202 TECHNOLOGY OF BACTERIA. 



been proposed since Pasteur first announced (1880) 

 that the microbe of fowl-cholera could be modified, 

 by special treatment, in such a manner that it no 

 longer produced a fatal form of the disease ; and 

 that fowls inoculated with this " attenuated virus " 

 were subsequently protected against the disease, 

 resisting inoculation with the most potent virus. 



Method of Pasteur. Pasteur found that the 

 poison of fowl-cholera was most virulent when ob- 

 tained from fowls which had died from a chronic 

 form of the disease, and that this virus could be 

 cultivated in chicken-bouillon for many successive 

 generations without any diminution of its potency, 

 if the interval between two successive inoculations 

 was not greater than two months. But when a 

 greater interval than this was allowed to elapse, 

 the disease produced by inoculation was of a less 

 serious character, and fewer deaths occurred. This 

 diminution of virulence became more marked in 

 proportion to the length of time during which a 

 culture-solution containing the microbe remained 

 exposed to the action of the atmosphere, and at 

 last all virulence was lost, as a result of the death 

 of the parasite. That this result is due to contact 

 with the oxygen of the air is shown by the fact 

 that virus enclosed in sealed tubes does not undergo 

 this modification, but retains its full virulence for 

 many months. According to Pasteur, the various 

 degrees of modification of virulence produced by 

 prolonged exposure to oxygen are preserved by 



