138 Dr. Schunck on the Colouring Matters of Madder. 



troducing hydrate of alumina into an alcoholic solution of alizarin, is 

 not decomposed by a concentrated solution of caustic potash. The 

 alcoholic solution gives with acetate of iron a dark purple precipi- 

 tate, with acetate of copper a light purple precipitate, with proto- 

 chloride of tin no precipitate, except on the addition of ammonia, 

 when a light red precipitate is produced. If alizarin and a piece of 

 mordanted cloth be introduced into boiling water and the boiling be 

 continued for- some time, the cloth becomes slowly dyed and the 

 mordants assume the tints peculiar to the so-called madder colours. 

 The alizarin slowly disappears in the same measure as the cloth 

 becomes dyed, even though less water had been taken than was suf- 

 ficient to dissolve the whole quantity. It is evident therefore that 

 in the process of dyeing, the alizarin, which is dissolved in the first 

 instance by the boiling water, is taken up by the mordants of the 

 cloth, that then a fresh quantity is dissolved by the water, which is 

 again absorbed, and so on, until all the alizarin has combined with 

 the cloth, or until the mordants can take up no more. Hence the 

 slowness with which madder dyeing is effected. No doubt can re- 

 main then, I think, that alizarin plays a great part in the production 

 of madder colours. That it does not produce the whole effect in 

 dyeing with madder I shall show afterwards. 



The method of obtaining alizarin in a state of purity I have dis- 

 covered so lately, that I have not yet been able to ascertain its com- 

 position and atomic weight. 



Alizaric acid. — At one period of my investigation I imagined that 

 alizarin and rubiacin might be separated by boiling the mixture with 

 perchloride of iron, in Avhich case I expected the rubiacin to dissolve 

 in the solution of the iron salt and the alizarin to remain behind in 

 combination with oxide of iron. I was not aware at that time that 

 alizarin is decomposed by perchloride of iron. Before I had discovered 

 this, however, 1 had taken about one hundred weight of madder, 

 treated it with boiling water, added acid to the fluid, separated the 

 brown precipitate by filtration, and treated the whole quantity of 

 precipitate with perchloride of iron. The fluid was, after the addi- 

 tion of acid and filtration, evaporated to a syrup. After allowing 

 the syrup to stand for some days, I found it filled with white crystals 

 bearing a resemblance to oxalic acid. I added water to the syrup, 

 filtered and washed until the perchloride of iron was removed. I 

 found that the crystals bore washing with cold water without dis- 

 solving in any considerable degree. I then dissolved them in hot 

 caustic alkali, filtered to separate some oxide of iron which remained 

 behind, and added sulphuric acid to the fluid while still hot. On the 

 fluid cooling there separated a quantity of long shining white cry- 

 stals, which were separated by filtration and washed. I now found 

 that they contained no oxalic acid, but that they bore a striking 

 resemblance to benzoic acid. Indeed some of the reactions arc the 

 same as those of benzoic acid, and it was only after having submitted 

 it to an elementary analysis that I became convinced that it was not 

 benzoic acid. I shall call it alizaric acid. It has the following pro- 

 perties : — 



Its taste is acid. When heated on platinum foil it melts and burns 



