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(or non-pathogenic) microbes between the filtration works and
the standpipes at which the samples for analysis are taken. It
is of importance, moreover, to make the examination at the
moment of maximum sterility: the fewer the microbes then
found, the less is the probability of pathogenic organisms being
present; and although the non-pathogenic may afterwards
multiply indefinitely, this is of no consequence in the primary
absence of the pathogenic. Professor Frankland describes in
detail the effect of various influences, in increasing or diminishing
the relative number of microbes before filtration; including
cold, which diminishes, and heat, which increases, the proportions.
Sunlight is a powerful germicide, but its effect upon
bacterial life in the great mass of Thames water is nearly if
not quite imperceptible. Speaking generally, it would appear
that the number of microbes in Thames water is practically
governed by rainfall, that is to say by the volume of the stream.
Certain tables' (at page 66) go to show " how very much the
bacterial quality of unfiltered water may differ from its chemical
quality. Thus, in chemical composition, the waterpumped from
the gravel near the banks of the Thames does not differ materially
from the Thames water itself, and it is generally turbid; but
whilst the raw Thames water contained, during the year, on
the average, 13,646 microbes per c.c., the water abstracted from
the natural gravel beds, by the Southwark Company, contained,
on an average, only 188 per c.c." Subsidence, moreover, " effects
a very material bacterial amelioration of the water." Thus, "in
November, when the river contained 29,260 microbes per c.c.,
the stored water of the Chelsea Company contained only 500
per c.c." To whatever causes the initial excess or deficiency of
microbes in the unfiltered water may be due, it is manifest
that a marvellous reduction is effected by efficient sand filtration,
as shown in the following table, which represents the
bacterial condition of the water issuing from the filter beds of
the three companies by which this parish is supplied, compared
with the raw material dealt with by these companies :—
I