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.

Manufacture at Le Bouchet.—­At Le Bouchet gun-cotton was made thus:—­200 grms. of cotton were steeped for an hour in 2 litres of a mixture of 1 volume concentrated nitric and 2 volumes sulphuric acid.  The cotton was then removed and pressed, whereby 7/10ths of the waste acids was recovered.  After this it was washed for one to one and a half hours in running water, strongly pressed again; allowed to lie for twenty-four hours in wood-ash lye; then well washed in running water; pressed, and finally dried on a wide linen sheet, through which was forced air heated to 60 deg.  C. The average yield from 100 parts of cotton was 165 parts of gun-cotton.  The strong pressings of the gun-cotton, while still impregnated with acids, caused subsequent washings to be difficult and laborious.

Granulation of Gun-Cotton.—­Gun-cotton is often required in the granulated form for use either alone or with some form of smokeless powder.  This is done under the patent of Sir Frederick Abel in the following manner:—­The gun-cotton from the poacher is placed in a centrifugal machine, very similar to the hydro-extractors before mentioned, and used for wringing out the acids.  In this machine