EXAMINATION OF WATER AND A IE. 249 



presence of the colon bacilli in water is, except possibly 

 in rare cases, only of importance as an indication of fecal 

 contamination, and, therefore, of the possibility of dan- 

 gerous infection with other bacteria. Formerly it was 

 considered that its presence indicated human fecal con- 

 tamination, but we now know that many of the animals 

 contain in their intestines colon bacilli so similar to those 

 found in human beings that we cannot differentiate 

 the one variety from the other; therefore, the finding of 

 colon bacilli in water must always be judged according 

 to the conditions surrounding the water-supply. Thus, 

 it may indicate cattle contamination from the barn or 

 surface water, human contamination, or, in certain 

 conditions, simply the accidental contamination of the 

 stream by wandering cattle or animals. Properly 

 judged, however, the examination for the colon bacilli 

 may yield results of considerable practical impor- 

 tance. Whenever, in water examinations, any special 

 variety of bacteria is found in unusual abundance, the 

 fact should be noted, for sometimes it maybe the cause 

 of some prevailing infectious disease ; thus the bacillus 

 pyocyaneus has been found in water producing diar- 

 rhoea with greenish discharges. 



The Obtaining of Water for Examination. Whenever 

 possible the inoculation of the gelatin or agar tubes 

 and the pouring of their contents into the Petri dishes 

 should be done immediately after gathering the samples, 

 otherwise the actual and relative numbers of the differ- 

 ent organisms will change. As a rule, the pathogenic 

 bacteria will decrease and the harmless water species 

 will increase. When the plates cannot be made imme- 

 diately the water held in the sterile vials should be 

 kept nearly at the freezing-point. Even at very low 



