JOHN H. NORTHROP 



221 



lowed by noting the time necessary to cause a certain degree of 

 liquefaction of the gelatin. 



A series of gelatin solutions, containing 5 per cent dry weight of 

 gelatin and adjusted to various hydrogen ion concentrations by means 

 of HCl, were prepared. The gelatin had previously been purified 

 as described by Loeb.^^ The conductivity of the solutions and the 

 time necessary for them to reach an easily determined degree of 

 liquefaction were then determined. The reciprocal of this time is 

 plotted in the curve as the rate. Fig. 3 and Table II show the result 

 of a typical experiment of this kind. It is clear that the rate of diges- 





^40 

 30 



20 



o 10 



iB- pH. 4 



100 

 75 

 50 

 25 



C 

 o 





o 



Fig. 3. Influence of pH on the rate of digestion and conductivity of gelatin 

 solutions. 



tion and the conductivity of the solution have their maximum value at 

 the same hydrogen ion concentration, and that the curves are nearly 

 parallel throughout. The rate of digestion decreases shghtly more 

 rapidly than the conductivity of the solution on the alkahne side of the 

 optimum and slightly less rapidly on the acid side. This peculiarity 

 was noted in all the experiments made and can hardly be ascribed to 

 experimental errors. It shows that the digestion on the alkaline 

 side of the optimum is slightly less rapid than would be predicted 

 from the conductivity data and that it is slightly more rapid on the 

 acid side. The divergence on the acid side is due to the fact that 

 in such strongly acid solutions the acid itself has some action on the 

 protein as was shown by control experiments without any pepsin. 

 The correction is too uncertain to be applied to the figures, however. 



26 Loeb, J., /. Gen. Physiol, 1918-19, i, 237. 



