JOHN H. NORTHROP 



117 



tained. Since there is always some uncertainty about the zero read- 

 ing the interval from to 5 was omitted and the time to cause a 

 change of from 5 to 10 and from 10 to 15 units was taken. The 

 values obtained in this way were then averaged and the result taken 

 as proportional to the reciprocal of the amount of pepsin present. 

 By comparing this figure with the corresponding one from a control 

 containing a standard amount of pepsin, the relative quantity of 

 pepsin present could be determined, since it was found that the time 



28 



ao 





10 



20 



30 



40 50 GO 



ninutGs 

 Fig. 1. 



70 



80 



80 



100 



necessary to cause any given change was inversely proportional to 

 the amount of pepsin present. That is, double the quantity of pepsin 

 requires half the time, etc. (Arrhenius' "QT" rule).'" 



Table I and Fig. 1 give the results with a series of solutions con- 

 taining different amounts of pepsin. It will be seen that the amount 

 of pepsin can be determined with an accuracy of ± 2 to 3 per cent, 



1° Arrenhius, S., Quantitative laws in biological chemistry, London, 1915. 



