[Footnote A: (594 x 1)/324= 1.83.]
[Illustration: FIG. 11.—TANK FOR DIPPING COTTON.]
[Illustration: FIG. 12.—THE COOLING PITS.]
The dipping is done in cast-iron tanks (Fig. 11), a series of which is arranged in a row, and cooled by a stream of cold water flowing round them. The tanks hold about 12 gallons, and the cotton is dipped in portions of 1 lb. at a time. It is thrown into the acids, and the workman moves it about for about three minutes with an iron rabble. At the end of that time he lifts it up on to an iron grating, just above the acids, fixed at the back of the tank, where by means of a movable lever he gently squeezes it, until it contains about ten times its weight of acids (the 1 lb. weighs 10 lbs.). It is then transferred to earthenware pots to steep.
[Illustration: FIG. 13.—COTTON STEEPING POT.]
~Steeping.~—The nitrated cotton, when withdrawn from the dipping tanks, and still containing an excess of acids, is put into earthenware pots of the shape shown in Figs. 12 and 13. The lid is put on, and the pots placed in rows in large cooling pits, about a foot deep, through which a stream of water is constantly flowing. These pits form the floor of the steeping house. The cotton remains in these pots for a period of forty-eight hours, and must be kept cool. Between 18° and 19° C. is the highest temperature desirable, but the cooler the pots are kept the better. At the end of forty-eight hours the chemical reaction is complete, and the cotton is or should be wholly converted into nitro-cellulose; that is, there should be no unnitrated cotton.
[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 Schüpphaus, 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.