JOHN H. NORTHROP 



791 



has emphasized the undoubted fact that in certain cases this mutual precipitation is 

 due to removal of the stabilizing ions rather than to any direct reaction between the 

 particles themselves. 



In general, suspensions of the same charges do not precipitate one another. It has 

 usually been assumed that the mutual precipitation of oppositely charged suspensions 

 is also an electrical phenomenon, and that agglutination occurs owing to the neutral- 

 ization of the charges. As Bancroft' has pointed out, however, the effect is not purely 

 a neutralization one, since the relative order of flocculation of a series of positive sus- 

 pensions by a series of negative suspensions is not always the same. It is evidently 

 necessary to consider the combination as separate from the neutralization. It is pos- 

 sible, also, that the difference maybe partially due to a difference in the critical poten- 





Agglufination -fff to C 

 - +0-H- 



FiG. 2. — Agglutination of bacillus of rabbit septicemia by egg albumin at different pH 



tials. Figure 2 represents the results of a series of experiments in which a suspension 

 of bacteria was agglutinated by the addition of egg albumin. The figure shows that 

 the agglutination is again determined solely by the potential. It also shows that the 

 potential of the particles is affected even where the protein has the same sign of charge 

 as the particles, since at pH 3 the addition of the (positive) egg albumin renders the 

 organisms still more positive. The fact that the effect of the potential is the result of 

 the combination rather than the cause is clearly shown in Figure 3, in which the 

 amount of "agglutinin" combined with the bacteria at various pH is compared with 

 the amount required to agglutinate. According to Michaelis, the antibody is positive 

 on the acid side of pH 5.0, so that a maximum effect would be expected between this 

 point and pH 3. This is the case with the agglutination, but no difference is noticeable 

 in the amount of antibody combined. 



Hitchcock^ was able to show by direct measurement that egg albumin was taken 

 up by collodion membranes irrespective of whether the membrane and albumin were 

 of the same or opposite charge. 



It will be noted in Figure 2 that the albumin stabilizes the suspension in some pH 



' Bancroft, W. D.: op. ciL, p. 226. New York and London, 1921. 

 2 Hitchcock, D. I.: J. General Physiol., 8, 61. 1925. 



