THE PHENOMENON OF AGGLUTINATION 



the supernatant fluid after agglutination has taken place. His ob- 

 servations in this respect have not found confirmation at the hands 

 of Friedberger and other workers, and it is generally agreed to-day 

 that the role of the salts is, as Bordet first assumed it to be, a purely 

 physical one. Bordet's opinion is often spoken of as the "two phase" 

 theory, in that he conceives the process of agglutination to consist of 

 two distinct occurrences, first, an absorption of the agglutinin by 

 the bacteria, and, second, an agglutination of the new complex by the 

 salt. It is not the agglutinin which causes agglutination, but by 

 union with the agglutinogen forms a complex which is altered in 

 "colloidal stability," and therefore is flocculable by the electrolyte. 



The opinion of Bordet becomes clearer as we consider the con- 

 ditions governing the flocculation of colloids in general. Without 

 wishing to enter in this place into detail regarding the nature of 

 colloidal suspensions, it nevertheless seems necessary in order to do 

 justice to this phase of the question to recall briefly the conditions 

 governing such flocculation. The so-called colloidal solutions are 

 not true solutions as the term is applied to dissociable substances, but 

 are looked upon as consisting of small particles in suspension. The 

 particles are similarly charged, as can be demonstrated by their 

 wandering when subjected to an electric current, and it is supposed 

 that it is this fact of similarity of charge which, in the "sol" state, 

 permits them to remain in suspension. For the similarity of the 

 charges of the individual particles prevents their mutual approxima- 

 tion. 



The state of suspension of these substances, then, represents a 

 delicately balanced equilibrium between the two forces of electrical 

 repulsion and of surface tension, an equilibrium which may be 

 disturbed by the action of a number of factors. Thus, studies on 

 inorganic colloids have shown, long before these considerations were 

 applied to the explanation of serum reactions, that the stability of 

 these suspensions could be disturbed both by electrolytes and by the 

 addition of other colloids. Thus they may be precipitated by vari- 

 ous salts, acids, and bases and, as Schultze 64 has shown, they react 

 with that ion of the electrolyte which carries an opposite charge to 

 that of the colloidal particles. For, although the colloidal units are 

 similarly charged, this may be either negative or positive, according 

 to the nature of the particular substance. In the case of the so-called 

 amphoteric colloids reaction may take place, according to Pauli, 65 

 with both ions of the electrolyte. 



The probable mechanism of the process is postulated by Pauli 

 in describing the precipitation of a colloidal metal by salts, acids, or 

 bases in the following way: 



64 Schultze. Journ. f. prakt. Chem., 25, 1882, and 27, 1883. 



65 Pauli. "Hofmeister's Beitrage," 1905, and "Physical Chemistry in 

 Medicine," Wiley & Son, N. Y., 1907. 



