'222 LOUIS PASTEUR. 



virulence in successive cultures is assured only when 

 no great interval has been allowed to elapse between 

 the cultures. For example, the second culture must 

 be sown twenty- four hours after the first, the third 

 twenty-four hours after the second, the hundredth 

 twenty- four hours after the ninety-ninth, and so on. 

 If a culture is not passed on to the following one 

 until after an interval of several days or several 

 weeks, and particularly if several months have 

 elapsed, a great change may then be observed in 

 the virulence. This change, which generally varies 

 with the duration of the interval, shows itself by the 

 weakening of the power of the contagium. 



If the successive cultures of fowl cholera, made at 

 short intervals, have such virulence that ten or twenty 

 inoculated birds perish in the space of twenty-four 

 or forty-eight hours, a culture which has remained, 

 say, for three months in its flask, the mouth of which 

 has been protected from the introduction of all foreign 

 germs by a stopper of cotton wool, which allows 

 nothing but pure air to pass through it this culture, 

 if used to inoculate twenty fowls, though it may 

 render them more or less ill, does not cause death in 

 any of them. After some days of fever they recover 

 both their appetite and spirits. But if this phenome- 

 non is extraordinary, here is one which is surely 

 in a different sense singular. If after the cure of 

 these twenty birds they are reinoculated with a very 



