JOHN H, NORTHROP AND PAUL H. DE KRUIF 



645 



times. The sediment finally obtained was then suspended in a volume of dis- 

 tilled water equal to one-half that of the original broth. For the determinations 

 one volume of this "standard" suspension was added to one or two volumes of the 

 other solutions used. Table II shows that no noticeable change could be detected 

 after the second washing. 



Effect of the Manner of Mixing and Time of Standing on the Potential. — 

 No difference could be detected in the results obtained when the suspension 

 was added to the solution or vice versa, provided the mixing was rapid and com- 

 plete. As a rule the suspension was squirted into the solution from a pipette and 

 mixed as thoroughly and rapidly as possible. No significant changes occurred in 

 the potential measurements over an interval of 2 days except in the case of silver 

 salts. The effect on the potential is, therefore, almost instantaneous in most 

 cases. This is also true of the effect of immune serum, and shows that the time 

 clement consists in the time required for the organisms to come into contact. ^^ In 



TABLE II. 



Efect of Washing on Rate of Migration of Type D Suspension. 

 100 cc. of broth culture. Type D, were centrifuged, suspended in distilled H2O, 

 centrifuged, and the process was repeated as noted. Migration was determined 

 as noted. 



No. of times washed 



Potential at pH 4.4 

 " " pH 3.0 



-28.0 

 + 1.8 



the case of suspensions treated with silver salts at a pH of 4 or more, the potential 

 drops rapidly and is very much lower after 24 hours. At the same time the sus- 

 pension turns black so that the effect is probably due to the reduction of the silver. 



EXPERIMENTAL RESULTS. 



The results of the experiments are shown graphically in Figs. 2 to 8. 

 The calculated potential in millivolts between the surface of the 

 organism and the surrounding liquid is plotted as ordinates, and the 

 salt concentration as abscissae. Since there is some doubt as to the 

 correctness of the formula connecting velocities to millivolts, the 

 actual velocities corrected for a potential drop of 1 volt per centi- 

 meter have also been given. The degree of agglutination is indicated 

 by the character of the line. In the experiments in which no pH value 



^' This conclusion had been reached by F. L. Gates (/ . E.xp. Med., 1922, .x.\.\v, 63) 



in a study of the time required for adsoq^tion of immune body. 



