SPECIAL MEDIA. 137 



process of sterilization to avoid excessive heating of the 

 medium. 



The relative degree of acidity produced, with or with- 

 out coagulation, with or without gas-production, and 

 with or without reduction of the litmus, in a series of 

 tubes of this medium containing the different carbohy- 

 drates serves to differentiate between related species of 

 bacteria. For instance, the colon bacillus produces an 

 acid reaction with coagulation and gas-formation with 

 some of the carbohydrates, while the typhoid bacillus 

 produces a lower degree of acidity with coagulation, but 

 without gas-production. Similarly, the different types 

 of the dysentery bacillus may be differentiated by means 

 of their effects on the different carbohydrates in this 

 medium. 



A complete list of the special media would be too 

 voluminous for a book of this size. For their descrip- 

 tion the reader is referred to the current literature. 

 Those that have been given above will suffice for ob- 

 taining a clear understanding of the principles of the 

 subject. In the chapters upon the Pathogenic Bacteria 

 such special media as have proved of use for purposes 

 of identification and differentiation are described in 

 detail. 



