194 LOUIS PASTEUR, 



to a thorough believer in the germ theory, and such 

 was the solution which, in reply to a question, I ven- 

 tured to offer nearly fifteen years ago to an eminent 

 London physician.* To exhaust a soil, however, a para- 

 site less vigorous and destructive than the really viru- 

 lent one may suffice; and if, after having by means of 

 a feebler organism exhausted the soil without fatal 

 result, the most highly virulent parasite be introduced 

 into the system, it will prove powerless, f 



The general problem, of which Jenner's discovery 

 was a particular case, has been grasped by Pasteur, in 

 a manner, and with results, which five short years ago 

 were simply unimaginable. How much " accident " 

 had to do with shaping the course of his inquiries I 

 know not. A mind like his resembles a photographic 

 plate, which is ready to accept and develop luminous 

 impressions, sought and unsought. In the chapter on 

 fowl-cholera is described how Pasteur first obtained 

 his attenuated virus. By successive cultivations of the 

 parasite, he showed that after it had been a hundred 

 times reproduced, it continued to be as virulent as at 

 first. One necessary condition was, however, to be 

 observed. It was essential that the cultures should 

 rapidly succeed each other that the organism, before 

 its transference to a fresh cultivating liquid, should not 

 be left long in contact with air. When exposed to air 

 for a considerable time the virus becomes so enfeebled 

 that when fowls are inoculated with it, though they 

 sicken for a time, they do not die. But this " attenu- 

 ated " virus, which M. Radot justly calls " benign," 

 constitutes a sure protection against the virulent virus. 

 It so exhausts the soil that the really fatal contagium 



* Sir Thomas Watson. 



f Recent researches suggest other explanations. 



