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growth of organisms. Smaller doses, 1 to 3 grains, produce
no visible effect, and therefore do not act as poisons. Minnows
will live for some days in fresh water to which 1 grain of
permanganate per gallon has been added, and for twelve
hours in a solution twice as strong. Bleaching powder,
moreover, disappears but slowly—taking some day—in
sewage, whereas manganate is immediately destroyed. The
conclusion drawn from the whole series of experiments is
that, apart from the consideration of cost, manganate is to
be preferred as a deodorant to bleaching powder. Sir
H. Roscoe advises the continued use, under specified conditions,
of manganate alone, the utility of sulphuric acid being
questionable, at an annual average cost of £40,000, although
this outlay will not prevent a foul condition of the river
occurring during the summer months, in drought or during hot
weather.
The question of aeration of the sewage is briefly
referred to. Free oxygen is to be had for nothing,
and the cost of pumping air need not be considerable.
Sir H. Roscoe believes that this may turn out to be a solution of
the problem. The rapid purifying effects of aeration on the
sewage were observed in the laboratory experiments, and it was
felt that this is a matter " well deserving of further inquiry."
The " general conclusions " are important, having regard to
the views of the Royal Commission on Metropolitan Sewage
Discharge, as set out in their Report (January, 1884), and to
the action taken by the Metropolitan Board. Sir H. Roscoe
remarks that he offers'' no opinion as to the processes of
precipitation by chemical treatment" (lime and green vitriol)
" as this question was not submitted to him by the Board."
" Looking, however, at the broad question of the permanent
disposal of the Metropolitan Sewage, and believing that the use
of deodorants ought to be regarded only as a temporary expedient,"
he feels " convinced that sooner or later the recommendations
of Lord Bramwell's Commission (the Royal