shown in the engraving. The press is thus almost instantaneously filled,
and the whole operation is completed in about an hour, the result being
a hard pressed cake containing about 45 per cent. of water, which can be
easily handled and disposed of as required. The same arrangement is in
use for dealing with sewage sludge, and the advantages of the compressed
air system over the ordinary pumps, as well as the ready and cleanly
method of separating the liquid, will probably commend itself to many of
our readers. We understand that from careful experiments on a large
scale, extending over a period of two years, the cost of filtration,
including all expenses, has been found to be not more than about 6d. per
ton of wet sludge. A number of specimens of waste liquors 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