94 THE POPULAR SCIENCE MONTHLY. 



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 "attenuated" 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 fails to find there the elements 

 necessary to its reproduction and multiplication. 



Pasteur affirms that it is the oxygen of the air which, by length- 

 ened contact, weakens the virus and converts it into a true vaccine. 

 He has also weakened it by transmission through various animals. It 

 was this form of attenuation that was brought into play in the case of 

 Jenner. 



The secret of attenuation had thus become an open one to Pasteur. 

 He laid hold of the murderous virus of splenic fever, and succeeded 

 in rendering it, not only harmless to life, but a sure protection against 

 the virus in its most concentrated form. No man, in my opinion, can 

 work at these subjects so rapidly as Pasteur without falling into errors 

 of detail. But this may occur while his main position remains impreg- 

 nable. Such a result, for example, as that obtained in presence of so 

 many witnesses at Melun must surely remain an ever-memorable con- 

 quest of science. Having prepared his attenuated virus, and proved, 

 by laboratory experiments, its efficacy as a protective vaccine, Pasteur 

 accepted an invitation, from the President of the Society of Agricult- 

 ure at Melun, to make a public experiment on what might be called 

 an agricultural scale. This act of Pasteur's is, perhaps, the boldest 

 thing recorded in this book. It naturally caused anxiety among his 

 colleagues of the Academy, who feared that he had been rash in clos- 

 ing with the proposal of the president. 



But the experiment was made. A flock of sheep was divided into 

 two groups, the members of one group being all vaccinated with the 

 attenuated virus, while those of the other group were left unvacci- 

 nated. A number of cows were also subjected to a precisely similar 

 treatment. Fourteen days afterward all the sheep and all the cows, 

 vaccinated and unvaccinated, were inoculated with a very virulent 



