COLLOIDAL THEORY OF ANTIBODIES 325 



strong evidence in its favour. For example, they find definite 

 zones of inhibition in the precipitation of B. coli by orthophosphoric 

 acid, a substance in which the existence of agglutinoids is out of 

 the question. Thus, in a series of tubes each containing 1-5 c.c. 

 of an emulsion of this organism in normal saline solution, agglu- 

 tination occurred in the tubes to which amounts from 118 centi- 

 grammes down to 4 centigrammes were added, and also in those 

 containing from i-i milligrammes to o-oi milligramme, but there 

 was none in which amounts from 4 centigrammes to 1*1 milli- 

 grammes were added. Further, there is a close analogy between 

 this phenomenon and the precipitation of gum mastic by ferric 

 chloride, for in each case the extent of the zone of inhibition 

 diminishes as the emulsions of the substance precipitated are made 

 stronger. 



The facts at our disposal are not yet sufficient to enable the 

 phenomena of agglutination of bacteria to be fully explained, but 

 the process may take place somewhat as follows : Normal bacteria 

 pass toward the anode when an electric current is passed through 

 the fluid, and are therefore to be regarded as particles carrying a 

 negative charge. The addition of agglutinin removes this charge, 

 and the particles become electrically inert. This is shown by the 

 fact that when placed in an electric current they do not move in 

 either direction. If, however, very large amounts of agglutinin 

 are added, the point of neutrality may be reached and passed, and 

 the bacteria acquire a positive charge, again becoming mutually 

 repellent. Remarkable facts which may have some bearing on 

 this subject have been brought forward by Neisser and Friede- 

 mann. Emulsions of mastic are, as we have seen, precipitated by 

 ferric hydroxide, and it has been shown that the addition of a 

 small amount of organic colloid, whether positive or negative or 

 inert like gelatin, protects these particles, so that they are no longer 

 agglutinated by colloids or electrolytes of opposite sign. A pheno- 

 menon probably similar is seen when salts which form a colloidal 

 precipitate are mixed in a solution of gelatin, when the mixture 

 remains transparent, the colloidal particles being prevented from 

 undergoing flocculation. Silver chromate, formed by the inter- 

 action of potassium chromate and silver nitrate, may be obtained 

 in a transparent and stable form by this process. Particles thus 

 protected are not carried in either direction by an electric current. 

 Neisser and Friedemann have shown that mastic emulsions are 

 " protected " by the addition of a little serum, leech extract, or 



