[Illustration:  FIG. 14.—­HYDRO-EXTRACTOR.]

Whirling Out the Acid.—­The next operation is to remove the excess of acid.  This is done by placing the contents of two or three or more pots into a centrifugal hydro-extractor (Fig. 14), making 1,000 to 1,500 revolutions per minute.  The hydro-extractor consists of a machine with both an inner cylinder and an outer one, both revolving in concert and driving outwardly the liquid to the chamber, from which it runs away by a discharge pipe.  The wet cotton is placed around the inner cone.  The cotton, when dry, is removed, and at once thrown into a large tank of water, and the waste acids are collected in a tank.[A]

[Footnote A:  Care must be taken in hot weather that the gun-cotton does not fire, as it does sometimes, directly the workman goes to remove it after the machine is stopped.  It occurs more often in damp weather.  Dr Schuepphaus, of Brooklyn, U.S.A., proposes to treat the waste acids from the nitration of cellulose by adding to them sulphuric anhydride and nitric acid.  The sulphuric anhydride added converts the water liberated from the cellulose into sulphuric acid.]

Washing.—­The cotton has now to be carefully washed.  This is done in a large wooden tank filled with water.  If, however, a river or canal runs through the works, a series of wooden tanks, the sides and bottoms of which are pierced with holes, so as to allow of the free circulation of water, should be sunk into a wooden platform that overhangs the surface of the river in such a way that the tanks are immersed in the water, and of course always full.  During the time that the cotton is in the water a workman turns it over constantly with a wooden paddle.  A stream of water, in the form of a cascade, should be allowed to fall into these tanks.  The cotton may then be thrown on to this stream of water, which, falling some height, at once carries the cotton beneath the surface of the water.  This proceeding is necessary because the cotton still retains a large excess of strong acids, and when mixed with water gives rise to considerable heat, especially if mixed slowly with water.  After the cotton has been well washed, it is again wrung out in a centrifugal machine, and afterwards allowed to steep in water for some time.

[Illustration:  FIG. 15_a_.—­THE BEATER FOR GUN-COTTON.]

Boiling.—­The washed cotton is put into large iron boilers with plenty of water, and boiled for some time at 100 deg.  C. In some works lead-lined tanks are used, into which a steam pipe is led.  The soluble impurities of unstable character, to which Sir F.A.  Abel traced the liability of gun-cotton to instability, are thereby removed.  These impurities consist of the products formed by the action of nitric acid on the fatty and resinous substances contained in the cotton fibres.  The water in the tanks should be every now and again renewed, and after the first few boilings the water should be tested with litmus paper until they are no longer found to be acid.