=Madder Extract.=—The fixation of the colouring matter of madder upon the fibre is attended with many difficulties; attempts were made to present it in the purest form possible, or in such a condition that the operation of dyeing was simplified. The commercial madder extracts consist either of liquid extracts of madder, or of the more or less pure colouring matter itself. Among the numerous solid or liquid madder extracts, without doubt the most important is the crude alizarin (the principal colouring matter of madder), made by the process discovered almost simultaneously by Rochleder and Pernod. This is produced by extracting either madder or garancin with hot water containing sulphuric acid; about 5 grammes of acid are mixed with 1 kilogramme of water and the madder boiled with this dilute acid in a lead-lined vessel. The liquid separated from the solid residue becomes turbid on cooling, and yellow flocks separate which consist of impure alizarin; no further purification is required for practical purposes. Crude alizarin is also obtained by treating madder with superheated steam. These preparations are important, not only to the dyer and calico printer, but also to the colour maker; from them the madder pigments can be made in a very simple manner. =The Constituents of Madder.=—Madder has been most thoroughly investigated: its colouring matter, alizarin, is now made artificially. In addition to woody fibre, madder contains sugar, mucilage, resin, a glucoside (_i.e._, a substance which can be decomposed into a sugar and another substance), and, in particular, two colouring matters known as alizarin and purpurin. Alizarin occurs ready formed in madder, and is also produced by the decomposition of the madder glucoside into alizarin and sugar. To obtain pure alizarin finely ground madder is extracted with boiling water; on the addition of sulphuric acid to the decoction alizarin separates together with other substances. The moist precipitate is boiled with a solution of aluminium chloride, and, after filtering, hydrochloric acid is added to the filtrate, when deep red flocks separate, consisting of a mixture of alizarin and purpurin. The colouring matters are further purified by dissolving in alcohol and adding freshly precipitated alumina, which unites with them; the alumina compounds are then boiled with strong soda solution, in which the purpurin dissolves. The residue consists of aluminium alizarinate mixed with resin, the latter is extracted with ether or benzene, and the residue then decomposed with hydrochloric acid, when alizarin is set free and is purified by recrystallisation.