JACQUES LOEB 



381 



is practically the HBr which was in combination with the gelatin, and 

 not free HBr left in the capillary spaces between the particles of pow- 

 dered gelatin after the treatment of the latter with HBr. The reader 

 will remember that in Experiment I (Table I) the acid was allowed 

 to drain off and that the gelatin solution was afterwards dialyzed for 

 about 20 hours against distilled water. In the other experiments, in 

 addition to this the last traces of acid were removed by one or more 

 additional washings with distilled water. It is a striking fact that 

 for pH > 4.7 the gelatin was always free from Br although it had 

 been treated with HBr. This coincidence of the point where Br be- 

 gins to appear in the gelatin with the value pH > 4.7 where it theo- 



TABLE II. 



retically should begin to appear speaks already against the assumption 

 that the Br number is the expression of free HBr not drained or dia- 

 lyzed or washed off. 



The direct proof lies, however, in a comparison between the pH 

 and the Br number. We select at random in Experiment III (Table I) 

 pH = 4.0. The Br number found in 25 cc. of gelatin solution is for 

 this pH 3.45 cc. 0.01 N Br. In the form of free acid this Br number 

 would represent a hydrogen ion concentration of jjqq n, which is 

 approximately too n, while the actual normality of the gelatin solu- 

 tion was 10^; i.e., less than one-tenth of 7^ (about 7 per cent). 

 Hence more than 90 per cent of the HBr existed in chemical com- 



