92 Manual of Veterinary llicrobiology. 



Pasteur in attenuating the virus of rabies. The cords 

 of rabid rabbits, suspended in vessels containing 

 pieces of caustic potash and maintained at a tempera- 

 ture of 20°, lose their virulence in seven days ; the 

 diminution takes place progressively, so that, by fix- 

 ing the successive degrees of virulence by inocula- 

 tion to the rabbit, we can obtain virus of gradually 

 increasing intensity. 



Attenuation is here the result of several factors : 

 desiccation, oxygen, and the temperature. The in- 

 fluence of the last agent is itself very great if we 

 can judge by the increased rapidity with which at- 

 tenuation is produced when the temperature is even 

 slightly increased ; the virulence is, in fact, obliterated 

 in seven days at 20°, in five days at 23°, and in 24 

 hours at 35°. 



6. Attenuaiion by antiseptics. — Chemical substances 

 which are toxic for bacteria can diminish the vitality 

 of the latter when they are employed in selected doses 

 and their contact maintained during definite periods 

 of time. 



The bacillus of symptomatic charbon is attenuated 

 to the point of becoming a vaccine in contact with 

 sublimate, at 1 to 5000, with carbolated glycerine, 

 eucalyptol, and thymol. Carbolic acid, at 1 to 800, 

 allows the multiplication of the charbon bacteridium 

 but prevents sporulation. Continuation of the con- 

 tact with this solution gradually weakens the viru- 

 lence. The same results are obtained, but much 

 more rapidly, with bichromate of potassium in the 

 proportion of from 1 in 2000 to 1 in 5000. 



The soluble substances secreted by germs can di- 

 minish the virulence of other germs ; thus bouillon 



