I22 
ted that samples do occur in which it is extraordinarily 
difficult to be sure that a correct interpretation has been 
arrived at. 
Although ground waters are not discussed in this little 
book, we consider it advisable to say just a few words on the 
subject. 
Probably the greatest objection to the method that we 
propose is, we do not know how far it applies to waters from 
such sources. The question is much more complicated than 
those met in dealing with surface waters, the difficulties being 
(7) to estimate the importance and significance of organisms 
that remain alive in the soil for long periods, and (77) to be 
quite certain when a pollution is caused by surface contam- 
ination, washed’ downwards through cracks or through a 
porous soil. In dealing with well waters the great point to 
look to is the quantity of feecal pollution, and the results ob- 
tained in glucose and lactose broth. Of course, when coli 
communis is itself present with a mixture of other organisms, 
there can be little doubt that contamination of a dangerous 
nature has taken place. But when, as is so often the case, 
most of the bacilli present consist of a mixture of class 2 with 
probably a few of class 3, how far is it safe to assume that this 
type of pollution is not serious? It would not be wise to 
place much importance on the power of resistance of the 
organisms present, for in well waters we are not dealing with 
the resistant majority, but with the whole lot resistant and 
susceptible alike. Not infrequently, a well water, in normal 
hot weather conditions, containing only one organism, and 
that a resistant one, will, with comparatively slight meteo- 
rological changes, suddenly be found to contain a very large 
number of coli communis. We are at present in absolute 
ignorance both as to where these organisms come from (for 
they certainly do not come from recent surface pollution), 
and also as to what is their precise significance. They cannot 
be looked upon as altogether harmless, because, supposing 
the theory is correct that these organisms remain alive in 
the damper layers of soil, there is no certainty that the organ- 
isms of enteric and cholera may not do so as well. Conse- 
quently one is bound to regard their presence with great 
