JACQUES LOEB 53 



bining possibly with two or more molecules of gelatin) will not cause 

 or may even prevent such an increase, without altering the con- 

 ductivity of the gelatin solution. The physical qualities of a gelatin 

 solution depend, therefore, aside from the degree of its electrolytic 

 dissociation and its hydrogen ion concentration upon a constitutional 

 factor varying with the valency of the ion with which the gelatin is 

 in combination. We intend to return to this question in a later 

 publication. 



IV. 



The writer was curious to find out how gelatin reacts with neutral 

 salts when brought to the isoelectric point. This was done by treating 

 gelatin for 30 minutes with m/256 acetic acid or with m/256 or m/512 

 HCl, etc., and then perfusing the gelatin repeatedly with 25 cc. of H2O 

 in the way described. That the gelatin had reached the isoelectric 

 point was ascertained by measurements of pH, of conductivity, osmotic 

 pressure, etc. When gelatin on either side of the isoelectric point is 

 treated with m/4 or m/8 NaCl or any other salt with univalent anion 

 or cation, it shows an increase in osmotic pressure, swelling, alcohol 

 number, after the salt is washed away ; when, however, gelatin rendered 

 isoelectric in the way described is treated with the same solution of 

 NaCl, all its physical properties, conductivity, osmotic pressure, 

 swelling, alcohol number, transparency, remain unaltered after the 

 salt is washed away. It is immaterial whether we treat such gelatin 

 with NaCl, Na2S04, CaCl2, or any other neutral salt; and it is also 

 immaterial whether we use high or low concentrations of these salts; 

 the physical qualities of the gelatin remain unaltered. Numerous 

 experiments were made in which the nature of the acid, the concentra- 

 tions, the number of washings, etc., used to reach the isoelectric point 

 varied, but all yielded the same result, which means that at the 

 isoelectric point gelatin is chemically inert, incapable of reacting with 

 neutral salts. When, however, the pH of the gelatin deviates only 

 slightly from that of the isoelectric point by becoming either > or 

 <4.7, the neutral salts react with gelatin in the way described in this 

 and previous papers. 



This is exactly what our theory demands, according to which gelatin 

 can dissociate electrolytically only as an anion when pH is > 4.7, as 



