x18 
covered ; so that, although it may be impossible to estimate 
by any ordinary means exactly how far the natural purifying 
forces have affected the water, if any sample shews distinct 
indication of being in any of these stages, we assume that 
a certain degree of departure from the original fecal arrange- 
ment has been reached. Consequently, we are nowadays not 
so much interested in the number of organisms per cubic 
centimetre, or even the number of fecal organism per c.c., as 
we are in the answer to the larger question—‘*‘ Has this 
particular sample passed the safety change?’’ We have, we 
think, demonstrated that the actual number of organisms 
present, fecal or otherwise, is subject to considerable variation 
under natural conditions, and is not a satisfactory guide to 
the real condition of the water. Thus, supposing a sample 
of water showing fecal organisms in 10 c.c., showed also that 
the acid and gas line in glucose and in lactose broth was the 
same, and that the varieties isolated belong to the susceptible 
group, one would be perfectly justified in being highly sus- 
picious of such a sample. On the other hand, a sample of 
water containing lactose fermenting organisms in ‘5 C.c., 
glucose fermenting organisms being present in ‘oI c.c., and 
the lactose fermenting organisms consisting of practically a 
pure culture of bacillus cloace, one would be prepared to 
say that such a sample was absolutely safe from the point of 
view of the possibility of conveying disease. In spite of. 
the number of fecal organisms present, an analyst in India 
would decide in his own mind that the sample was taken 
from a lake that was probably nearly dry, the water of which 
had been exposed for 6 or 8 months to a tropical sun, the 
bacilli being the residue of a pollution months old. Samples 
of this kind are quite common in the East. These examples 
demonstrate the utter futility ofa hard-and-fast numerical 
standard of purity of waters. Each separate sample should 
be considered on its own merits, taking into consideration all 
the information that it is possible to obtain. The more one 
knows about the natural processes of self-purification of 
water, the more certain one is that it is necessary to study 
the kind of organisms rather than their number in order to 
arrive at a satisfactory conclusion. 
