JOHN H. NORTHROP AND PAUL H. DE KRUIF 667 



millivolts, and there is no stable zone in concentrated salt. The 

 effect of the strong acid is due partly to destruction of the antibody 

 and partly to the fact that the combination of the antibody with the 

 bacteria is less complete in acid solutions. ^^ 



The results also show that the effect of all monovalent cations (ex- 

 cept hydrogen) was identical both as regards potential and agglutina- 

 tion. The valency and nature of the anion have no effect. This is 

 the usual result when the particles are negative. The bivalent cations 

 agglutinate in much lower concentration. The trivalent curves are 

 not comparable owing to changes in the pH. 



SUMMARY. 



1. The addition of proteins or serum to suspensions of bacteria, 

 {Bacillus typhosus or rabbit septicemia) at different pH widens the 

 acid agglutination zone and shifts the isoelectric point to that of the 

 added substance. 



2. The amount of serum required to agglutinate is much less near 

 the acid agglutination point of the organisms. 



3. The addition of immune serum prevents the salt from decreasing 

 the cohesive force between the organisms, and agglutination therefore 

 is determined solely by the potential, provided excess immune body 

 is present. Whenever the potential is decreased below 15 millivolts 

 the suspension agglutinates. 



'^ This point is taken up in the following paper. 



