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Madder contains two closely allied colouring matters, namely, alizarin and purpurin.

Red.—Alizarin, anthrapurpurin, flavopurpurin, purpurin, alizarin Bordeaux, alizarin garnet R, alizarin maroon, alizarin S, cloth red, diamine fast red, anthracene red, chrome red, chrome Bordeaux, salicine red, erio chrome red, emin red, milling red.

Purpurin is a glucoside of 1,2,4, trioxyanthraquinone.

These chemists isolated two compounds which they named alizarin and purpurin.

The purpurin, discovered among the colouring-matters of madder in 1826, is a trihydroxy-anthraquinone; it can be prepared by the oxidation of alizarin, as shown by De Lalande in 1874.