30 Mac Leod, The Place of Science in History. 



as an attenuated virus. Such an attenuated virus can 

 be employed to render a man or an animal immune, that 

 is to say, to prevent him from contracting the malady, or 

 even as a veritable medicine to cure the malady. Let 

 us add further that it is possible in a great number of 

 cases to prevent certain diseases, for example cholera, 

 from spreading and claiming new victims by destroying 

 the microbes or preventing their dissemination. The 

 new medical science created by Pasteur has already 

 accomplished wonders ; it is full of promises for the 

 future. 



In the above we have considered Pasteur's work 

 chiefly from the point of view of its practical application. 

 There remains to say a few words of the scientific prin- 

 ciple which constitutes its real foundation. 



Let us ask ourselves why organic matter, such as broth 

 or milk, decomposes (or ferments) when it is exposed to 

 ordinary air. Are the germs of the microbes which bring 

 about these fermentations always present in the atmos- 

 phere ? Or are the microbes engendered in the substance 

 which ferments, by what is called spontaneous generation, 

 the air only entering in to furnish the oxygen necessary for 

 this generation ? Both theories have found defenders, and 

 at the time when Pasteur began his investigations many 

 scientists still believed in the spontaneous generation of 

 microbes. After long researches Pasteur was able to 

 demonstrate that the germs of the microbes were present 

 in the atmosphere and floated in it in the form of dust. 

 Whenever this dust was successfully eliminated, fermenta- 

 tion did not take place. Among the numerous experi- 

 ments by which Pasteur proved this truth, we mention 

 one which it is easy to repeat. In a glass flask we place 

 a certain quantity of a liquid which, under ordinary con- 

 ditions, decomposes rapidly, for example broth. We boil 



