226 LOUIS PASTEUR. 



microbe. A culture has a duration of some days ; in 

 twenty-four hours it is not terminated. The air which 

 comes in contact with it is then entirely employed 

 in nourishing and largely reproducing the microbe. 

 During the longer intervals of culture, the air acts 

 only as a modifier, and at last there arrives a moment 

 when the virulence is so much weakened as to become 

 nil. 



This very extraordinary fact is, then, established 

 that the virulence may be entirely gone while yet 

 the microbe lives. The cultures offer the spectacle of 

 a microbe indefinitely cultivable, yet, on the other 

 hand, incapable of living in the bodies of fowls, and 

 in consequence deprived of virulence. May not this 

 domesticated microbe, as M. Bouley calls it, be com- 

 pared to those inoffensive microbes of which there 

 are so many in nature ? May not our common mi- 

 crobes be those organisms which have lost their former 

 virulence? But may not these harmless microbes, 

 become infectious in some particular circumstances ? 

 And if there are fewer virulent maladies now than 

 there were in times past, might not the number of 

 these maladies again increase ? 



Questions multiply as the facts relating to the 

 attenuation of a virus suggest inductions, awaken 

 ideas, and throw new lights upon a problem which, 

 until within these last few years, has remained so 

 obscure. Formerlv it was believed that these viruses 



