246 BACTERIOLOGY. 



which were present in the water at the moment the 

 water was examined. Any bacteria, however, which, 

 though living in the water, were unable to grow in the 

 media or at the temperatures employed would, naturally, 

 not reveal themselves by the growth of colonies, nor 

 would bacteria clinging together in bunches count as 

 more than a single member. As to the value of learn- 

 ing the number of bacteria in water, we must admit that 

 a single determination of the number of living bacteria 

 in any sample is now known to be of little avail unless 

 the conditions under which the water exists are well 

 known or the number of bacteria is enormous. Thus, 

 for instance, the water in an Adirondack lake might 

 contain in a cubic centimetre far more bacteria than 

 that of a well which was slightly contaminated with 

 typhoid bacilli from human sources. If, however, we 

 knew the usual condition of the well water and the 

 usual number of bacteria present in it, any sudden 

 increase would, of course, give us a strong suspicion, 

 but nothing more than a suspicion, of dangerous con- 

 tamination. In the same way in a stream into which 

 a sewer empties, if we find a great many more bacteria 

 in the stream some distance below the point of entrance 

 of the sewer than there were above, we would have 

 every reason to believe that the increase of bacteria 

 found in the stream below was due to the bacteria added 

 to the stream by the sewer; if we drank that water we 

 would know from the examination we were drinking 

 not only a portion of the sewage but of the bacteria 

 contained therein. It is true, that with our present 

 knowledge, derived from previous bacteriological studies, 

 we would be almost as certain of these facts before as 

 after the bacteriological examination. The determina- 



