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