THE VACCINE OF SPLENIC FEVER. 245 



virulent small-pox a year after its vaccination ? 

 Finally, taking into consideration the commercial and 

 economic view of the life of a sheep if such an ex- 

 pression may be used the average scarcely exceeds 

 three years. The duration, then, of the immunity 

 that vaccination confers is about a third of the dura- 

 tion of the animal's life. 



