isplacement 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
great advantage over the usual method of working ordinary centrifugal
nitrating apparatus, because the acid being removed before the centrifugal
is run, practically all danger of firing therein disappears, and a greater
proportion of the waste acid is recovered.

In some cases the acids and water may be supplied by perforated pipes,
lying along the edges of the nitrating vessel, and these edges may, if
desired, be themselves made inclined, like the sides of the troughs _f_.
In the case of effecting nitration in centrifugals as above, the
displacing sulphuric acid and water may thus be supplied round the edges
of the machines, or removal troughs such as _f_ may be used. It will be
obvious that any inert liquid of suitabl