Dr. Schunck on the Colouring Matters of Madder. 135 



pure colouring matter, a circumstance which has likewise been 

 doubted. In order to obtain it in a state of purity, the red flocks 

 which are deposited by the boiling watery solution are separated by 

 filtration and dissolved in boiling alcohol. To the boiling solution, 

 which has a brownish-yellow colour, hydrate of alumina is added, 

 and the boiling is continued for some time. The alizarin combines 

 with the alumina, forming a dark-red compound, while the fluid loses 

 its colour. Fresh alumina is added, until no more colouring matter 

 can be thereby separated. A great part of the fat remains dissolved 

 in the alcohol, while a part combines with the alumina. The coloured 

 alumina is separated by filtration and washed with alcohol for some 

 time. It is then treated with a weak boiling solution of caustic potash, 

 which dissolves the excess of alumina and all the fat which may 

 have combined with the alumina, but leaves the compound of ali- 

 zarin and alumina undissolved, merely changing its colour from red 

 to dark purple. This process is repeated several times, until the 

 alkaline fluid is no longer red, but of a pure purple colour. The 

 residue is treated with muriatic acid, which dissolves the alumina 

 and leaves the alizarin behind in crystalline flocks of an orange colour, 

 which are washed with water to remove the acid and then dissolved 

 in alcohol. The alcoholic solution on evaporation gives shining, 

 prismatic, orange-coloured crystals of alizarin, which may, if neces- 

 sary, be purified by a second crystallization from alcohol. 



The mass left undissolved by boiling water consists of rubiacin 

 and two distinct kinds of fat. I have only been able to discover 

 one method of extracting the rubiacin from this mixture. This 

 method is founded on the solubility of rubiacin in perchloride and 

 pernitrate of iron. It is immaterial which of these two salts be 

 taken. Persulphate of iron would not answer the purpose. If the 

 mixture of rubiacin with the two fats be treated with a somewhat 

 concentrated boiling solution of perchloride or pernitrate of iron, a 

 solution of a deep reddish-brown colour is obtained, while a brown 

 residue remains insoluble in an excess of the iron salt solution, 

 consisting of one of the two fats in combination with oxide of iron. 

 The fluid is filtered, and on the addition to it of muriatic acid, a 

 yellow flocculent precipitate is obtained, which is separated by filtra- 

 tion and washed until all the iron salt and the excess of acid are 

 removed. This precipitate consists of rubiacin, the second of the 

 two fats, and of a new body which I shall call rubiacic acid. The 

 latter substance does not exist ready-formed in madder, but is pro- 

 duced by the action of the persalt of iron on rubiacin. This action 

 consists in the rubiacin taking up a certain number of atoms of 

 oxygen from the persalt of iron, and the acid which is thus formed 

 combines with peroxide of iron, producing a compound soluble in 

 water with a reddish-brown colour, and decomposable by any strong 

 acid. Part of the rubiacin however escapes this action, and is pre- 

 cipitated together with the rubiacic acid and fat on the addition of 

 muriatic acid. The precipitate is treated with boiling alcohol, which 

 dissolves the rubiacin and the fat, and leaves behind the rubiacic 

 acid in the shape of a yellow powder. This process is repeated 



