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° 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.

[Illustration: FIG. 15_b_.--WHEEL OF BEATER.]

~Pulping.~--The idea of pulping is also due to Abel. By its means a very
much more uniform material is obtained. The process is carried out in an
apparatus known as a "Beater" or "Hollander" (Fig. 15, _a, b_). It
consists of a kind of wooden tank some 2 or 3 feet deep of an oblong
shape, in which a wheel carrying a series of knives is made to revolve,
the floor of the tank being sloped up so as to almost touch the revolving
wheels. This part of the floor, known as the "craw," is a solid piece of