AGGLUTINATION AND DISSOLUTION. 135 



Third : — If we add the fresh serum of a normal animal to cholera 

 serum previously heated to 55 degrees, and which consequently 

 no longer effects a granular transformation but still clumps the 

 vibrios, we find that the bactericidal power is restored to the pre- 

 ventive serum, that is, it again produces granules; and yet heated 

 preventive serum alone is an excellent culture medium, and normal 

 serum alone has only slight bactericidal power. In other words, 

 the two constituents of the mixture, separately, are slightly, if at 

 all, bactericidal; when united, however, they act energetically on the 

 vibrio. Normal serum restores to the preventive serum that sub- 

 stance which heat has destroyed, but it is incapable of restoring 

 this property when itself heated to 55 degrees. It is rather striking 

 to note how very small an amount of preventive serum, whether 

 fresh or heated to 55 degrees, will endow normal serum with strong 

 bactericidal activity. From these facts we concluded that the 

 intense destructive power against bacteria present in immune serum 

 is due to the action of two distinct substances, one of which is 

 characteristic of immunized animals, is endowed with specificity, 

 is capable of acting in a very small dose, and resists heat; and the 

 other of which is present in normal as well as immunized animals, 

 is destroyed by heating to 55 degrees, is not in itself specific, and 

 has only a slight activity when not associated with the first substance. 

 Without indulging in hypotheses as to the intimate mechanism of 

 the action of the two substances, we offered as a probable explana- 

 tion that the specific substance by immobilizing and clumping 

 bacteria renders them more susceptible to the bactericidal sub- 

 stance (alexin) present in the serum of normal as well as of immun- 

 ized animals. 



It may easily be understood then why the injection in normal 

 animals, of either fresh or heated cholera serum, gives rise to a 

 specific bactericidal power in their serum;* the specific substance 

 unites in the injected animal with the alexin already present. The 

 serum obtained from an animal after such an injection contains, 

 therefore, the two substances, the presence of both of which is neces- 

 sary to affect the vibrio seriously as indicated by granular metamor- 



* We noted in 1895 that if a normal guinea-pig is injected with preventive 

 serum active against the vibrio Metchnikovi, the serum of this animal becomes 

 bactericidal for the vibrio Metchnikovi, but not for the cholera vibrio. 



