JOHN H. NORTHROP 115 



tions, however, as hydrochloric, there is a rapid decrease in the con- 

 ductivity — due to the removal of acid ions by combination with 

 the hberated amino groups. This change is so much larger than the 

 increase due to the acid groups set free by the protein that the 

 increase in conductivity due to the latter is more than compensated. 

 It has already been shown "^ that the actual rate of digestion is approxi- 

 mately the same in all acids at the same reaction so that the differ- 

 ences in the changes in the conductivity cannot be ascribed to 

 differences in the rate of digestion. 



It was found that the maximum change occurs in strong solutions 

 of egg albumin titrated to pH 2.6 with hydrochloric acid. This 

 solution was therefore used. The conductivity was followed by 

 means of the apparatus described by Taylor and Acree.^ The elec- 

 trodes were of the dipping type and were immersed in the solution in 

 order to make a reading. It was found that the percentage change 

 in conductivity was constant for a given quantity of pepsin, irre- 

 spective of the absolute value of the original conductivity. The 

 readings and figures are therefore given in terms of the increase in 

 the scale readings on the bridge, which for small readings are equiva- 

 lent to the percentage change. ' The measurements were carried out 

 as follows. 



25 cc. of a 3 per cent egg albumin solution were pipetted into a 

 series of large "Non-sol" test-tubes and suspended in a water bath 

 at 37° ± 0.02°. The electrodes were immersed in the solution and 

 1 cc. of the pepsin solution was added. The external resistance was 

 then set so as to give a bridge reading of 500; i.e., the middle of the 

 bridge. The change in conductivity was now followed by the bridge 

 readings. These figures are related to the actual change in resistance 



X A 



of the solution by means of the formula — = — ttt r where X = 



^ R 1,000 - A 



resistance of the solution, R is the external resistance, and A is the 



bridge reading. 



The figures given in Table I are the increase in the value of A and 



are very nearly proportional to the percentage increase in the 



resistance. 



8 Northrop, J. H., /. Gen. Physiol, 1918-19, i, 607. 



^ Taylor, W. A., and Acree, S. F., /. Am. Chem. Soc, 1916, xxxviii, 2396. 



