I20 
Concerning the subject of acid and gas line in glucose 
and lactose broth we have said practically all that is 
necessary in Chapter VI. The conclusion we arrived at 
was that in all cases when the acid and gas line is similar 
in broth media the pollution is recent, that is to say, the 
fecal arrangement of the organisms is still present. The 
wider the divergence, the safer the water, and the older the 
pollution. : 
Turning now to the subject of the three classes of 
lactose fermenting organisms, v2z. (class I) the susceptible, 
(class 2) the intermediate, and (class 3) the resistant organisms, 
the following points may be mentioned :— 
In his own mind the analyst will subdivide the samples 
into (1) those which contain susceptible organisms, (2) those 
which are made up almost entirely of class 3, or the resistant 
organisms, and (3) those which contain a mixture of the two 
or more classes, but in which class 2 predominate. Let us 
deal with each of these separately. 
(1) A sample containing coli communis and other members 
of the susceptible group will present very little difficulty 
in arriving at a conclusion. Coli communis represents un- 
desirable pollution, because it rapidly dies out from water, 
therefore its presence is suspicious. The following points 
will also be observed in samples of this nature :— (7) Pro- | 
bably 3, 4 or 5 different varieties of bacilli in the 10 colo- 
nies sub-cultured will be observed, the large number of 
varieties suggest the true fecal arrangement. (77) Nearness 
of acid and gas line in glucose and lactose broth. (272) 
Possibility of finding streptococci in water. (iv) Scarcity 
of bacillus lactis zrogenes. 
With this amount of confirmatory evidence one will 
have no difficulty in coming to the conclusion that the pollu- 
tion is recent, and therefore dangerous. In tropical waters, 
samples containing susceptible organisms are distinctly rare, 
consequently, one has frequently to rely on the confirmatory 
evidence alone in giving an opinion. In rivers, however, where 
contamination along the bank is common they are more fre. 
quently met with, and-always indicate dangerous pollution. 
