164 THEORY OF COLLOIDAL BEHAVIOR 



by the addition of a salt. The isoelectric point of a protein is a 

 constitutional property of the protein which need not be and 

 probably, as a rule, is not affected by the addition of a neutral 

 salt, since it is that hydrogen ion concentration at which a protein 

 dissociates equally as an acid and as a base. 



Electrical endosmose, anomalous osmosis, and kindred phen- 

 omena are due to the fact that there is a P.D. between the liquid 

 and the walls of the membrane through which the liquid diffuses. 

 It is often assumed that this P.D. is due to the adsorption of ions 

 by the membrane whereby the charge of the adsorbed ion is 

 transferred to the membrane. The writer tested this idea by 

 experiments with membranes which had received a coating of a 

 protein. He found that at the isoelectric point of the protein 

 which forms the coating no electrical transport of water occurs 

 either in electrical endosmose or in anomalous osmosis. 1 This 

 agrees with the idea that the charge of the liquid inside the pores 

 of the membrane is due to the Donnan equilibrium between 

 membrane and liquid. 



Experiments on anomalous osmosis were made to test the idea 

 whether or not salts can transfer an electrical charge to solid 

 particles of gelatin as acids or alkalies can. In order to test this 

 idea these experiments were made with liquids of pH 4.7, i.e., at 

 the isoelectric point of gelatin. In this case the gelatin is not 

 charged through acid or alkali and no electrical transport of water 

 occurs at this point in either electrical endosmose or in anomalous 

 osmosis. If now a salt, like CaCl 2 or Na 2 SO 4 , were able to trans- 

 fer a charge to the gelatin, this should betray itself by an electrical 

 transport of water through the gelatin membrane at pH 4.7 in 

 experiments on anomalous osmosis. It was found that salts, like 

 LiCl, NaCl, KC1, MgCl 2 , CaCl 2 , BaCl 2 , Na 2 S0 4 , and others, at 

 pH 4.7, leave the isoelectric gelatin uncharged, and that no 

 electrical transport of liquid occurs at pH 4.7 in the presence of 

 these salts. When, however, solutions of salts with trivalent 

 cations, such as LaCl 3 or CeCl 3 , or with tetravalent anion, like 

 Na 4 Fe(CN) 6 , (all of pH 4.7) were used, the film of isoelectric 

 gelatin assumed a charge; this charge was positive in the case of 

 CeCl 3 or LaCl 3 and negative in the case of Na 4 Fe(CN) 6 . It was 



^OEB, J., J. Gen. PhysioL, vol. 2, p. 557, 1919-20; and in unpublished 

 experiments. 



