420 -S^ OTERIOL OGV. 



greater or smaller than in that from another, and yet 

 no difference may be seen to result from their employ- 

 ment. For this reason the proper study of any water, 

 from this point of view, should begin' with the establish- 

 ment of what may be called its normal mean number 

 of bacteria, as well as the character 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 to predict approximately the normal 

 bacteriological condition of water at any season. Marked 

 deviations from these " means," either in the quantity or 

 quality of the organisms present, can then be considered 

 as indicative of the existence of some unusual disturbing 

 element, the nature of which should be investigated. 

 Similarly, it is impossible to formulate an opinion of 

 much value from a single chemical analysis of a water, 

 for the results thus obtained indicate only the state 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 

 water throughout the immediate neighborhood. 



The interpretation of the results of both chemical 

 and bacteriological analysis of a sample of water ac- 

 quires its full nature only through comparison, either 

 with "means" that have been determined for this 

 water, or with the results of simultaneous analyses of a 

 number of samples from the other sources of supply of 

 the locality. 



The aid of the bacteriologist is frequently sought in 

 connection with investigations upon waters that are 

 supposed to be concerned in the production of disease, 

 particularly typhoid fever, either in isolated cases or in 



