ESSAYS ON BACTERIOLOGY. 79 



bodies of certain animals. Of this metliod the vac- 

 cination of Jenner is probably an example. 



Third. The injection of products of the specific 

 bacteria, these products being obtained from cultures 

 or from the bodies of infected animals. They may be 

 used when still toxic, or, more frequently, after dimin- 

 ution or removal of their toxicity. 



The principal means of reducing toxicity and in- 

 creasing vaccinating values are heat and mixture with 

 certain animal juices, particularly thymus extract, or 

 with tri-chloride of iodine and some other agents in- 

 jurious to bacterial poisons. 



The so-called chemical vaccines belong to this class. 

 They have a very wide field of application. In some 

 instances partial immunity -has followed the introduc- 

 tion of this class of substances into the stomach. 



Fourth. Injection of blood-serum or other fluids 

 from animals artificially rendered immune from the 

 disease. As the blood-serum from immunized ani- 

 mals may possess not only prophylactic but curative 

 properties, it has been called curative or healing 

 serum. 



Usually various methods can be successfully em- 

 ployed in producing artificial immunity from a given 

 disease. 



It is a general rule that a given microbe and its 

 products are capable of couferring immunity only 

 from the disease which is caused by that microbe. 



The conferred immunity may be increased in de- 



