192 LOUIS PASTEUR, 



After Koch's investigation, no doubt could be en- 

 tertained of the parasitic origin of this disease. It 

 completely cleared up the perplexity previously existing 

 as to the two forms — the one fugitive, the other perma- 

 nent — in which the contagium presented itself. I may 

 here remark that it was on the conversion of the per- 

 manent hardy form into the fugitive and sensitive one, 

 in the case of bacillus subtilis and other organisms, that 

 the method of sterilising by ' discontinuous heating ' 

 introduced by me in February 1877 was founded. The 

 difference between an organism and its spores, in point 

 of durability, had not escaped the penetration of 

 Pasteur. This difference Koch showed to be of para- 

 mount importance in splenic fever. He moreover 

 proved that while mice and guinea-pigs were infallibly 

 killed by the parasite, birds were able to defy it. 



And here we come upon what may be called a hand- 

 specimen of the genius of Pasteur, which strikingly 

 illustrates its quality. Why should birds enjoy the 

 immunity established by the experiments of Koch ? 

 Here is the answer. The temperature which prohibits 

 the multiplication of bacillus anthracis in infusions is 

 44° Cent. (Ill Fahr.). The temperature of the blood 

 of birds is from 41° to 42°. It is therefore close to 

 the prohibitory temperature. But then the blood- 

 globules of a living fowl are sure to offer a certain 

 resistance to any attempt to deprive them of their 

 oxygen — a resistance not experienced in an infusion. 

 May not this resistance, added to the high tempera- 

 ture of the fowl, suffice to place it beyond the power 

 of the parasite ? Experiment alone could answer this 

 question, and Pasteur made the experiment. By the ap- 

 plication of cold water he lowered the temperature of 

 a fowl to 37° or 38°. He inoculated the fowl, thus 

 chilled, with the splenic fever parasite, and in twenty 



