448 MR. SCHUNCK ON RUBIAN AND ITS PRODUCTS OF DECOMPOSITION, 
= 50’44 per cent. The formula H 4 Og+PbO requires 49-90 per cent, oxide of 
lead. 
The formula here given is the same to which I was led by my former experiments, 
and it now receives a new confirmation from the relation in which it stands to that of 
rubian. 
The formation of alizarine from rubian admits of a very easy explanation. By 
simply losing 14 equivs. of water, 1 equiv. of rubian is converted into 4 equivs. of 
alizarine, as the following equation shows : — 
C .6 H 34 03 o= 4 (C .4 H 3 04 ) + 14HO. 
The action of sulphuric acid in the preparation of garancine from madder now be- 
comes more intelligible. It consists simply, as far as the practical effect is concerned, 
in the conversion of rubian into alizarine, 
MM. Wolff and Strecker, in a late paper ‘ On the Red Colouring Matters of Mad- 
der*,’ 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 He Oe, 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 analysed was impure. Even 
when perfectly well crystallized, alizarine may contain an amount of impurity suffi- 
cient 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 investigation I obtained from madder a specimen 
of alizarine in perfectly well-defined crystals, containing apparently no foreign sub- 
stance, 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, equiva- 
lent to 68*41 per cent, of carbon. 
The remainder of the substance was recrystallized from alcohol, 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 
* Ann. der Chem. u. Pharm., vol. Ixxv. p. 1. 
