great advantage over the usual method of working ordinary centrifugal
nitrating apparatus, because the acid being removed before the centrifugal
is run, practically all danger of firing therein disappears, and a greater
proportion of the waste acid is recovered.

In some cases the acids and water may be supplied by perforated pipes,
lying along the edges of the nitrating vessel, and these edges may, if
desired, be themselves made inclined, like the sides of the troughs _f_.
In the case of effecting nitration in centrifugals as above, the
displacing sulphuric acid and water may thus be supplied round the edges
of the machines, or removal troughs such as _f_ may be used. It will be
obvious that any inert liquid of suitable specific gravity may be used
instead of sulphuric acid, as a separation layer.

~Moulding.~--By means of the small measuring tank above referred to, the
gun-cotton pulp is drawn off from the stuff chest, and run into moulds of
the shapes and sizes required. Thence a large proportion of the water is
drawn off by means of tubes connected with the vacuum engine, the moulds
having bottoms of fine wire gauze, in order to prevent the pulp from
passing through. Hydraulic pressure of about 34 lbs. on the square inch is
then applied, which has the effect of compressing the pulp into a state in
which it has sufficient consistency to enable it to be handled with care,
and also expels a portion of the remaining water.

~Compressing.~--The moulded gun-cotton is now taken to the press house,
which is situated at some distance from the rest of the factory. Here the
moulds are subjected to powerful hydraulic pressure, from 5 to 6 tons per
square inch, and is compressed to one-third of its previous bulk. The
slabs or discs thus formed are kept under pressure for a short time, not
exceeding a minute and a half, to give the requisite density. It should,