562 PATHOGENIC BACTERIA. 



dose necessary was 20 c.cm. of a serum having a potency 

 of 1 : 10. If too little serum was given, the course of 

 the disease was slowed, the animal improved for a time 

 and then suffered a relapse, and died in from thirteen to 

 seventeen days. The serum also produced immunity, 

 but of only ten to fourteen days' duration. Immunity 

 lasting three weeks was conferred by inoculating a mon- 

 key with an agar-agar culture heated to 6o° C. If too 

 large a dose of such a culture was given, however, the 

 animal was enfeebled and remained susceptible. 



Of Yersin's serum, which is prepared by immunizing 

 horses in the usual manner to the toxins and cultures of 

 the bacillus, 5 c.cm. doses have been found to confer an 

 immunity lasting for about a fortnight. Larger doses 

 confer a longer immunity. For the treatment of the de- 

 veloped disease enormous doses of 50 and even 100 c.cm. 

 seem to be necessary to produce the desired results, evi- 

 dently indicating that the serums thus far obtained are 

 weak. 



