304 BACTERIOLOGY. 



the production of the desired degree of vir- 

 ulence the spinal cord is dried in air free from 

 water and carbonic acid ; by that means the 

 virus is weakened. The duration of the pro- 

 cess of attenuation determines the amount of 

 decrease in virulence. If animals are inocu- 

 lated w r ith the weakest material and then with 

 a slightly stronger, and so on, it becomes pos- 

 sible eventually to inoculate safely with the 

 fresh and most virulent cord of a rabbit. The 

 animals thus vaccinated no longer die from 

 rabies ; they have even become immune to a 

 more virulent rabic virus than is likely to 

 occur in nature. 



In all these cases, cultures or viruses were 

 used which, though attenuated, were still active, 

 but in 1887 Hueppe discovered in the case of 

 chicken cholera and Wildseuche, and Chauveau 

 in 1889 in the case of anthrax that even cul- 

 tures altogether attenuated, which had become 

 purely saprophytic and no longer excited 

 any sort of pathogenic action could never- 

 theless confer immunity against virulent cul- 

 tures. 



In all the inoculations hitherto described the 

 material used in inoculation, however obtained, 

 and whatever degree of attenuation it might 

 possess, was identical in kind with the infective 

 substance. Protective inoculation would seem 



