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the temperature, moreover, being low : 1,178 tons of the
manganate had been used. Deodorization was resumed July
16th, and continued until October 13th, 893 tons having been
used. About 52 million gallons of crude but deodorized sewage
was discharged at the two stations on the flood tide daily. The
value of the chemicals used in 1888 was £24,357, as compared
with £42,467 in 1887, a saving of £18,110, part of which was
due to cheapening of the manganate. The amount expended in
1887, moreover, was less by £38,600 than in 1886. Thus the
expenditure in 1888, as compared with the expenditure in 1886,
shows a reduction of £56,700 in the item of chemicals, without
apparent deterioration of the river. The manganate of soda
was found to be not less effective in its action than when
sulphuric acid is added, as was done in 1887. Sir Henry Roscoe
states that it is only by oxidation that putrescent sewage, which
contains little or no oxygen, can be purified. One gallon of
pure river water contains nearly four times as much oxygen
as that supplied by three grains of manganate of soda. A much
larger quantity would not suffice to convert the whole of the
fcecal matters into harmless forms. The oxygen in the
manganate is quickly consumed in removing the putrescent
odour of the sewage, and in preparing for a process of natural
oxidation by the further supply, by the river, of free oxygen, of
which the deodorized sewage contains none. The chemicals
then, as used, can effect little improvement in the river.
I mprovement is to be sought in some process of effluent aeration.
This matter requires further experiment. The purification of
the river is chiefly effected by living organisms, requiring free
oxygen for their growth, which produce the changes ascribed to
animal life. Vegetable organisms (algæ, etc.), always present
in river water, evolve oxygen during their life, and are potent
instruments in the natural purification of the river. Both forms
of life act together in ridding the water of dead organic matter
—soluble and insoluble—thereby bringing it into a healthy
conditiou. The summer rainfall of 1888 was greater, and the
temperature lower, than in 1885-6-7; influences favouring the
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