[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