POISONS AND THEIR PREVENTION 185 



depends upon training the animal to gradually 

 withstand larger and larger doses of the venom ; 

 and considering the intensely toxic character of 

 the substance which had to be handled, the danger 

 was ever present of the animal succumbing to 

 venom poison before its serum had acquired the 

 requisite pitch of protective power to render it 

 of service as an anti-toxin. Dr. Calmette tells us 

 that he carried out a very large number of experi- 

 ments before he met with success. But it is not 

 necessary here to discuss his various efforts ; suffice 

 it to say that at length his labours were rewarded, 

 and the following extract from one of his memoirs 

 describes the methods which he adopted for this 

 purpose : 



"The best method of procedure for the purpose of 

 vaccinating large animals destined to produce anti- 

 venomous serum consists in injecting them from the 

 outset with gradually increasing quantities of the venom 

 of the cobra mixed with diminishing quantities of a one- 

 in-sixty solution of hypochlorite of lime.* The condition 

 and the variations in the weights of the animals are care- 

 fully followed, in order that the injections may be made 

 less frequently if the animals do not thrive well. Quan- 



* More recently the snake venom employed by Dr. Calmette 

 for the immunisation of his horses consists of a mixture of 

 colubrine and viperine poisons, the former making up about 80 per 

 cent, of the mixture. A solution of this mixture is heated at 

 about 73 C. for half an hour and then filtered, and injected into 

 horses. 



