938 TANNIC ACID AND BODIES ALLIED TO IT. 



more than its basic water. It is formed, as already stated, 

 on boiling a solution of meconic acid with a pretty strong 

 acid; also by heating dry meconic acid to 392, when 

 carbonic acid is disengaged till the temperature rises to 

 446, when the meconic acid is found to be entirely con- 

 verted into comenic acid. In this transformation, the former 

 hydrated acid loses 2 atoms of water and 2 atoms of carbonic 

 acid : C 14 H 4 O 14 =C 12 H 2 O 8 and H 2 O 2 and C 2 O 4 . 



Comenic acid dissolves in 16 parts of boiling water ; its solu- 

 tion decomposes the alkaline carbonates, and reddens salts of 

 peroxide of iron. At 572, it is decomposed and resolved into 

 water, carbonic acid and pyromeconic acid. The comenates 

 contain all two atoms of base, one of which is occasionally 

 water. 



Pyromeconic acid, HO,C 10 H 3 O 5 , a monobasic acid, which 

 presents itself as a colourless sublimate, composed of brilliant 

 flattened plates, when crystallized comenic acid is submitted to 

 dry distillation : C 12 H 4 O 10 =C 10 H 3 O 5 and HO and C 2 O 4 . Its 

 taste is styptic, with a bitter after-taste ; it fuses between 

 248 and 257 into an oily liquid, and sublimes without leaving 

 any residue. It is very soluble both in water and alcohol. It 

 reduces a solution of gold, and communicates a red colour to 

 solutions of peroxide of iron. Its salt of lead is a white pre- 

 cipitate, of the composition, PbO,C 10 H 3 O 5 . 



SECTION III. 



TANNIC ACID AND BODIES ALLIED TO IT. 



The formula of tannic acid or tannin dried at 212 is 3 HO 

 -}-C 18 H 5 O 9 : it is a tribasic acid. 



Tannic acid occurs in the bark of all the varieties of Quercus 

 and many other trees, and in gall-nuts, from which it is pro- 

 cured in greatest purity. It is prepared, after Pelouze, in a 

 percolator or apparatus of displacement, fig. 95, the lower 

 opening of which is closed by a plug of cotton, and the vessel 

 entirely filled with broken gall-nuts, which are more suitable 

 for the experiment than the powder of gall-nuts. Over the 

 gall-nuts as much aqueous ether is poured as the vessel will 

 contain, and the mixture left to digest for several hours. 

 The liquid is then permitted to run off into the caraffe below, 



