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 displacement 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.