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~Steeping.~--The nitrated cotton, when withdrawn from the dipping tanks,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 deg. and 19 deg. 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
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 riv