Dr. Schunck on the Colouring Matters of Madder. 139 



with a smoky flame, leaving no residue. When heated in a tube it 

 is completely volatilized, without leaving the least residue. The 

 vapours condense in the colder parts of the tube, forming long white 

 needles. This sublimate differs however in composition from the 

 acid itself, and I shall therefore call it pyroalizaric acid. When 

 heated in a tube with caustic lime it is decomposed and a yellow oil 

 distils over, having a smell very nearly resembling that of benzin. It 

 is soluble in boiling water ; the solution has an acid taste and reddens 

 litmus paper. A concentrated boiling solution crystallizes on cool- 

 ing. It is easily soluble in alcohol. Concentrated sulphuric acid 

 dissolves it in the cold. On boiling the solution no blackening takes 

 place, and the alizaric acid distils over and crystallizes on the cooler 

 parts of the tube. It is easily soluble in alkalies. A concentrated 

 alkaline solution gives crystals on the addition of a strong acid. 

 The solution in ammonia gives no precipitates with the chlorides of 

 barium and calcium. If to the solution in water chalk be added 

 until all effervescence has ceased, the solution gives a crystallized 

 lime salt on evaporation. This lime salt is decomposed on being 

 strongly heated in a tube, becomes black, and gives an oil with an 

 aromatic odour which on cooling crystallizes. The aqueous solution 

 gives with perchloride of iron a yellowish-white precipitate, with 

 sugar of lead a white precipitate, with nitrate of silver alone no pre- 

 cipitate, but on the addition of ammonia a white curdy precipitate, 

 which after some time becomes crystalline. The reaction with sugar 

 of lead distinguishes it from benzoic acid, which is not thereby 

 precipitated. Its greater solubility in water and its acid taste are 

 also distinguishing characteristics ; but in most respects it will be 

 seen that the resemblance between the two is very striking. It is 

 very probable that this acid is also formed from alizarin by means 

 of nitric acid, but the small quantity of pure alizarin hitherto at my 

 disposal has prevented me from ascertaining whether this is the case. 



From some analyses which I have made of this acid, I infer that 

 its composition is expressed by the formula C 14 H 3 7 , and that the 

 formula for pyro-alizaric acid is C 14 H 3 5 , in which case alizaric 

 acid contains one equivalent of hydrogen less and three equivalents 

 of oxygen more than benzoic acid, which is C, 4 H G 4 . 



Rubiacin. — This is the substance which I called alizarin in the 

 paper which I read on this subject last year. I have since discovered 

 that it is not identical with Robiquet's alizarin, and have therefore 

 given it a new name. Rubiacin is most easily obtained in a state 

 of purity from rubiacic acid. Rubiacin and rubiacic acid are mu- 

 tually convertible, as they only differ from one another by a certain 

 number of atoms of oxygen. Rubiacin is converted into rubiacic 

 acid by the action of the persalts of iron, and rubiacic acid may be 

 reconverted into rubiacin by reducing agents, such as sulphuretted 

 hydrogen. In order then to obtain pure rubiacin, it is best to take 

 a solution of pure rubiacate of potash, to which a slight excess of 

 caustic potash has been added, and to pass sulphuretted hydrogen 

 gas through the solution for some time. Chloride of barium is then 

 added, which produces a dark purple precipitate, consisting of rubi- 



