400 MR. SCHRNCK ON RUBIAN AND ITS PRODUCTS OF DECOMPOSITION. 
mined the quantities of alizarine which were obtained by decomposition of weighed 
quantities of the acid with sulphuric acid and with caustic soda. 
1-7210 grm. rubianic acid, dried at 100° C., gave, when decomposed with sulphuric 
acid, 0*7310 grm. dry alizarine=42*47 per cent. 
1*2020 grm. rubianic acid yielded, when decomposed with pure caustic soda, on 
precipitation with sulphuric acid, 0*5430 grm. alizarine = 45* 17 per cent. 
Assuming the formula C52 H29 O27 to be correct, 100 parts of acid should, according 
to theory, aflford 43*44 of dry alizarine. IfSTRECKER’s formula for alizarine, C20H6O6, 
were the correct one, then the only possible formula for rubianic acid, though the 
calculated composition would then not agree very well with that found in my 
experiments, would be C52H27O27, in which case the quantity of alizarine derived 
from 100 parts of acid would be 62*70. It will be seen, therefore, that the view 
which I have adopted of the constitution of the acid derives considerable support 
from these determinations. 
A comparison of the composition of rubian and rubianic acid shows that the latter 
can only be derived from the former by means of oxidation. Jn order however to 
remove all doubt on this point, I made tise following experiment. A solution of 
rubian was divided into two equal parts. One half vras boiled to expel all the air it 
might contain, then mixed w'ith a certain quantity of caustic soda, and immediately 
put into a bottle, which the liquid just sufficed to fill, and which was then closed air- 
tight. The other half was mixed with the same quantity of caustic soda and left 
exposed to the air. After twenty-one days, both liquids were examined in the same 
manner. The soda was supersaturated with acetic acid, the liquid was again ren- 
dered alkaline with ammonia, and then chloride of barium was added, which gave in 
each case a red precipitate. This precipitate was treated as usual with sulphuric acid 
and carbonate of lead, and the filtered liquid was evaporated to dryness. From that 
part of the solution which had been enclosed in the bottle I obtained in this manner 
a quantity of a substance resembling rubian, which was probably rubidehydran, but 
not a trace of rubianic acid ; whereas, by adding baryta water to the liquid filtered 
from the precipitate with chloride of barium, leaving the mixture to stand exposed 
to the air for sometime, filtering and treating the substance left on the filter with 
sulphuric acid as usual, I obtained a ssnali quantity of the acid. On the other hand, 
the precipitate with chloride of barium from the second half of the solution, which 
had been left in an open vessel, yielded at once 0*60 grm. of rubianic acid as well as 
a quantity of rubidehydran, but the liquid filtered from this precipitate, on being 
mixed with baryta water and treated as before, gave no more acid. Hence it follows 
that the presence of oxygen is as essential as that of alkalies to the formation of this 
acid. 
As regards the manner in which the formation of rubianic acid takes place, we 
may suppose it to be effected in two ways. Assuming 1, equivalent of rubian to 
absorb 10 equivalents of oxygen, it may then yield 1 equivalent of rubianic acid. 
