JACQUES LOEB 249 



of copper gelatinate (pH being about 6.3). When we wash off the 

 excess of copper acetate solution, the blue color remains. When we 

 treat such copper gelatinate with an acid to increase the hydrogen ion 

 concentration, we notice that the gelatin loses its blue color as soon as 

 its pH ^ 4.7. If after the acid treatment gelatin is perfused six 

 times with distilled water, the gelatin keeps its blue color when 

 pH > 4.7, and it is possible to demonstrate the presence of ionized 

 copper in such gelatin with the ferrocyanide test. When the pH of 

 the gelatin is ^ 4.7, the ferrocyanide test shows that the gelatin is 

 free from ionized copper. The water with which this gelatin was 

 washed contains the copper, while the water with which the gelatin 

 with a pH > 4.7 was washed is free from copper, except that which 

 was originally contained in the capillary spaces between the granules 

 of gelatin and which was carried away with the first perfusions. 



Fig. 7 gives the curves of gelatin first treated for 30 minutes with 

 m/16 copper acetate and subsequently with from m/8 to m/8192 HCl. 

 For pH ^ 4.6 all the values of the curves are as low as they are at the 

 isoelectric point, and for the same reason; namely, copper gelatinate 

 as well as the gelatin at the isoelectric point being sparingly soluble. 

 (It is well to keep in mind that at the isoelectric point the gelatin 

 contains no ionizable copper, while for pH > 4.7, the gelatin is in 

 combination with ionic copper.) On the left, more acid side from the 

 isoelectric point the gelatin exists in the form of gelatin chloride and 

 gelatin acetate. 



The experiment proves, therefore, that copper gelatinate cannot 

 continue to exist when pH ^ 4.7, and that the gelatin gives off its 

 copper at such pH. 



The same experiment was made with CeCls and Pb acetate4, and 

 almost identical curves were obtained as in the case of copper ace- 

 tate, Ce as well as Pb forming sparingly soluble gelatinates. In the 

 case of K4FeCN6 the sparingly soluble gelatin4 FeCNe was formed 

 only on the acid side of the isoelectric point as our theory demands. 

 Fig. 8 gives the curves. The gelatin was first treated with different 

 concentrations of HCl and then with m/16 K4FeCN6 in the manner 

 described for the other cases. 



