AGGLUTININS 43 



or precipitated by the addition of minute amounts 

 of salts (electrolytes). This of course is entirely 

 analogous to the need of salts in the agglutination 

 of bacteria by sera. In the latter reaction the 

 agglutinins carry a positive, the bacteria a negative 

 charge. The resulting combination, therefore, does 

 not precipitate from the menstruum supposedly 

 because there is still sufficient difference in the 

 electric potential. When salts are present the 

 kations so alter the electric conditions, of the colloi- 

 dal particles, i.e., of the agglutinin-bacterium com- 

 bination, that their surface tension is increased. 

 In order to overcome this the particles get together, 

 presenting in a clump less surface tension than if 

 they remained as individual particles. Some experi- 

 ments by Field indicate that the pro zone may be 

 explained on the assumption that the bacteria and 

 agglutinins behave as colloids. It has already 

 been stated that the union of agglutinin and bac- 

 terium does not precipitate because, possibly, there 

 is still sufficient electric potential; the combination 

 carries a negative charge. Field believes that with 

 very large amounts of agglutinin (as in the pro 

 zone) the bacteria load themselves with so much 

 agglutinin that the combination now carries a con- 

 siderable positive charge. The surface tension there- 

 fore is not sufficient to cause a clumping to occur. 

 Naturally, the presence of salts does not alter the con- 

 dition, as the kations also carry a positive charge. 

 Group Agglutinins. For some time after their 



