Alizarin has the formula C₁₄H₆O₂(OH)₂. It forms fine red crystals which are very little soluble in cold but more easily in hot water; they dissolve readily in strong alcohol to a yellow solution. Alizarin dissolves in alkaline liquids, the solution is dichroic; it appears dark purple by transmitted light and pure blue by reflected light. When the alkaline solution of alizarin is brought in contact with freshly precipitated alumina the colouring matter is completely thrown down and a beautiful red lake is formed. The second madder colouring matter is purpurin. This is more soluble in water than alizarin, it also dissolves in alum solutions; its alkaline solutions are not dichroic. The formula of purpurin is C₁₄H₅O₂(OH)₃. In addition to these two colouring matters, madder contains a third, known as rubiacin, which also forms red compounds. It appears to be fixed upon fabrics dyed with madder in company with the other colouring matters. Dyeing and printing with the colouring matters of madder are among the most difficult processes in dyeing; a long series of operations is required to fix the colour permanently on the fabric. The beautiful deep red colour known as Turkey red is produced by madder; it is distinguished from other vegetable colours by its great fastness. By means of madder various other shades besides pure red may be produced. This very valuable substance is largely used in dyeing and calico printing to produce permanent colours. CHAPTER LV. MADDER LAKES. The madder lakes are equally as permanent as the madder colouring matters dyed upon fabrics. On this account and because of their handsome colour they are highly prized by painters. The best qualities of madder are necessary to produce fine lakes; it is more convenient to use garancin or madder extract. Only when the colouring matter is tolerably free from the foreign substances which accompany it can a pure red madder lake be produced; the poorer qualities of madder produce a lake which is not a pure red and is never bright. The good repute of the madder lakes of certain makers is in great part due to careful choice of the raw materials. This choice and care in the process are the secrets of the manufacturers who produce good madder lakes. The author has found that the finest lakes can be obtained without difficulty from the alizarin, separated in the before-mentioned manner from madder by means of sulphuric acid. The small cost of obtaining crude alizarin by this process enables it to be used on a commercial scale. The labour involved in the process is far outweighed by the excellent qualities of the lakes produced.