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.]

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