JACQUES LOEB 477 



We can say that the sign and order of the values of the potentials 

 are the same in Tables V and VI indicating that the p.d. across the 

 membrane is essentially of the nature of a diffusion potential. The 

 negative p.d. across the membrane in Table V is, however, always 

 greater than the corresponding p.d. across the membrane in Table VI. 



The drop in the curves in Fig. 3 beyond a concentration of m/64 

 is due again to the diminution of e through the increase in the concen- 

 tration of salts. 



///. The Transport Curves at the Isoelectric Point. 



The main purpose of this paper is the investigation of the transport 

 curves at the isoelectric point. At this point the gelatin is not ionized 

 and salts cannot cause a charge of the particles unless they alter the 

 pH or cause the formation of complex protein salts which undergo an 

 electrolytic dissociation. This latter seems to occur when salts 

 with trivalent (or tetravalent ?) cations or salts with tetravalent 

 anions are added to isoelectric gelatin, since the addition of 

 this kind of salts has a similar effect as the addition of acid 

 or alkali respectively to isoelectric gelatin. No such effect seems, 

 however, noticeable in the case of salts of the type of NaCl, CaCl2, 

 or Na2S04. These latter salts influence the transport curves at the 

 isoelectric point in approximately the same way as does cane-sugar 

 or grape sugar; i.e., only osmotically. The transport curves for these 

 latter salts show no electrical effect at the isoelectric point but only the 

 osmotic effect (Fig. 4). In these experiments at the isoelectric point 

 special care was taken that the gelatin fihn of the membrane was at 

 the isoelectric point at the beginning of the experiment; i.e., that the 

 pH was 4.7. The gelatin used for the film formation was isoelectric 

 and in addition the collodion-gelatin bags were kept in water which 

 had been brought to pH 4.7, by adding acetic acid. The salt solu- 

 tions also were carefully brought to pH 4.7. 



Table VII shows that the p.d. across the membrane is very high 

 at pH 4.7.8 jf^ therefore, at this pH there exists only a small p.d, 



8 If we compare the p.d. across the membrane for the NaCl and CaClj and 

 CeCla solutions with the diffusion potentials (which are not given in this paper), 

 it is seen that the p.d. across the membrane is greater than the diffusion potentials 

 for the same solutions, as if the membrane retarded the motion of the cations. 

 The same was found when the pH was 3.0. It is, therefore, obvious that this 

 cation retention is not caused by the charge of the gelatin. 



