' IMMUNITY 165 



for obtaining an attenuated virus of anthrax, as 

 he had successfully employed in chicken-cholera. 

 A difficulty was soon encountered, for in cultiva- 

 tions of the bacillus with free access of air spore- 

 formation readily takes place, and the spores are 

 well known to have an extraordinary power of 

 retaining their virulence. Pasteur found that the 

 bacilli ceased to develop at 45 C., and he believed 

 that spore-formation ceased at 42 43 C., the 

 bacilli continuing to develop by fission only. The 

 cultivations were, therefore, kept at this tempera- 

 ture, and at the end of eight days the bacilli were 

 found to have lost their virulence, and were quite 

 inert when inoculated in guinea-pigs, sheep, or 

 rabbits. This total destruction was, however, 

 preceded by a gradual mitigation, so that a virus 

 could be obtained, by taking it at the right time, 

 which only gave a mild disease, and afforded 

 subsequent protection. 



At Melun, in 1881, the protective inoculation 

 against anthrax was put to a practical test. Sheep 

 and oxen were inoculated with the mitigated virus, 

 and then with a virulent form ; at the same time 

 other sheep and oxen were inoculated with the 

 virulent form without previous vaccination as a 

 control experiment. The unprotected sheep died 

 without exception ; the unprotected oxen suffered 

 from cedematous swellings at the seat of inocu- 

 lation, and a rise of temperature; but all the 

 protected animals remained healthy. 



