HIS LITE AND LABOURS. 195 



fails to find there the elements necessary to its repro- 

 duction and multiplication. 



Pasteur affirms that it is the oxygen of the air 

 which, by lengthened 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. Xo 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 re- 

 main an ever-memorable conquest of science. Hav- 

 ing prepared his attenuated virus, and proved by labora- 

 tory experiments its efficacy as a protective vaccine, 

 Pasteur accepted an invitation from the President of 

 the Society of Agriculture 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 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 



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