_Concluding remarks, Patents, &c._ In extension of the above it may be added that, on the large scale, the distillation is usually carried on in cast-iron retorts, similar in size, shape, and character to those employed in the manufacture of coal-gas, and of which five, or more, are commonly set horizontally in the same furnace. (See _engr._) Each retort has its mouth (_a_), through which the ‘charge’ is introduced, closed with a movable door, which is securely fastened in its place, in the manner shown in the engr.; and is furnished, at the upper part of its further end, with an iron pipe (_c_), to carry off the evolved vapours to the condenser or receiver. The latter consists of two large square wooden chambers (_B, C_), lined with lead, and either fitted with movable covers, secured by water-joints, or with doors in the side, to permit of the easy removal of the sublimed salt. The first receiver communicates with the second by means of a large lead tube (_d_) near its centre, and by another tube (_d′_), somewhat smaller, and nearer the bottom, but above the surface of the stratum of water in the second receiver, before alluded to. These chambers have also a lead pipe (_e, e_), stopped during the process with a plug or cock of lead, to allow of the liquid product of the distillation, &c., to be drawn off, or run into another receiver or cistern, at will. Both chambers are placed on strong wooden supports, or scaffolding, to bring them on a level with the retorts. When the impure sulphate or other ammonia-salt is used in the manufacture of the sesquicarbonate (which is generally the case), the resulting salt being impure and discoloured, is resublimed in iron pots (_f, f, f_), furnished with movable leaden heads, which are kept cool by a current of air passing over them; a little water being introduced into the subliming pots to render the product translucent. The heat is applied either by means of a flue passing from the retort-furnace (_A, b_), or by a water bath heated in the same manner; the latter being the preferable method, as the temperature should not be greater than about 200° Fahr., and need not exceed 150° to 155°. These pots are arranged in sets, as shown at _D_ in the engraving. [Illustration] The charge of a retort usually consists of about 70 to 72 _lbs._ of sulphate of ammonia or 57 to 58 _lbs._ of the hydrochlorate to 1 _cwt._ of chalk; or in these proportions. The product is about 40 _lbs._ of the crude salt, which, by careful resublimation, yields about 39 _lbs._ of marketable carbonate of ammonia. Carbonate of ammonia, like the chloride and sulphate, is now scarcely ever prepared on the small scale, that of commerce being not only cheaper, but sufficiently pure for all the purposes of medicine and the arts.