Bacteriological Examination. 173 



present. The bacteriologist in such cases can declare 

 the innocence of the water with justifiable certainty. 

 When high numbers are found the interpretation is less 

 simple, since they may exceptionally be due to the multi- 

 plication of certain peculiar water forms. Large counts, 

 however, under ordinary conditions, 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 washings 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 degrees, since organisms which 

 thrive at the body temperature, and particularly those 

 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 interpre- 

 tation of the quality of a water, owing to the fact that they 

 do not yield adequate data by which abnormal and inac- 

 curate results may be separated from those which are 



