INFECTION AND RESISTANCE 



"The introduction of such electrolytes into a colloidal suspension 

 is of course accompanied by a certain amount of dissociation. In 

 consequence the weakly charged particles of the colloid collect about 

 the ions of opposite charge until a sufficient accumulation of the 

 particles leads to an electrical neutralization of the ion, and the ag- 

 gregation, if of sufficient size, will sink to the bottom, forming a 

 precipitate." 



In regard to the mutual influences exerted upon each other when 

 two colloids are mixed, it has been shown by Biltz, Hardy, and 

 many other observers that oppositely charged colloids precipitate 

 each other, though this is not an absolute rule, as experiments by 

 Professor Stewart Young, of Stanford, have shown. Thus colloidal 

 gold and platinum will be precipitated by such colloids as ferric 

 oxid or aluminium oxid. When such a precipitating colloid is 

 added to another oppositely charged suspension in quantities too 

 small to bring about flocculation, moreover, the addition of a quan- 

 tity of salt, likewise too small to precipitate alone, will in many cases 

 bring about the flocculation. 



These and other phenomena of colloidal reaction have found 

 close analogy in antibody-antigen studies, and have given support to 

 the interpretation of the latter in the sense of Bordet. 



To return to the consideration of bacterial agglutination, we have 

 spoken of the dependence of the reaction upon the presence of salts, 

 and have seen that the researches of Friedberger and others have 

 refuted the assumption that the action of the salt in bringing about 

 agglutination depends upon chemical union of the salt with the 

 bacteria. It is probable, therefore, that here, as in other colloidal 

 precipitations, the function of the salt is to be regarded purely as 

 an electrophysical phenomenon. 



The analogy becomes still closer when we consider the researches 

 of Bechold, 6 ' 6 Neisser and Friedemann, 67 Sears and Jameson, 68 and 

 others, which have shown that bacteria in suspension are to be com- 

 pared very closely with true colloidal suspensions in that the bac- 

 terial cells carry a definite and uniform electrical charge. 



Bacteria in salt solution emulsion, for instance, wander to the 

 anode, thus giving evidence of their carrying a negative charge. 

 This charge may be altered by adding to the emulsions definite con- 

 centrations of acids or bases, a reversal of the charge taking place 

 under the influence of NaOH or other hydroxids. Just how this is 

 brought about is by no means clear, but it is not impossible that 

 there is a selective absorption of OH ions by the bacteria, which 

 therefore take on the charge of the ion. 



66 Bechold. Zeitschr. f. physik. Chem., 48, 1904. 



67 Neisser and Friedemann. Munch, med. Woch., Vol. 51, pp. 465, 827, 

 1904. 



68 Sears and Jameson. "Thesis for M. A. Stanford University," 1912. 



