d 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