e 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,
when removed, be compact, and just sink in water, and should perceptibly
yield to the pressure of the fingers. There are perforations in the press
blocks, to allow of the escape of gases, if formed, by reason of
sufficient heat being generated. The men working the press are placed
under cover, behind strong rope mantlets having eye tubes which command a
view of the press.

~Packing.~--The finished slabs and discs are dipped into a solution of
soda and carbolic acid, and packed in special wood metal-lined cases. When
it is to be sent abroad, the metal lining, which is made of tinned copper,
is soldered down, but both the outer wooden and inner metal cases are
fitted with air-tight screw-plugs, so that when necessary water can be
added without unfastening the cases.

~Reworked gun-cotton~ does not make such good discs as new pulped gun-
cotton, probably because the fibrous tenacity of the gun-cotton has been
destroyed by the amount of pressure it has previously undergone, so that
when repulped it resembles fine dust, and a long time is required to press
it into any prescribed form. It is generally boiled for eight hours to
open up the fibre and remove alkali, then broken up by hand with wooden
mallets, pulped, and then used with fresh gun-cotton in the proportion of
1 to 5 parts.

~Ma