324 EXPLANATION OF SPECIFIC INHIBITION 



case of toxin, which is exactly equal to toxoid in this respect, and 

 Dreyer and Jex-Blake bring forward very strong evidence against 

 this view. Some .of the more important of their facts may be 

 summarized thus : If it were true, the more the serum is weakened 

 by heat (and the greater the production of agglutinoid) the greater 

 should be the zone of inhibition, and vice versa. This they found 

 not to be the case, for a serum that had had its agglutinating power 

 very largely destroyed by heat might show a very small zone of 

 inhibition, whilst another which had been hardly injured might 

 have a very large one. Further, a serum might show a zone with 

 an emulsion of bacteria in saline solution, but not in broth. 



The alternative explanation of all these phenomena is based on 

 facts observed in the mutual precipitation of colloids, for here 

 again exactly similar zones of inhibition are seen. For example, 

 as Neisser and Friedemann have shown, a suspension of particles 

 of mastic in water (made by dropping an alcoholic solution into 

 water) takes on a negative charge, and can be precipitated by 

 positive colloids or by positive ions. Thus, if ferric chloride be 

 added precipitation occurs, and if the dose be increased it gradually 

 becomes more and more rapid and abundant until a certain amount 

 of ferric chloride is present, after which it becomes less and less 

 until no reaction takes place at all. And similar facts have been 

 observed in numerous other cases of interaction of colloids. Their 

 explanation is somewhat as follows : When the two colloids are 

 present in such an amount that their electrical charges mutually 

 annul one another, and they are therefore formed into aggregates 

 which tend to run together, the addition of fresh colloid bearing 

 an electric charge disturbs the conditions, and may, by electrifying 

 the masses already formed, cause mutual repulsion, and, if suf- 

 ficient amount of the second colloid be added, a re-solution of all 

 .the masses. Hence solutions, say, of an electro-negative colloid 

 may be dependent on (a) the presence of mutually repellent mole- 

 cules or aggregates of molecules in an inert fluid, or (b) on the 

 presence of these molecules in a fluid also containing a large 

 number of molecules of opposite sign, whereas in the intermediate 

 mixtures the conditions for precipitation are present. Here the 

 precipitate is soluble in excess of both substances, just as pre- 

 cipitum is soluble in excess either of precipitin or of its antigen. 



This theory of the action of agglutinins has been investigated 

 and strongly supported by Biltz, Neisser and Friedemann, Pauli, 

 and others, and Dreyer and Jex-Blake have in particular given 



