486 BAGTERIOLOGY. 



number 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 em- 

 ployment. 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 pre- 

 vailing species; and in order to do this the investiga- 

 tions 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 pres- 

 ent, 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 chem- 

 ical 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 

 aud bacteriological analysis of a sample of water ac- 

 quires 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 the other sources of supply 

 of the locality. 



The aid of the bacteriologist is frequently sought in 

 connection with investigations upon waters that are sup- 



