922 ' NEUTRAL COLOURING MATTERS. 



to a state for crystallization. The orcin is purified by treatment 

 with animal charcoal and repeated crystallization. It forms 

 colourless, four-sided prisms, of a sweet but disagreeable taste, 

 soluble in water and alcohol, easily fused and volatile. Its 

 most characteristic property is the becoming of a deep violet 

 colour, when exposed to the joint action of ammonia and air, 

 owing to the formation of orcein. Orcin forms a compound 

 with oxide of lead, of which the formula is C 18 H 7 O 3 -f 5PbO. 



Orcein. The orcein of archil is of a fine red colour, slightly 

 soluble in water, but colouring it strongly, and wholly precipi- 

 tated from it by the addition of a neutral salt. It is very 

 soluble in alcohol, which it colours scarlet, and from which 

 water precipitates it. It is scarcely soluble in ether. Orcein 

 dissolves easily in potash or ammonia, giving it a magnificent 

 purple colour, the colour of ordinary archil ; from this solution, 

 the colouring matter may be separated by the addition of an 

 excess of common salt. An alkaline orceinate gives with me- 

 tallic salts, lakes of a fine purple of different shades, which, 

 however, lose much of their lustre in drying. Dr. Kane finds 

 the orcein of archil to be often a mixture of two substances, dif- 

 fering in their proportion with the age of the archil, which he 

 names alpha-orcein and beta-orcein ; the last is produced by the 

 oxidation of the first, and is the orcein of Robiquet and other 

 chemists. Their formulae are : 



Alpha-orcein . . C 18 H 10 NO 5 (Kane). 

 Beta-orcein . . C 18 H 10 NO 8 (Liebig, Kane). 



The last was dried before analysis at 212. The formulae of 

 their compounds with oxide of lead are : that of alpha-orcein, 

 C 18 H 10 NO 5 + 3PbO, and that of beta-orcein, C 18 H 10 NO 8 

 + 3PbO. The two orceins are identical in all their essential 

 chemical properties ; have the same solubility in water, alcohol 

 and ether. The formula of anhydrous ore in being C 18 H 7 O 3 , 

 that substance requires only to combine with I atom of am- 

 monia, together with 2 or 5 atoms of oxygen, to form the one 

 or other variety of orcein. 



In addition to the two orceins, the archil of commerce con- 

 tains erythrokic acid and azoerythrin, of which the former 

 admits of two modifications ; besides the yellow matter of 

 Heeren. 



