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116
Commenting on these results, Prof. Frankland
says "These experiments show that, on the 15th
May, the germicidal effect of sunlight on Thames
microbes was nil at depths of one foot and upwards
from the surface of the water. It cannot, therefore,
excite surprise that the effect of sunshine upon
bacterial life in the great river mass of the Thames
should be nearly, if not quite, imperceptible."
Such a conclusion appears to assume that water
which is at any given instant at an inert depth
below the surface always remains at that depth.
The disturbing influences of heating of the surface
of the water, eddies in stream, &c., would, however,
favour the assumption that the strata of water are
constantly interchanging, and that the liquid
which is at one instant at an inert depth may
shortly after be much nearer the surface of the
stream, i.e., subject to the germicidal influence of the
sun. In the experiments an exposure of four hours
and a half under the artificial conditions of the
intervention of glass between the sun's rays and the
water, was able to reduce the microbes by rather
more than 50 per cent. The balance of evidence
would appear to favour the view that the sun has a
very considerable influence in purifying the
Thames water.
The reason for the failure of the filter beds in the
very cold weather is obscure. Mr. Fuller, official
bacteriologist to the Massachusetts Board of Health,