42 BACTERIOLOGICAL EXAMINATION OF WATER. 



the flora of sewage with great care. As a result of their work a 

 number of organisms have been described which appear to be so 

 characteristic of sewage as to justify the term sewage-bacteria in 

 contradistinction to water-bacteria. In the present state of 

 our knowledge, therefore, it appears most advantageous, from a 

 hygienic point of view, to arrange the micro-organisms found in 

 water in the following three classes : 



Class I. Micro-organisms which are usually found in pure 



waters viz., the so-called water-bacteria. 

 Class II. Micro-organisms which are common in sewage, but 



are rarely found in pure waters. 

 Class III. Micro-organisms which are the cause of specific disease 



in human beings, and which have been isolated from water 



supplies. 



CLASS I. 



MICRO-ORGANISMS WHICH ARE FOUND IN 

 PURE WATERS. 



A very large number of water-bacteria have been isolated, but 

 the biological characteristics of many of them are so vague that 

 it will serve no useful purpose to attempt to describe them. 

 For hygienic work it seems better to try and arrange the water- 

 bacteria into groups of certain well-defined types, and then to 

 mention the variations from the type which have been 

 described. 



GROUP I. 



In this group are included the microbes which produce a 

 green colour in nutrient media ; they may be again sub-divided 

 into those which liquefy and those which do not liquefy gelatine. 



BACILLI WHICH LIQUEFY' GELATINE AND PRODUCE A GREEN 

 FLUORESCENCE IN NUTRIENT MEDIA. 



B. Fluorescens Liquefaciens. 



This microbe is exceedingly common in water from all sources, 

 and has the following characteristics : 



Colonies on Gelatine Plates. After twenty-four hours at 20 C. 



