carbonates. The precipitates thus formed are in the state of an
impalpable powder, and in the original Clark process many hours were
required for their subsidence in large settling tanks, which had to be
in duplicate in order to permit of continuous working. By Mr. Porter's
process, however, this is obviated by the use of filter presses, through
which the chalky water is passed, the precipitate being left behind,
while, by means of a special arrangement of cells, the softened and
purified water is discharged under pressure to the service tanks. Large
quantities can thus be dealt with, within small space, and in many cases
no pumping is required, as the resistance of the filtering medium being
small, the ordinary pressure in the main is but little reduced. One of
the apparatus exhibited is designed for use in private mansions, and
will soften and filter 750 gallons a day. In such a case, where it would
probably be inconvenient to apply the usual agitating machinery, special
arrangements have been made by which all the milk of lime for a day's
working is made at one time in a special vessel agitated by hand, on the
evening previous to the day on which it is to be used. Time is thus
given for the particles of lime to settle during the night. The clear
lime water is introduced into the mixing vessel by means of a charge of
air compressed in the top of a receiver, by the action of water from the
main, the air being admitted to the milk of lime vessel through a
suitable regulating valve. A very small filter suffices for removing the
precipitate, and the clear, softened water can either be used at once,
or stored in the usual way. The advantages which would accrue to the
community at large from the general adoption of some cheap method of
reducing the hardness of water are too well known to need much comment
from us.

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