THE VACCINE OF SPLENIC FEVER. 235 



attenuation which was practised with the microbe of 

 cholera these germs of splenic fever may be exposed 

 for years to the air without losing their virulence, 

 always ready to reproduce themselves without any 

 appreciable change, and to manifest their effects in 

 the bodies of animals. How can it be hoped to dis- 

 cover a vaccine of splenic fever by the method used 

 with the contagium of fowl cholera, since the splenic 

 fever virulence, at the end of twenty-four hours, is con^ 

 centrated in a spore ? Before the oxygen of the air has 

 had time to attenuate the contagium, the virulence of 

 the parasite would be encased in these spores. Yet 

 this objection did not appear insuperable to Pasteur. 

 Since (said he to himself), under its filamentous form, 

 the microbe of splenic fever is quite analogous to the 

 microbe of fowl cholera, may not the problem of ex- 

 posing the splenic microbe to the air be reduced to 

 the following one : to determine the conditions which 

 would prevent the production of spores ? The diffi- 

 culty would thus be surmounted ; for, once we have 

 got rid of the spores, the splenic filaments might be 

 maintained in contact with air for any length of time, 

 and we might then no doubt fall back upon the con- 

 ditions which had produced the attenuation of the 

 cholera microbe. 



Pasteur and his two assistants gave themselves up 

 to this research. Days passed and experiments were 

 multiplied. Pasteur became more and more en- 



