JACQUES LOEB 



377 



These experiments furnish the proof that the effect of acid (HBr) 

 upon the physical properties of gelatin is the unequivocal function of 

 the amount of gelatin bromide formed; the probable reason for this 

 being that pure gelatin (as it exists at the isoelectric point) is prac- 

 tically insoluble (and undissociated) , while gelatin bromide is soluble. 



We must now furnish the proof that not only is there a parallelism 

 between the curve for the bromine number on the one hand and the 

 curves for the physical properties of gelatin treated with HBr, but 



Br number 0.5 



1.5 ?,0 lb iO 3.5 40 4.5 50 5.5 6.0 



Fig. 9. The abscissae represent the bromine number, the ordinates the values 

 for osmotic pressure found in three different experiments with 1 per cent gelatin 

 solution previously treated with HBr and washed four times with water. Notice 

 agreement between values. 



that to each definite Br number belongs a definite and absolute value 

 for osmotic pressure, conductivity, swelling, and viscosity. We can 

 do this by plotting the results of different experiments with the 

 values for Br numbers as abscissae and the values for osmotic pres- 

 sure, swelling, etc., as ordinates. In this case the values obtained for 

 osmotic pressure in the different experiments should differ only within 

 the limits of the accuracy of our measurements. 



In Fig. 9 the curves for three different experiments with four wash- 

 ings each are plotted for osmotic pressure and for swelling. The 



