252 PASTEUR: THE HISTORY OF A MIND 



pig as surely as a drop of anthrax blood. It is, there- 

 fore, to the bacteridium that the virulence belongs. 

 Behold a conclusion of the first rank firmly established, 

 avoiding the objection which could be made to the 

 corresponding conclusion of Koch, because Pasteur, 

 at this time, knew how to make with certainty an in- 

 definite series of cultures, while Koch learned to do 

 this only later. Such is the advantage of technique. 



This first step taken, we can ask ourselves how the 

 bacteridium acts. Does it secrete a soluble poison 

 which spreads about it in the liquid, as it undoubtedly 

 spreads in the tissues of an attacked animal to produce 

 the disease and kill it? No, for the liquid of the culture, 

 filtered through a porous membrane and injected in 

 any desired quantity into a rabbit, barely makes it sick. 

 This time it was Davaine's experiment but carried on 

 under convincing conditions because the experiment was 

 not with a complex liquid like the blood, but with an 

 artificial culture of the bacteridia. 



Finally there remains the hypothesis that the bac- 

 teridium itself produces a virus in the form of living 

 granules which it disseminates in the liquid or in the 

 tissues, and which alone is the active agent. This 

 hypothesis accepts the bacteridium: its only object is 

 to connect the bacteria with the virus, whereas the actual 

 current of events is, on the contrary, to connect the 

 virus with the bacteria. But it matters not! To this 

 objection, Pasteur and Joubert responded that nothing 

 suggestive of virulent granules can be seen in the liquid 

 of a bacterial culture even when examined with the 

 highest magnifications. 'There are only well-defined 

 filaments, floating in the midst of a perfectly clear liquid. 

 We have the right to consider this reply as insufficient, 

 since nothing is to be seen in the yellow serosity of some 

 pustules of sheeppox, yet we know that it is virulent. 



