148 PROTOPLASMIC ACTION AND NERVOUS ACTION 



separated from a weaker solution by a porcelain partition; 

 the solution on the more dilute side is then positive, 

 because of the diffusion field; the solid substance of 

 the partition is positive and the water in the pores is 

 negative; accordingly the latter is drawn toward the 

 more dilute solution. Anomalous osmosis of this kind 

 has long been known; Graham (1854) observed, for 

 example, that K2SO4 showed positive osmosis in alkaline 

 solution and negative in acid when a bladder membrane 

 was used; NaCl, on the contrary, showed positive osmosis 

 in acid solution and was indifferent in neutral solution.^ 

 These changes in the direction of transport are now 

 recognized as depending on the influence of the ions on 

 the charged condition of the structural surfaces. Many 

 surfaces are rendered positive by H ions and polyvalent 

 cations; the adjacent water-layer, being then negative, 

 moves in a corresponding direction in the potential- 

 gradient between the two surfaces of the membrane. 

 Water may thus move in one or the other direction 

 according to the electrolyte content. The influence of 

 ions on the direction of transport is shown with especial 

 clearness in the experiments of Perrin^ and others on 

 electrical endosmose. The influence of ions on anomalous 

 osmosis has recently been studied by Loeb, using gelatine- 

 permeated membranes of collodion; the effect varies 

 with the Ph of the solution and becomes minimal at the 

 isoelectric point of gelatine. Isoelectric membranes can, 



^ Graham, Phil. Trans., CXLIV (1854), 117. For a recent account 

 of negative osmosis cf. Bartell, Journal of the American Chemical Society y 

 XXXVI (1914), 646; Bartell and Hocker, ihid., XXXVIII (1916), 

 1029, 1036; Bartell and Madison, Journal of Physical Chemistry^ 

 XXIV (1920), 444, 593- 



' Perrin, loc. cit. 



