ng 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.]

In carrying out this invention a rectangular lead-lined or earthenware
tank _a_ is employed, having a false bottom _b_, supported by ribs _c'_,
over the real bottom _c_, which slopes down to a draining outlet pipe _d_,
provided with a perforated grid or plate _e_, adapted to prevent choking
of the outlet. Suitably supported near the top of the vessel _a_ are
provided two troughs, _f_ having depending aprons _g_, a pipe _h_ has two
branches _h'_, leading to the troughs, _f_. This pipe _h_ is adapted to be
connected by a rubber pipe either to the outlet pipe _k'_ of the sulphuric
acid tank _k_ or the water supply pipe _l_. The nitrating acids are
supplied through the pipe _m_. A charge of mixed nitrating acids is
introduced into the vessel _a_ say up to the level _n_, and the dry
cellulose thrown into the acids in small quantities at a time, being
pushed under the surface in the usual way.

[Illustration: FIG. 21.--PLAN OF THOMSON'S APPARATUS, _a_, Tank; _b_,
False Bottom; _c'_, Ribs; _e_, Grid; _f_, Troughs; _g_, Aprons; _h_ and
_h'_, Pipes to Troughs _f_; _k_, Sulphuric Acid Tank; _m_, Pipe to
Nitrating Acids Tank; _o_, Perforations of Troughs; _p_, Cock to remove
Acid.]

A thin layer, say half an inch, of a suitable liquid, preferably sulphuric
acid, of a gravity not exceeding that of the waste acid to be produced, is
run carefully on the top of the acids by means of the troughs _f_, which
are perforated as shown at _o_, so that the sulphuric acid runs down the
aprons _g_, and floats on the nitrating acids. The whole is then allowed
to stand till nitration has been completed. Water is then supplied to the
troughs by way of the pipes _l_, _h_, and _h'_, and is allowed to float
very gently over the surface of the sulphuric acid, and when a sufficient
layer has been formed, the cock _p_ at the bottom of the apparatus is
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 d