378 



AMPHOTERIC COLLOIDS. Ill 



variations lie within the Hmit of error. In the experiments plotted 

 in Fig. 10 the number of washings of gelatin varied. In spite of the 

 difference in the treatment we notice that for the same Br number 

 practically the same value of osmotic pressure was found in all 

 experiments. Since the curves for the other properties are parallel 

 to the curve for osmotic pressure, it is not necessary to reproduce 

 all the curves. 



We therefore reach the conclusion that the variation of the physical 

 properties of gelatin under the influence of HBr is an unequivocal func- 

 tion of the number of gelatin bromide molecules formed and that col- 

 loidal speculations not based on the laws of classical chemistry are 

 neither needed nor warranted. 



300 

 275 

 250 

 225 

 200 

 175 

 150 

 1 25 

 lOOl 



75 



50Kt 







25it 



: pr<;S5aT2 



o — Not washed 



■• One washing 



• Two washings 



< — rour „ 

 D---5rx 



Br number 05 1.0 1.5 10 25 10 i5 4o 45 50 5.5 6.0 6,5 7.0 75 80 8.5 9.0 9.5 IQO 



Fig. 10. Abscissae represent bromine number, ordinates, values for osmotic 

 pressure found in different experiments with 1 per cent gelatin solution previously 

 treated with HBr and washed a different number of times. 



IV. Titration of Gelatin with NaOH. 



The following facts constitute an important link in the proof for a 

 chemical theory of the action of electrolytes (in our case HBr) upon 

 the physical properties of gelatin. 



We notice that in all the curves the gelatin was found to be abso- 

 lutely free from Br at the isoelectric point as well as on the alkaline 

 side from the isoelectric point; i.e., it was free from Br whenever 

 pH > 4.7. Yet this gelatin had been treated with HBr. The fact 

 that HBr cannot combine or remain in combination with gelatin for 



