294 LOUIS PASTEUR. 



some time in contact with the air, before passing from 

 one culture to the following one, the virulence of the 

 cultures becomes enfeebled. Thus, then, as in fowl 

 cholera, attenuated cultures of the microbe can be ob- 

 tained. Unlike what happens with cholera, however, 

 the cultures of the microbe of the saliva, exposed 

 to the contact of air, perish very quickly. Two or 

 three days of keeping suffice for the parent cultivation 

 to lose all virulence. The seed, taken in any quantity 

 from it, does not fertilise a new cultivation. But, 

 before perishing, this culture passes through very differ- 

 ent degress of progressively weakened virulence, and 

 it is easy with these cultivations to render rabbits ill 

 without causing their death. Once cured, they resist 

 all inoculation which would be mortal for others. 

 The oxygen of the air is manifestly the transformer of 

 this virulent virus into vaccine virus ; for, if the viru- 

 lent blood or cultivations remain inclosed in their 

 tubes, sealed from all entrance of the air, they retain 

 not only for some hours, but for months, their life and 

 their original virulence. 



But though these results were as new as they were 

 unexpected, and though one cause of confusion in the 

 study of this terrible problem was removed, yet these 

 first researches were not marked by any progress in 

 the etiology of hydrophobia. The question remained 

 wholly unsolved. 



