BACTERIOLOGICAL STUDY OF WATER. 441 



to present variations in their chemical composition, with- 

 out being unfit for use, so may the number of bacteria 

 per volume in water from one source be always greater 

 or smaller than in that from another, and yet no differ- 

 ence may be seen to result from their employment. For 

 this reason the proper 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 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 bacterio- 

 logical condition of water at any season. Marked de- 

 viations 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 acquires its 

 full value only through comparison, either with " means' 7 

 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 



