288 LOUIS PASTEUR. 



effects as to cause it to become a vaccine for this latter 

 species. 



' The importance of this result cannot fail to be 

 perceived by everyone, for it contains the secret of a 

 new method of attenuation, which can be applied to 

 some of the most virulent viruses. We will give an 

 example and an application of it. 



' If a pigeon be inoculated in the pectoral muscle 

 with the microbe of swine fever, the pigeon dies in an 

 interval of six or eight days, after having shown the 

 apparent exterior symptoms of fowl cholera. 



' When a second pigeon is inoculated with the blood 

 of the first, a third with that of the second, and so on 

 in succession, the microbe acclimatises itself to the 

 pigeon. The symptoms of forming itself into a ball, 

 and of somnolence, which are the habitual character- 

 istics of the disease, appear in a much shorter time 

 than with the first pigeons of the series. Death 

 likewise comes on more rapidly. Finally, the blood 

 of the last pigeons exhibits much more virulence in 

 the pig than even the most infectious products of 

 a pig that has died of what is called spontaneous 

 foyer. 



'The passage of the swine fever microbe through 

 rabbits leads to quite a different result. Rabbits ino- 

 culated with the infectious products of a pig that lias 

 died of the fever, or with the cultivations of them, are 

 always made ill and most frequently die. 



