Dr. Schunck on Rubian and its Products of Decomposition. 359 



Colouring Matters of Madder*/ have given another formula for 

 alizarine, which they prefer on account of the relation in which 

 they suppose this substance to stand to naphthaline. This 

 formula is C 20 H 6 O 6 , which requires in 100 parts — 



Carbon 68-96 



Hydrogen 3*45 



Oxygen 27'59 



In confirmation of this formula they adduce one analysis, in 

 which they obtained from 0-0650 grm. alizarine 0-163 carbonic 

 acid, equivalent to 68-4 per cent. If it be permitted to deduce 

 any safe inference from the analysis of so small a quantity of 

 substance, I should be inclined to say that the substance ana- 

 lysed was impure. Even when perfectly well crystallized, aliza- 

 rine may contain an amount of impurity sufficient to affect its 

 composition. This impurity generally consists of verantine. A 

 large admixture of the latter substance entirely prevents alizarine 

 from crystallizing, but a small quantity merely gives the crystals 

 a brownish or reddish tinge. Alizarine can never be considered 

 as perfectly pure unless it exhibits a pure dark yellow colour 

 without admixture of red. In proof of this statement I may 

 adduce the following experiments. In the course of my investi- 

 gation I obtained from madder a specimen of alizarine in per- 

 fectly well-defined crystals, containing apparently no foreign 

 substance, but having a brownish-red colour instead of the dark 

 yellow characteristic of pure alizarine. 



0-3530 grm. of these crystals, dried at 100° C, gave 0-8855 

 carbonic acid, equivalent to 68-41 per cent, of carbon. 



The remainder of the substance was recrystallized from alco- 

 hol, and 



0-2940 grm. now gave 0-7360 carbonic acid, equivalent to 

 68-27 per cent, of carbon. 



On dissolving the rest in alcohol and adding to the solution 

 acetate of copper, a dark reddish-brown precipitate of verantine- 

 oxide-of-copper fell. From the dark purple solution, after being 

 treated in the manner before described, there were obtained some 

 beautiful dark yellow crystals of alizarine, which on analysis 

 yielded the following result : — 



0-1270 grm., dried at 100° C, gave 0-3245 carbonic acid, 

 equivalent to 69-68 per cent, of carbon. 



It appears therefore that alizarine cannot be separated from 

 the last portions of impurity by recrystallization merely. 



Were the above formula the correct one, it would be difficult 

 to account for the circumstance that the purer the substance the 

 greater is the excess, not only of hydrogen, but also of carbon. 



* Ann. de Client, u. Phurm., vol. lxxv. p. 1. 



