250 PASTEUR: THE HISTORY OF A MIND 



the virus in opposition to the microbe these persons now 

 asked: Why are not bacteria present in all virulent diseases? 

 However, an answer to this question was just beginning 

 to be found in a simple perfection of technique which had 

 demonstrated the presence of organisms where their ex- 

 istence was suspected, but where they had not been seen. 



Their discovery was easy in anthrax, where they pass 

 into the blood, even before death. It is more difficult, 

 even in anthrax, to trace them in the organs of the body. 

 There were only very imperfect methods for doing that: 

 the treatment of the tissue with potash, as Davaine 

 advised, or with acetic acid, as in von Recklinghausen's 

 method. We cannot be too grateful to C. Weigert for 

 the great service he rendered in 1875 by teaching us 

 how to stain bacteria with basic anilin colors, and 

 thus to make them visible in the tissues. Two years 

 later, Koch achieved a new advance in technique 

 by teaching us to study unstained structures micro- 

 scopically with a very subdued light and stained objects 

 with a flood of light [structure-picture vs. color-picture]. 



We see, from this brief exposition, that the science was 

 mature, and that, moreover, it was thoroughly equipped 

 for new discoveries. What did it lack? Faith, the 

 conviction that it would not be deceived on entering 

 these new paths, and that there were genuine bacterial 

 diseases. It is this demonstration that Pasteur gave. 



YII 



PASTEUR: THE BACTERIDIUM IS THE SOLE CAUSE 



OF ANTHRAX 



To this question : Is it a virus? Is it a microbe? Pas- 

 teur was happily in a better position to reply than any 

 one would have been in 1877. From his Etudes sur 



