LOUIS PASTEUR 143 



some of the cultures were injected into fowls. Most 

 important of all, he found that after the fowls had been 

 treated with these old cultures they would not take 

 chicken cholera even when injected with fresh and viru- 

 lent germs. Therefore partly by chance came the dis- 

 covery of the process of vaccinating poultry against 

 cholera by use of depleted cholera germs or possibly 

 by use of the dead products remaining in old cultures of 

 these germs. 



Thus Pasteur began his efforts to reduce the vigour of 

 anthrax germs so that perchance they might not pro- 

 duce anthrax of usual destructiveness. Many highly 

 illuminating experiments were performed. Finally, by 

 growing anthrax bacteria in beef broth at high tempera- 

 tures, it was found that they flourished for a time, then 

 slowly died out. By using some of these cultures when 

 the bacteria were much depleted, it was found that sheep 

 could be given mild attacks of anthrax from which they 

 recovered. After their recovery they were given fully 

 active anthrax germs, from which the sheep promptly 

 developed bad cases of anthrax and died. Pasteur 

 then tried a first vaccination of depleted bacteria, and 

 when the sheep had recovered, gave a second mild at- 

 tack by use of bacteria much less depleted than those 

 first used, but far from normal vigour. The sheep and 

 cattle upon which this experiment was tried took suc- 

 cessive mild attacks of anthrax. Thereafter, fully 

 virulent anthrax bacteria failed to produce the disease, 

 and Pasteur announced his triumph in producing pro- 

 gressive vaccination with successful results. 



So important was this discovery that Pasteur was 



