CHANGES WROUGHT IN PATHOLOGY. Ill 



particles of A. Every new addition of B will encounter 

 new conditions in this regard, and, as experience has 

 taught, usually conditions less favorable so far as pre- 

 cipitation is concerned. The result is that under these 

 conditions of partial saturation more precipitating ma- 

 terial is used up than when all is added at once, and 

 that a series of intermediate bodies between the pure 

 substances A and B and the fully neutralized mixture 

 AB are formed. These intermediate bodies are built 

 according to the type xAyB t in which x and y vary 

 within certain limits. 



A third possibility that interests us is the following. 

 The aggregates formed through neutralization of the par- 

 ticles of two oppositely charged colloids are often held 

 loosely together through slight electric forces. If an 

 excess of one or the other colloid is added to such a 

 precipitate, the new particles will, because of their elec- 

 trical charge, enter into competition with the attraction 

 forces existing in the neutral aggregates, and by diminishing 

 their size and electrifying the particles cause the pre- 

 cipitate to go back into solution. As soon as a certain 

 quantitative relation exists between the two colloids, the 

 precipitate will therefore attain a maximum and will go 

 back into solution as soon as one or the other colloid 

 is present in excess. This is a familiar and well-studied 

 relation existing between colloids. 



The experimental facts that have just been recited 

 were arranged so as to be ready for immediate application 

 to one of the most important and interesting chapters 

 of medicine, the immunity reactions. We shall deal 

 more particularly with that difficult and much-argued 

 relation between toxin and antitoxin. On thiss ubject 



