218 ELEMENTS OF WATER BACTERIOLOGY 



when including a normal variety of forms indicate the 

 presence of an excess of organic matter, derived in all 

 probability either from sewage or from the fresh wash- 

 ings of the surface of the ground. In either case danger 

 is indicated. 



A still closer measure of polluting material may be 

 obtained from the numbers of colonies which develop 

 on litmus-lactose-agar at 37°, since organisms which 

 thrive at the body temperature, and particularly thoso 

 which ferment lactose, are characteristic of the intestinal 

 tract and occur but rarely in normal waters. 



Gage (Gage, 1907) has shown that by counts at 20, 30, 

 40, and 50° C, information may be quickly obtained 

 which is of great assistance in judging the character 

 of the water. 



" Modern methods of bacterial examination of water, 

 consisting usually of determinations of the numbers of 

 bacteria by means of plates incubated at room tempera- 

 ture, and of tests for the presence or absence of one or 

 two specific types, occasionally lead to an erroneous 

 interpretation of the quality of a water, owing to the 

 fact that they do not yield adequate data by which 

 abnormal and inaccurate results may be separated from 

 those which are truly indicative of purity or pollution. 

 Furthermore, as several days must elapse before the 

 bacterial tests can be completed, the results when 

 obtained may have passed their usefulness. If, however, 

 we can so modify our procedure that the varied char- 

 acter of the bacteria in waters of different classes may 

 be quickly and accurately recognized, the value of 

 bacterial water analysis will be enormously increased. 



