Transformation, etc., of Pathogenic Microbes. 89 



bacteridia by the preceding process is the effect of 

 the heat and not of the oxygen, as was advanced by 

 Pasteur. 



The least virulent varieties produce a mild disease 

 Avhich leaves behind it immunity for the varieties less 

 attenuated, and we thus have vaccines of different 

 degrees of strength which we can bring into action 

 in succession. 



The attenuated virus of fowl cholera and of bac- 

 teridian charbon can be restored to their normal 

 virulence by passing them through the organism, 

 first, of young individuals, and then through those 

 of individuals of gradually increasing age. 



2. Attenuation by compressed oxygen or air. — M. 

 Chauveauhassucceededin attenuating the bacteridium 

 of charbon by subjecting it to contact with pure oxy- 

 gen at a pressure of two and a half atmospheres dur- 

 ing fifteen days and over, at a temperature of 35° to 

 36°. This attenuation is transmitted through suc- 

 cessive generations of the bacteridia. He has, in 

 this way obtained an energetic vaccine which con- 

 fers immunity against charbon without giving rise to 

 accidents in the vaccinated, the vaccinating property 

 being wholly retained whilst the virulence becomes 

 progressively enfeebled until it entirely disappears. 



3. Attenuation by heat. — Heat is a powerful means 

 of destruction of germs; we also find it among the 

 number of attenuating agents. It is to Toussaint 

 that the honor belongs of having first demonstrated 

 this property of heat in connection witli charbonous 

 blood. His methods were improved, and Chauveau 

 applied himself to the task of heating small quanti- 

 8 



