The apparatus constructed by Thalwitzer for the manufacture of oil black (Fig. 34) consists of a plate, _A_, with a rim, _a_, fastened to a vertical axis, _b_. This is carried by bearings in _B_, and is kept in rotation by the cog-wheels, _d_ and _f_, moved from the shafting, _O_. The plate is cooled by water supplied by the pipe, _g_, it flows away by _h_ and the annular vessel, _D_. The lamps, _e_, are connected with the common oil reservoir by _n_. _H_ is a scraper fastened to _B_ by _x_. CHAPTER XLIII. INDIAN INK. Indian ink consists of purified soot mixed with gum, as binding medium, and a little camphor and musk. It is generally believed that Indian ink is made in China by a process which is still a secret; this opinion is supported by the fact that the ink made in Europe is almost always inferior to the Chinese. It is quite possible that a substance which produces a particularly fine black is used in China to prepare the soot used for the ink, and which we do not yet know or do not use on account of its high price. But it appears that the difficulty of producing an ink equal in quality to the Chinese lies less in the quality of the soot —for by the methods of purification already given we can make almost chemically pure carbon—than in the extremely careful mechanical treatment of the soot with the other constituents of the ink. The genuine Chinese ink has a peculiar smell, which, when not hidden by musk, distinctly recalls the smell of burning camphor; the camphor tree is a native of China, so that it is not impossible that the soot of the wood of this tree is used for the ink. The soot from camphor alone would be too expensive, so that it is probable that camphor wood is used from which camphor has been prepared, or that camphor soot is mixed with the soot of a cheaper substance. In fact several varieties of Chinese ink show distinctly the smell peculiar to soot produced by fat. The binding material is animal glue, which is most thoroughly ground with the carbon; the secret of making good Indian ink lies in the careful grinding with the binding medium. According to Piou, who lived for a long time in China and learned the process there used, the black employed for the ink is made from the resins of the pine and other trees. The soot is sieved through silk, boiled with glue and water, and kneaded by hand with a little oil until the mass has become completely homogeneous. It is then left for some days, heated, and pressed into the moulds. This description is not very satisfactory; the products obtained are far inferior to those made by the ordinary European process.