GALLIC ACID. 941 



at 212: HO,PbO.2C 7 HO 3 -f HO; the last atom of water is 

 lost at 320. Of another gallate of lead : 2PbO,C 7 HO 3 . Of 

 acid gallate of ammonia, the composition is expressed by 

 NH 4 O,C 7 HO 3 + 2HO,C 7 HO 3 ; it loses nothing at 212. These 

 are the only salts of gallic acid of which the composition is 

 certainly known, and they are not sufficient to determine 

 whether or not gallic acid is bibasic. 



The metamorphosis of tannic acid into gallic acid, under the 

 influence of boiling dilute sulphuric acid has already been 

 adverted to. The same change occurs in an aqueous extract of 

 gallnuts exposed to air for several months. By the absorption 

 of 8 atoms of oxygen, 1 atom of hydrated tannic acid might be 

 converted into 2 atoms of hydrated gallic acid, 4 atoms of carbonic 

 acid, and 2 atoms of water : C 18 H 8 O 12 and O 8 =2C 7 H 3 O 5 and 

 4CO 2 and 2 HO. Much ellagic acid is also formed in the trans- 

 formation of the tannin of gallnuts, in the air. To prepare 

 gallic acid, a strong extract of gallnuts in cold water may be pre- 

 cipitated in the cold by sulphuric acid ; the thick mass be mixed 

 with dilute sulphuric acid, expressed while still humid, and in- 

 troduced in this state into a mixture of sulphuric acid with two 

 parts of water at the boiling temperature. The liquid is boiled 

 for some minutes and then allowed to cool ; crystals of gallic 

 acid are deposited, which may be purified by crystallizing again 

 from water, converting the new product, which is still coloured, 

 by means of acetate of lead into an insoluble gallate of lead, 

 which last is washed, then diffused through water, and decom- 

 posed by a stream of sulphuretted hydrogen gas ; the sulphuret 

 of lead, which is then formed, assists in carrying down the 

 colouring matter. 



Gallic acid crystallizes on cooling from a hot solution, in thin, 

 silky needles. It requires 100 parts of cold water to dissolve 

 it, but is soluble in 3 parts of boiling water. It is also very 

 soluble in alcohol, and to a small extent in ether. When pure 

 it does not precipitate a solution of gelatine. The aqueous 

 solution of this acid may be kept apart from air without change, 

 but with access of oxygen it undergoes decomposition, a brown 

 matter being deposited, while carbonic acid is evolved and the 

 surface becomes covered with mouldiness. This decomposition 

 is greatly promoted by the presence of a mineral acid, or of an 

 alkali in the solution. With salts of peroxide of iron gallic 



