618 BACTERIOLOGY. 



for just as normal waters from different sources are 

 seen to present variations in their chemical composition, 

 without being unfit for use, so may the relative number 

 and variety of species of bacteria in water from one 

 source be always greater or smaller than in that from 

 another, and yet no difference may be seen to result 

 from their employment. For this reason systematic 

 study of any water, from this point of view, should 

 begin with the establishment of what may be called its 

 normal mean number of bacteria, as well as the charac- 

 ter of the prevailing species ; and in order to do this 

 the investigations must cover a long period of time 

 through all the seasonal variations of weather. From 

 data obtained in this way it may be possible without 

 analysis to predict approximately at any season the 

 bacteriological condition of the water studied. Marked 

 deviations from these " means," either in the quantity 

 or quality of the organisms present, can then be con- 

 sidered as indicative of the existence of some unusual, 

 disturbing element, the nature of which should be 

 investigated. It is impossible to formulate an opinion 

 of much value from either a single chemical or bac- 

 teriological analysis of a water, or from both together 

 in many cases; for the results thus obtained indicate 

 only the condition of the water at the time the sample 

 was procured, and give no indication as to whether it 

 differed at that time from its usual condition, or from 

 the normal condition of the waters of the immediate 

 neighborhood. 



The interpretation of the results of both chemical 

 and bacteriological analyses of a sample of water ac- 

 quires its full value only through comparison, either 

 with " means " that have been determined for this 



