JACQUES LOEB • 253 



with anions; that on the more acid side from its isoelectric point it 

 combines only with anions but not with cations; and that at the 

 isoelectric point, pH = 4.7, it combines with neither anion nor cation. 

 This confirms our statement made in a previous paper that gelatin 

 can exist only as an anion on the alkaline side from its isoelectric 

 point and only as a cation on the more acid side of its isoelectric 

 point, and practically as neither anion nor cation at the isoelectric 

 point. 



2. Since at the isoelectric point gelatin (and probably amphoteric 

 colloids generally) must give off any ion with which it was com- 

 bined, the simplest method of obtaining amphoteric colloids approxi- 

 mately free from ionogenic impurities would seem to consist in bring- 

 ing them to the hydrogen ion concentration characteristic of their 

 isoelectric point {i.e., at which they migrate neither to the cathode 

 nor anode of an electric field) . 



3. It is shown by volumetric analysis that when gelatin is in com- 

 bination with a monovalent ion (Ag, Br, CNS), the curve representing 

 the amount of ion-gelatin formed is approximately parallel to the 

 curve for swelhng, osmotic pressure, and viscosity. This fact proves 

 that the influence of ions upon these properties is determined by the 

 chemical or stoichiometrical and not by the "colloidal" condition 

 of gelatin. 



4. The sharp drop of these curves at the isoelectric point finds its 

 explanation in an equal drop of the water solubility of pure gelatin, 

 which is proved by the formation of a precipitate. It is not yet pos- 

 sible to state whether this drop of the solubility is merely due to lack 

 of ionization of the gelatin or also to the formation of an insol- 

 uble tautomeric or polymeric compound of gelatin at the isoelectric 

 point. 



5. On account of this sudden drop slight changes in the hydrogen 

 ion concentration have a considerably greater chemical and physical 

 effect in the region of the isoelectric point than at some distance from 

 this point. This fact may be of biological significance since a num- 

 ber of amphoteric colloids in the body seem to have their isoelectric 

 point inside the range of the normal variation of the hydrogen ion 

 concentration of blood, lymph, or cell sap. 



6. Our experiments show that while a slight change in the hydrogen 



