INTRODUCTION. XXXVii 



a hundred times reproduced, it continued to be as 

 virulent as at first. One necessary condition was, 

 however, to be observed. It was essential that the 

 cultures should rapidly succeed each other that the 

 organism, before its transference to a fresh cultivating 

 liquid, should not be left long in contact with air. 

 When exposed to air for a considerable time the virus 

 becomes so enfeebled that when fowls are inoculated 

 with it, though they sicken for a time, they do not 

 die. But this ' attenuated ' virus, which M. Kadot 

 justly calls 'benign,' constitutes a sure protection 

 against the virulent virus. It so exhausts the soil 

 that the really fatal contagium fails to find there the 

 elements necessary to its reproduction and multipli- 

 cation. 



Pasteur affirms that it is the oxygen of the air 

 which, by lengthened contact, weakens the virus and 

 converts it into a true vaccine. He has also weakened 

 it by transmission through various animals. It was 

 this form of attenuation that was brought into play 

 in the case of Jenner. 



The secret of attenuation had thus become an open 

 one to Pasteur. He laid hold of the murderous virus 

 of splenic fever, and succeeded in rendering it, not 

 only harmless to life, but a sure protection against the 

 virus in its most concentrated form. No man, in my 

 opinion, can work at these subjects so rapidly as 

 Pasteur without falling into errors of detail. But this 



