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- 
