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although the actual amount of this organic matter is
often quite insignificant, yet it may be of the most
objectionable character on account of its origin.
Thus the water, both of the Thames and the Lea,
receives, above the points where it is abstracted for the
purposes of the Metropolitan supply, various contributions
of organic matter of animal origin, such as the
drainage from manured land, the effluents from sewage
works, and even raw sewage itself. This animal
matter may, at any time, be accompanied by zymotic
poisons dangerous to health, and although the chances
of such substances reaching the water-consumer are
enormously reduced, both by the care which is exercised
in excluding the flood-waters from the reservoirs,
and especially by the efficient filtration to which
the water is subjected before distribution; yet, in
spite of these protective measures, there is no absolute
guarantee that the noxious ingredients, which may
at any time be present, are wholly removed. Most
of the Companies are endeavouring to substitute as
far as possible water from subterranean sources for
raw river water, whilst the Board of Conservators
have almost completely stopped the discharge of
untreated town sewage into the Thames.
Table A gives the proportion of organic impurity
present in the waters taken from the Thames, the Lea,
and the deep-wells respectively, from 1868 to 1892.
From the figures there given it will be seen that the
Thames water distributed during the year 1892 was