s from factories
with the residual matters pressed into cakes, and also of the purified
effluents, are exhibited. These will prove of interest to many, all the
more so since in some instances the waste products are converted into
materials of value, which, it is stated, will more than repay for the
outlay incurred.

[Illustration: Fig. 3. Fig 4.]

Another application of the filter press is in the Porter-Clark process
of softening water, which is shown in operation. We may briefly state
that the chief object is to precipitate the bicarbonates of lime and
magnesia held in solution by the water, and so get rid of what is known
as the temporary hardness. To accomplish this, strong lime water is
introduced in a clear state to the water to be softened, the quantity
being regulated according to the amount of bicarbonates in solution. The
immediate effect of this is that a proportion of the carbonic acid of
the latter combines with the invisible lime of the clear lime water,
forming a chalky precipitate, while the loss of this proportion of
carbonic acid also reduces the invisible bicarbonates into visible
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