=Cudbear= is a dirty purple powder with an ammoniacal odour and salt taste. It is made on a large scale in Holland. It differs from archil only in containing the colouring matter in the solid form. =Litmus= is generally made from lichens of the genus _Variolaria_, though varieties of _Rocella_ and _Lecanora_ are often used. It is not made in quite the same manner as archil and cudbear. Together with the ammonia, potash is added to the powdered lichen. The mixture is left until it has become violet, when urine, potash and lime are added at intervals to continue the decomposition until the colour is changed to blue and the whole mass has become a paste. Gypsum and lime are then added, the mixture formed into rectangular plates, and brought into the market, described by a number, according to the amount of gypsum added. Pure litmus should leave little solid residue when dissolved in water, the solution should be a fine violet. On account of its want of stability this colouring matter is not used in dyeing. It can be used for colouring foods since it is innocuous; but indigo carmine has much greater colouring power and at the same time gives fine shades. The only use of litmus at the present time is as an indicator to decide whether a solution is acid, neutral or alkaline. In a quite neutral solution litmus is violet. Acids turn the solution to red and alkalis to pure blue. In preparing litmus tincture regard must be given to the fact that the aqueous solution of the colouring matter is always alkaline; acid must be cautiously added until the colour is violet, and the least addition of acid or alkali suffices to change it to red or blue. CHAPTER LVIII. RED WOOD LAKES. Red wood is obtained from species of _Cæsalpinia_ growing in South America. The following varieties are distinguished: Fernambuco or Brazil wood, Bahia wood, St. Martha wood, Lima wood and Sapan wood, all of which contain the same colouring matter, but in different quantities. Red wood comes into commerce either as thick logs of a fine red colour or as finely-rasped chips. Fernambuco and Sapan woods are accounted the best varieties. Although the colouring matters contained in these woods have been investigated for a long time, an accurate knowledge of them has only recently been obtained. The pure colouring matter of red wood, brasilin, forms orange-coloured needle-shaped crystals, soluble in water, alcohol and ether. On careful heating it partially sublimes undecomposed. Brasilin most probably results from the alteration of a substance contained in the inner less coloured portion of the wood; this compound, brasileïn, has been obtained in colourless needles; its solution acquires a red colour on boiling, and produces the pure colouring matter.