942 TANNIC ACID AND BODIES ALLIED TO IT. 



acid assumes a deep blue tint ; the black precipitate which falls, 

 left in contact with gallic acid, is gradually reduced to the state 

 of protoxide, but not when a salt of the ferrosoferric oxide has 

 been employed in its formation. 



When crystallized gallic acid is dissolved in concentrated 

 sulphuric acid, and the solution heated, it assumes a crimson 

 tint at a temperature above 284. If allowed to cool in this 

 state, and cold water afterwards added, an abundant precipitate 

 is formed of a reddish brown colour and crystalline aspect, 

 which appears to be gallic acid that has lost an atom of water, 

 its composition after drying being C 7 H 2 O 4 (Robiquet). It is 

 insoluble in water, but dissolves easily in alkalies, and has some 

 analogy, according to Robiquet, to the colouring matter of 

 madder. It is decomposed by dry distillation, giving small 

 cinnabar-red prismatic crystals. 



All the g dilates are remarkable for the facility with which 

 they absorb oxygen, when in contact with an excess of alkali ; 

 carbonic acid is then formed and a brown matter insoluble in 

 water. In a mineral water alkaline from the presence of lime 

 or magnesia, an addition of gallic acid causes the liquid gra- 

 dually to become turbid from the formation of a black precipi- 

 tate, although no iron be present, owing to the decomposition 

 of the gallic acid itself, 



Pyrogallic acid, C 6 H 3 O 3 , or C 8 H 4 O 4 (the equivalent being 

 doubtful), is prepared by heating briskly either tannic or gallic 

 acid, previously well dried, in a retort by means of a spirit-lamp 

 till coloured empyreumatic products come over. The pyro- 

 gallic acid is obtained as a crystalline sublimate, which may be 

 purified by a second sublimation at a gentle heat. It forms 

 white plates or needles, fuses at 239, boils at 410, and sub- 

 limes without alteration. It does not redden litmus ; its taste 

 is bitter and slightly astringent. It dissolves in 2^ parts of 

 water at 55.4 (13 centig.) ; the solution absorbs oxygen and 

 deposits a brown powder. It is equally soluble in alcohol and 

 ether. When heated briskly above 482 (250 centig.), pyro- 

 gallic acid blackens, and is converted into water and metagallic 

 acid. 



Pyrogallate of lead, a white precipitate formed on adding a 

 solution of pyrogallic acid to acetate of lead, is PbO,C 6 H 6 O 3 , 



