JACQUES LOEB 



247 



Figs. 4 and 5 confirm a fact already referred to in connection with 

 Fig. 1 . The curves for the swelHng of gelatin for pH < 4.7 are parallel 

 to the curves representing the amount of gelatin Br and gelatin CNS 

 formed. Fig. 6 gives the relation of the curve for swelling to the 

 curves for the other physical properties of gelatin treated with acid 

 and then with m/8 NaBr. These experiments prove definitely that 

 on the acid side from the isoelectric point gelatin (and probably am- 

 photeric colloids in general) can combine only with anions, on the 



i?egion of 

 Gelatin -CN5 



laoelectric 



point. 

 Pure geiatin 



■Relion of_ 

 NHA-GelaTinaTe 



90 

 80 

 70 

 60 

 50 

 40 

 30 

 20 



10 







10 



8 



6 



4 



2 







HNOs 

 cone, used 



pH 



M_ M_M_3M_M 3I1_M _M_M_. 

 32 64 126 512 256 I0Z4 512 I0Z4 im ' 



3.5 3.8 35 3.9 41 4.3 4,4 4.7 49 5.5 6.2 6.3 64 6.5 



Fig. 5. Gelatin treated with different concentrations of HNO3, washed, then 

 treated with m/8 NH4CNS, and washed again. Further explanation of figure cor- 

 responds to that for Fig. 4. 



more alkaline side from the isoelectric point only with cations, and 

 at the isoelectric point with neither. 



Action of Heavy Metals and Polyvalent Ions. 



When we treat finely powdered gelatin with a solution of m/10 

 copper acetate, the powder assumes a blue color due to the formation 



