INTRODUCTION. XXXV 



mitting the fever to coine to a head, and then remov- 

 ing the fowl, wrapped in cotton-wool, to a chamber 

 with a temperature of 35. The strength of the 

 patient returned as the career of the parasite was 

 brought to an end, and in a few hours health was 

 restored. The sharpness of the reasoning here is 

 only equalled by the conclusiveness of the experiment, 

 which is full of suggestiveness as regards the treat- 

 ment of fevers in man. 



Pasteur had little difficulty in establishing the 

 parasitic origin of fowl cholera ; indeed, the parasite 

 had been observed by others before him. But by his 

 successive cultivations, he rendered the solution sure. 

 His next step will remain for ever memorable in the 

 history of medicine. I allude to what he calls ' virus 

 attenuation.' And here it may be well to throw out 

 a few remarks in advance. When a tree, or a bundle 

 of wheat or barley straw, is burnt, a certain amount of 

 mineral matter remains in the ashes extremely small 

 in comparison with the bulk of the tree or of the straw, 

 but absolutely essential to its growth. In a soil lacking, 

 or exhausted of, the necessary mineral constituents, the 

 tree cannot live, the crop cannot grow. Now contagia 

 are living things, which demand certain elements of life 

 just as inexorably as trees, or wheat, or barley ; and it 

 is not difficult to see that a crop of a given parasite may 

 so far use up a constituent existing in small quantities 



