opened, and the acid slowly drawn off, water being supplied to maintain
the level constant. It is found that the rate of displacement of the acids
is a factor which exerts a considerable influence on the properties of the
resulting nitro-cellulose, and affords a means of regulating the
temperature of displacement. A rate of displacement which has been found
suitable is about two inches in depth of the vessel per hour when treating
highly nitrated celluloses, but this rate may, in some cases, be
considerably increased. The flow of water at the top of the apparatus is
regulated so that a constant level is maintained. By this means the water
gradually and entirely displaces the acids from the interstices of the
nitro-cellulose, the line of separation between the acids and the water
being fairly sharply defined throughout. The flow of water is continued
until that issuing at the bottom is found to be free from all trace of
acid. The purification of the nitro-cellulose is then proceeded with as
usual, either in the same vessel or another.

In the process above described, the object of the introduction of a small
layer of sulphuric acid is mainly to prevent the fuming which would
otherwise take place, and is not essential, as it is found it can be
omitted without any deleterious effect. In order to use the mixed acids in
the most economical manner, the waste acid from a previous operation may
be used for a first nitration of the cellulose; being afterwards displaced
with fresh acids which carry the nitration to the required degree before
they are in turn displaced by water. The apparatus may be used merely for
the removal of the acid, in which case the nitration is carried out in
other vessels in the usual way, and the nitro-cellulose removed to the
displacement apparatus where it is just covered with waste acid, and the
displacement then proceeded with as above described. In some cases the
process is carried out in an ordinary nitrating centrifugal, using the
latter to effect preliminary drying after acid extraction. This gives a