_Props._ Red prisms; sublimes at 419° F.; odourless, tasteless, and neutral to test-paper; sparingly soluble in water, even at the boiling temperature; soluble in alcohol and ether; not decomposed by hydrochloric acid; dissolved, without decomposition, by strong sulphuric acid; soluble in solutions of the alkalies and their carbonates; acids precipitate alizarin from its alkaline solutions in orange-coloured flakes; alumina decolorises an alcoholic solution of alizarin, forming a red lake. =ALIZARIN, ARTIFICIAL.= C_{14}H_{8}O_{4}. This colour was first obtained by Graebe and Liebermann in 1869 from anthrachinon, an oxidation product of anthracen, this latter being a substance which is formed during the destructive distillation of coal-tar. These chemists converted anthracen into antichinon by means of nitric acid. The crude anthracen is previously purified by treatment with benzoline (petroleum spirit), aided by heat, and by being subjected to the action of the centrifugal machine to fusion, and to sublimation. According to the original method of preparing alizarin, the anthrachinon was first converted into a dibromide of anthrachinon by treatment with bromine, and this bromated compound, by further treatment either with caustic potash or soda at a temperature of 180° to 200° C., converted into alizarin-potassium (or alizarin-sodium if caustic soda has been used), from which the alizarin is set free by means of hydrochloric acid. Alizarin is now procured from anthrachinon by treatment at a temperature of 260° C., with concentrated sulphuric acid of 1·84 sp. gr., the anthrachinon being converted into a sulpho-acid; this acid is next neutralised with carbonate of lime, the fluid decanted from the deposited sulphate of lime, and carbonate of potash added to it, with the object of throwing down all the lime. The clear liquid is then evaporated to dryness, the resulting saline mass is converted into alizarin-potassium by heating it with caustic potash. From the alizarin-potassium thus obtained the alizarin is set free by the aid of hydrochloric acid. In another method the preparation of anthrachinon is avoided, and anthracen employed directly, by first converting it, by means of sulphuric acid and heat, into anthracen sulphonic-acid. After having been diluted with water, the solution of this acid is treated with oxidising agents (peroxides of manganese, lead, chromic acid, nitric acid), and the acid fluid is afterwards neutralised with carbonate of lime. When peroxide of manganese has been used, the manganese is also precipitated as oxide. The oxidised sulpho-acid having been previously converted into a potassium salt, the latter being heated with caustic potash, alizarin is obtained. The details of these two processes will be found set forth in the terms of the patent taken out by Messrs Caro, Graebe and Liebermann, further on.