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 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Steeping.—­The nitrated cotton, when withdrawn from the dipping tanks, and still containing an excess of acids, is put into earthenware pots of the shape shown in Figs. 12 and 13.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.