HIS LIFE AND LABOURS. 193 



four hours it was dead. The argument was clinched 

 by inoculating a chilled fowl, permitting the fever to 

 come to a head, and then removing the fowl, wrapped 

 in cotton-wool, to a chamber with a temperature of 45. 

 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 collusiveness of the ex- 

 periment, which is full of suggestiveness as regards the 

 treatment of fevers in man. 



Pasteur had little difficulty in establishing the para- 

 sitic origin of fowl-cholera; indeed, the parasite had 

 been observed by others before him. But, by his suc- 

 cessive cultivations, he rendered the solution pure. His 

 next step will remain for ever memorable in the history 

 of medicine. I allude to what he calls ' virus attenua- 

 tion.' 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 

 in the body, but essential to the growth of the parasite, 

 as to render the body unfit for the production of a 

 second crop. The soil is exhausted, and, until the lost 

 constituent is restored, the body is protected from any 

 further attack of the same disorder. Some such expla- 

 nation of non-recurrent diseases naturally presents itself 

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