6 feet per minute, and as the length of the band travelled amounts to 126
feet, the operation of drying takes twenty-one minutes. One and a quarter
lb. are weighed out and placed in a tin box; a truck, fitted to receive a
number of these boxes, carries it along a tramway to a cool room, where it
is allowed to cool.

~Dipping.~--Mixed acids are used in the proportion of 1 to 3, specific
gravity nitric acid 1.52, and sulphuric acid 1.84. The dipping tank is
made of cast iron, and holds 220 lbs. of mixed acids, and is surrounded on
three sides by a water space in order to keep it cool. The mixed acids are
stored in iron tanks behind the dipping tanks, and are allowed to cool
before use. During the nitration, the temperature of the mixed acids is
kept at 70° F., and the cotton is dipped in quantities of 1-1/2 lb. at a
time. It is put into a tin shoot at the back of the dipping tank, and
raked into the acids by means of a rabble. It remains in the acids for
five or six minutes, and is then removed to a grating at the back, pressed
and removed. After each charge of cotton is removed from the tank, about
14 lbs. of fresh mixed acids are added, to replace amount removed by
charge. The charge now weighs, with the acids retained by it, 15 lbs.; it
is now placed in the pots, and left to steep for at least twenty-four
hours, the temperature being kept as low as possible, to prevent the
formation of soluble cotton, and also prevent firing. The proportion of
soluble formed is likely to be higher in hot weather than cold. The pots
must be covered to prevent the absorption of moisture from the air, or the
accidental entrance of water, which would cause decomposition, and
consequent fuming off, through the heat generated by the action of the
water upon the strong acids.

The excess of acids is now extracted by means of hydro-extractors, as at
Stowmarket. They are worked at 1,200 revolutions per minute, and whirled