niS LIFE AND LABOURS. 195 



to find there the elements necessary to its reproduction 

 and multiplication. 



Pasteur affirms that it is the oxygen of the air 

 which, by lengthened contact, weakens the vims 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 ob- 

 tained in presence of so many witnesses at Melun 

 must remain an ever-memorable conquest of science. 

 Having prepared his attenuated virus, and proved by 

 laboratory experiments its efficacy as a protective 

 vaccine, Pasteur accepted an invitation from the Presi- 

 dent of the Society of Agriculture at Melun to make a 

 public experiment on what might be called an agricul- 

 tural 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 closing 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 unvaccinated. A 

 number of cows were also subjected to a precisely 

 similar treatment. Fourteen days afterwards, all the 



