Microhic Diseases Individually Considered. 193 



bon cultures is very stable ; this is due to the pres- 

 ence of the spores which are little subject to change. 

 When it is desired to attenuate these cultures it is 

 necessary to begin by preventing the formation of 

 spores. Pasteur succeeded in this by cultivating the 

 charbon bacilli at the temperature of 42° to 43°. 

 Multiplication of the bacilli still continues, but spores 

 are no longer formed. Now, if such cultures are 

 kept in contact with the air their virulence rap- 

 idly diminishes ; after twelve days they no more kill 

 adult guinea pigs, and vaccinate the rabbit and the 

 sheep ; the power of vegetation, however, still per- 

 sists, and is absolutely extinguished only at the end of 

 one and a half months, on an average. The bac- 

 teridium which has become asporogenous at the tem- 

 perature of 42° to 43° then loses its virulent properties, 

 retaining only those of an ordinary saprogenic mi- 

 crobe, and, finally, it loses all vitality. 



The culture loses its pathogenic power little by lit- 

 tle ; it ceases to be fatal first for the large animals, 

 then for small adults, and finally for small animals 

 only a few days old. ISTow, each degree of virulence 

 can be perpetuated separately by cultivating at 42° 

 to 43° the different varieties obtained, each of these 

 varieties transmitting its special virulence to its de- 

 scendants. The degree of attenuation of each of 

 these varieties can be preserved if the precaution be 

 taken to frequently transfer to fresh culture media ; 

 but it is not possible to entirely avoid the attenuation 

 which finally takes place. In order to definitely fix 

 these varieties it suffices to return them to 37° ; they 

 then form spores possessing, in embryo, the special 

 17 



