228 LOUIS PASTEUR. 



there will be found vaccines to suit them all, special 

 care being taken to resort to the employment of two 

 successive vaccines of unequal power, employed after 

 an interval of ten or fifteen days. The first vaccine 

 may always be chosen of a degree of weakness which 

 will not in any case cause death, and yet of sufficient 

 strength to prevent dangerous consequences from the 

 second vaccine, which would in some cases be fatal 

 if employed at once, and to enable it to act as a 

 vaccine against the most virulent virus. 



With regard to the preparation of vaccines, and the 

 ascertaining of their proper strength, it is necessary to 

 make trials upon a certain number of fowls, even at 

 the risk of sacrificing a few in these preliminary 

 experiments. Beyond such questions of manipula- 

 tion there remains still a scientific question. How 

 are the effects of vaccination to be conceived ? What 

 explanation can be given of the fact that a benign 

 disease can preserve from a more serious and deadly 

 one ? Pasteur long sought for the solution of this 

 problem. Without flattering himself that he has un- 

 ravelled the difficulty, he has nevertheless amassed 

 facts which, amid these physiological mysteries, per- 

 mit us to frame a hypothesis which can satisfy the 

 mind. Pasteur believes, for example, that the vaccine, 

 when cultivated in the body of the animal, robs the 

 globules of the blood, for example, of certain material 

 principles which the vital actions take a long time to 



