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235
for laboratory experiments, which were subsequently carried
out. Between August 27th and 5th of October, experiments
were carried out with the manganate and acid. Deodorization
of the sewage was effected, and a general impression formed that
the manganate does not act prejudicially on the river.
Diagrams are given showing, in the form of curves, the
amount of dissolved oxygen in the river water at Crossness at
different periods of the year; the changes in the volume of
fresh water coming into the river, as indicated by the quantity
of sea-salt present at low tide, the temperature, and the rainfall.
" Comparing the general aspect of the curves for the three
years, 1885-6-7, it does not appear that those for 1887 show any
material indications of a worse state of things than those for the
two previous years. And it may well be doubted . . .
whether the state of the river in 1886, when upwards of
£80,000* was spent in chemicals, was perceptibly better than
that in 1887, when £42,000 was spent, or even better than that
in 1885, when only £29,000 was spent in deodorants.
Indeed, the idea forces itself on one's mind that either the effects
of the chemicals are imperceptible, or that the methods of
measuring those effects are imperfect."
The Laboratory Experiments show that the sewage which
had been treated with bleaching powder contained, in common
with that treated with manganate, lower forms of life in
quantity; but that the higher forms, such as algæ, were conspicuous
by their absence in the first and by their presence in
the second case. The bleaching powder (9 grains per gallon
of sewage) completely and permanently stops the growth of all
organisms visible under a high power of the microscope;
smaller doses have similar effects differing in degree. The
powder, moreover, is very inimical to animal (fish) life, so little
as one-third grain per gallon of water being sufficient in time
to kill minnows. Neutral manganate of soda added to sewage,
9 grains per gallon, does not prevent but appears to retard the
* In a previous report (vide page 231) the cost of chemicals used is put at
£40,000 only.
Q 2