to be dry the bottom of the drying cylinder is
removed, and the cotton pushed out from the top by means of a piece of
flat wood fixed on a broom-handle. It is then packed away in galvanised-
iron air-tight cases, and is ready for the next operation. At some works
the cotton is dried upon shelves in a drying house through which hot air
circulates, the shelves being of canvas or of brass wire netting. The hot
air must pass under the shelves and through the cotton, or the process
will be a very slow one.

~Dipping and Steeping.~--The dry cotton has now to be nitrated. This is
done by dipping it into a mixture of nitric and sulphuric acids. The acids
used must be strong, that is, the nitric acid must be at least of a
gravity of 1.53 to 1.52, and should contain as little nitric oxide as
possible. The sulphuric acid must have a specific gravity of 1.84 at 15 deg.
C., and contain about 97 per cent. of the mono-hydrate (H_{2}SO_{4}). In
fact, the strongest acids obtainable should be used when the product
required is gun-cotton, i.e., the highest nitrate.

The sulphuric acid takes no part in the chemical reaction involved, but is
necessary in order to combine with the water that is liberated in the
reaction, and thus to maintain the strength of the nitric acid. The
reaction which takes place is the following:--

2(C_{6}H_{10}O_{5}) + 6HNO_{3} = C_{12}H_{14}(NO_{3})_{6} + 6 H_{2}O.
324 378 = 594 108.
Cellulose. Gun-Cotton.

Theoretically,[A] therefore, 1 part of cellulose should form 1.8 part of
gun-cotton. Practically, however, this is never obtained, and 1.6 lb. from
1 lb. of cellulose is very good working. The mixture of acids used is
generally 1 to 3, or 25 per cent. nitric acid to 75 per cent. sulphuric
acid.

[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