Drying the Cotton.—­This operation is performed in either of two ways.  The cotton may either be placed upon shelves in a drying house, through which a current of hot air circulates, or dried in steam-jacketed cylinders.  It is very essential that the cotton should be as dry as possible before dipping in the acids, especially if a wholly “insoluble” nitro-cellulose is to be obtained.  After drying it should not contain more than 0.5 per cent. of moisture, and less than this if possible.  The more general method of drying the cotton is in steam-jacketed tubes, i.e., double cylinders of iron, some 5 feet long and 1-1/2 foot wide.  The cotton is placed in the central chamber (Fig. 10), while steam is made to circulate in the surrounding jacket, and keeps the whole cylinder at a high temperature (steam pipes may be coiled round the outside of an iron tube, and will answer equally well).  By means of a pipe which communicates with a compressed air reservoir, a current of air enters at the bottom, and finds its way up through the cotton, and helps to remove the moisture that it contains.  The raw cotton generally contains about 10 per cent. of moisture and should be dried until it contains only 1/2 per cent. or less.  For this it will generally have to remain in the drying cylinder for about five hours.  At the end of that time a sample should be taken from the top of the cylinder, and dried in the water oven (100 deg.  C.[A]) for an hour to an hour and a half, and re-weighed, and the moisture then remaining in it calculated.

[Footnote A:  It is dried at 180 deg.  C. at Waltham Abbey, in a specially constructed drying chamber.]

[Illustration:  FIG. 10.—­COTTON DRIER.]

It is very convenient to have a large copper water oven, containing a lot of small separate compartments, large enough to hold about a handful of the cotton, and each compartment numbered, and corresponding to one of the drying cylinders.  The whole apparatus should be fixed against the wall of the laboratory, and may be heated by bringing a small steam pipe from the boiler-house.  It is useful to have a series of copper trays, about 3 inches by 6 inches, numbered to correspond to the divisions in the steam oven, and exactly fitting them.  These trays can then be taken by a boy to the drying cylinders, and a handful of the cotton from each placed in them, and afterwards brought to the laboratory and weighed (a boy can do this very well), placed in their respective divisions of the oven, and left for one to one and a half hours, and re-weighed.

When the cotton is found 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.