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Now although it is essential that the data submitted with each sample should be
sufficient to enable the analyst to estimate roughly the length of exposure, in not a
few cases the result obtained gives very valuable evidence, as to the amount, of sunning
and storage that the water has undergone. A reference to the analyses of the water
of lakes, for the first quarter dealt with in this report [vide table X (a)], will show the
frequency with which analyses occur, demonstrating, in our opinion beyond the shadow
of doubt, that one is dealing with a thoroughly well sunned water. Thus we think
it my be taken as certain that when a water shows faecal bacilli that belong entirely
to class III, we are dealing with a water that has been exposed to the action of sun
and storage for some time.
We have already stated. that samples should be divided in the mind of the analyst
into two rough groups, those that have been exposed to sunlight, and those that have
not. In the first of these groups will fall the larger proportion of samples. These are
again roughly divided into three smaller groups, according to the result that the
analyses give, namely, (1) those that contain organisms belonging to class I
(the susceptible organisms); (2) those that contain practically no other organisms but
those in class III (the resistant organisms); (3) those that contain mixtures of two
or more classes.
Let us deal with each of these separately—
(1) Those containing class I.—The samples which contain colic communes and
the members of the susceptible group, present very little difficulty in arriving at a
conclusion, as regards the undesirable nature of the pollution. In such samples,
derived from river or lake, there can be no doubt we are dealing with recent, and
therefore objectionable contamination. At the same time if faecal contamination has
been recent, susceptible organisms (those in group 1) will be mixed with those of
groups 2 and 3 , roughly in, the proporlion in which they occur in f œc es . Hence we
have the double reason for suspecting th.e water; namely, (1) the presence of suscep-
tible organisms, which shows that the contamination must have been recent and (2)
the confirmatory evidence of getting the unaltered proportions of faecal organisms.
I n a c t ua l pr a c t i c e c ol i c c ommuni s i s s o s e l dom me t wi t h, t ha t t he l a t t e r c ondi t i on of
affairs is often all we have to guide us. In rivers, organisms in class I are more
commonly met with, because pollution is constantly being added all along the banks,
and when found certainly indicate pollution of a most serious and dangerous nature.
(2)
Those containing class III.—It is probably not realised how extremely
often, in a tropical country, organisms belonging to class III alone are found. in
water samples. Personally we doubt whether such condition of affair is ever recorded
in Europe. But a glance at the analyses of the several supplies, during the
quarter of July, August and September, is absolutely convincing on this point. It is
quite common to obtain samples of water which contain nothing else but bacillus
grunthal. or bacillus Cloacæ or a mixture of the two. When this occurs, one may
be practically sure that we are dealing with a water from a tank or river that is
getting very low, and also that these organisms represent the residue of a pollution
months old.
(3) Those containing mixtures of two or more classes .—The chief difficulty that
will be met with by the analyst using this method, will be to arrive at an opinion from
samples showing a mixture of organisms in class II, or mixture of class II and
class III. It is in dealing with such samples that it is absolutely necessary to know
the particulars concerning rainfall, etc., that have been enumerated in the early part of
this chapter. Mixtures of organisms are to be expected in rainy weather and flooded
rivers. In attempting to arrive at a conclusion one has to be guided by (1) the
number of faecal organisms present in the sample; (2) the number of different species
present in the 10 colonies isolated; (3) whether the organisms show a tendency to
belong to those at the top or at the bottom of the list given in class II ; and (4)
whether class III are present in considerable quantity. Thus, in a water containing'
a hundred fœ cal organisms per cc., known to have been taken from a lake or
river within two or three days of a heavy downpour of rain, we were to find
four or five different varieties of organisms all belonging to group 2, we should be
justified in considering, that, although no organisms belonging to class I happen to be
isolated, that the circumstances were highly suggestive of recent faecal contamination.
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