of the acids. It was found that if water was carefully run on to the
surface of the acids in which the nitro-cellulose is immersed, and the
acids be slowly drawn off at the bottom of the vessel, the water displaces
the acid from the interstices of the nitro-cellulose without any
undesirable rise in temperature, and with very little dilution of the
acids. By this process almost the whole of the acid is recovered in a
condition suitable for concentration, and the amount of water required for
preliminary washing is very greatly reduced. The apparatus which is used
for the purpose consists of a cylindrical or rectangular vessel
constructed with a perforated false bottom and a cock at its lowest point
for running off the liquid. Means are also provided to enable the
displacing water to be run quietly on to the surface of the nitrating
acids.[A]

[Footnote A: In a further patent (Eng. Pat. 7,269, 1903, F.L. Natham),
J.M. Thomson and W.T. Thomson propose by use of alcohol to replace the
water, used in washing nitro-cellulose, and afterward to remove the
alcohol by pressing and centrifuging.]

The apparatus is shown in Fig. 2O, side elevation, and in Fig. 21 a plan
of the nitrating vessel and its accessories is given. In Fig. 20 is shown
in sectional elevation one of the trough devices for enabling liquids to
be added to those in the nitrating vessel without substantial disturbance.

[Illustration: FIG. 20.--SECTIONAL ELEVATION OF THOMSON'S APPARATUS, _a_,
Tank; _b_, False Bottom; _c_, Bottom; _c'_, Ribs; _d_, Draining Outlet;
_e_, Grid; _f_, Troughs, with Aprons _g_; _h_, Pipe, with Branches _h'_,
leading to Troughs, _f_; _k'_, Outlet Pipe of the Sulphuric Acid Tank _k_;
_l_, Water Supply Pipe; _m_, Pipe to supply of Nitrating Acids; _o_,
Perforations of Trough _f_; _p_, Cock to remove Acid.]