpretation 

 should be Umited to those cases in which the impermeabihty is actually 

 determined by a lack of diffusibiUty (of a molecule) ; where the ulti- 

 mate cause of the potential Ues in the presence of a colloid in the 

 aqueous solution; where the membrane may be imagined as a system 

 of fine canals through which all substances (except the colloid) circu- 

 late freely: and where, especially, the solvent circulating in these 

 canals is the same as in the adjacent solutions. In such a case 

 Donnan's interpretation is quite irreplaceable, and as such it has 

 assumed an important place in the theory of colloidal systems. 



