BACTERIOLOGICAL STUDY OF WATER 639 



chemical composition, without necessarily 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 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 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 considered 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 bacteriological 

 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 acquires its 

 full value 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 other 

 sources of supply of the locality. 



The aid of the bacteriologist is frequently sought in con- 



