THE STABILITY OF BACTERIAL SUSPENSIONS. 



IL The Agglutination of the Bacillus of Rabbit Septicemia 

 AND OF Bacillus typhosus by Electrolytes. 



By JOHN H. NORTHROP and PAUL H. De KRUIF. 



{From the Laboratories of The Rockefeller Institute for Medical Research.) 

 (Received for publication, May 22, 1922.) 



It is a very old observation that the stability of suspensions is 

 markedly affected by the addition of electrolytes.' Under certain 

 conditions the particles remain separate, while under other conditions 

 they adhere to each other. In the latter case the particles settle 

 rapidly and the suspension is said to be coagulated or agglutinated.^ 

 Since under certain conditions the particles remain distinct and in 

 others collect into large masses, it is evident that there is a force which 

 tends to hold them together and another force which tends to keep 

 them apart. If the attractive force is greater than the repulsive force, 

 the particles agglutinate. It was early found that nearly all sub- 

 stances in suspension are electrically charged with reference to the 

 surrounding liquid, and it was suggested by Jevons that the repulsion 

 due to this charge was the repelling force. This conception was sub- 

 stantiated by Hardy, who found that suspensions of denatured pro- 

 teins coagulated at the point at which they carried no electric charge. 

 Hardy called this the isoelectric point. Hardy's experiments have 

 been greatly extended by Michaelis and his coworkers. It is prob- 

 able, however, that the precipitation of proteins and the agglutina- 



^ For a review of the literature on this subject see Burton, E. F., The physical 

 properties of colloidal solutions, London, New York, Bombay, Calcutta, and 

 Madras, 2nd edition, 1921. In regard to proteins see Michaelis, L., Die Wasser- 

 stoffionenkonzentration, Berlin, 1914. 



^ The rapid settling of the coagulated suspension is a secondary phenomenon 

 due to the increase in size of the particles and governed presumably by Stokes' 

 law. The primary phenomenon is the adherence or repulsion of the individual 

 particles. 



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