CHAPTER VIII. 



QUALITATIVE ANALYSIS continued. 



CLASS II. 



THIS class contains the micro-organisms which are common in 

 sewage, but are rarely found in pure water supplies. Many of 

 them are derived from the intestinal canal of human beings and 



o 



animals ; others are associated with putrefactive processes. The 

 recognition and proper interpretation of these organisms is a 

 matter of the first importance. It often happens during the exami- 

 nation of water supplies suspected to have caused zymotic disease, 

 that chemical analysis, enumeration of micro-organisms, and 

 search for pathogenic organisms, all fail to give sufficient 

 data upon which a reliable opinion may be based as to the 

 purity of the supply. Under these conditions the detection of 

 the organisms in this class will either enable a proper decision 

 to be made, or suggest a rigid examination of the supply 

 when possibly unsuspected sources of contamination may be 

 detected. 



Bacillus Coli Communis. 



The most important organism in the sewage class is the B. coli 

 communis, often called Escherich's bacillus, after the name of its 

 discoverer. Studies of sewage and normal faeces, however, have 

 shown that the B. coli usually described in the text-books is the 

 type of a large group containing many varieties ; the cultural 

 characteristics, morphology, and serum reactions of the typical 

 bacillus will be first considered, after which the more important 

 varieties will be compared with the type. The typical B. coli 

 communis is usually described as follows : 



Gelatine Plates. In the depth the colonies are circular or 



