ATTENUATION OF VIRUS. 205 



method. The attenuated virus, when used to in- 

 oculate a culture-fluid, develops more or less 

 rapidly, according to the degree of attenuation. 

 Bacilli heated for the longest time, and those sub- 

 jected to the highest temperature, are the longest 

 in showing signs of development. 



Method of Chauveau. Chauveau has attempted 

 to test experimentally the question whether sus- 

 ceptible animals might not resist infection by a 

 small number of active bacilli, and acquire im- 

 munity as the result of such inoculation. His 

 results were favorable to the view that this is true 

 as regards anthrax, at least ; and Salmon has since 

 adduced satisfactory evidence that it also applies 

 to fowl-cholera. The method adopted by Chau- 

 veau consisted in diluting infected blood from the 

 guinea-pig until a cubic centimetre of the mixture 

 contains, as nearly as can be computed, the num- 

 ber of bacilli desired. A given quantity of this 

 fluid was injected into the jugular vein of a sheep. 

 Sheep of native French breeds were invariably 

 killed when the number of bacilli introduced into 

 the circulation was about one thousand. In an 

 experiment in which two hundred and fifty bacilli 

 were injected into each of five sheep, all with- 

 stood the dose, and four showed immunity when 

 reinoculated at the end of six weeks. Immunity 

 against symptomatic anthrax was also procured by 

 the same procedure. Salmon, who has tested this 

 method in fowl-cholera, has arrived at the follow- 

 ing conclusions : 



