ELLAGIC ACID. 943 



according to Berzelius, and also Pelouze, but PbO,C 8 H 8 O 4 ac- 

 cording to the later analysis of R. C. Campbell. 



Metagallic acid, melangallic acid (Berzelius). The formula 

 of the anhydrous acid is C 6 H 2 O 2 (Pelouze) ; its probable atomic 

 weight: HO,C 12 H 3 O 3 . This body remains as a fixed residue 

 in the retort when gallic acid or tannic acid is heated by an oil 

 bath to 482, till all the volatile products escape. It is then 

 dissolved in a solution of alkali and precipitated by an acid to 

 obtain it in a state of purity. It is a black powder, insoluble in 

 water, which forms soluble compounds with alkalies of a deep 

 black colour. 



Tannic and gallic acids, although differing so much in com- 

 position, afford the same products when decomposed by heat. 

 This is explained by supposing tannic acid a compound of gallic 

 and pyrogallic acids. In fact 3 atoms of tannic acid contain the 

 elements of 6 atoms of gallic acid, and 2 atoms of pyrogallic 

 acid. 



3 (C I8 H 8 I2 )=6 (C 7 H 3 5 )+2 (C 6 H 3 3 ). 



Ellagic acid. The gallic acid which forms in an infusion of 

 gallnuts exposed to air, is always accompanied by a grey 

 powder, which being insoluble, may be purified by boiling 

 water; it is the acid in question. Ellagic acid dissolves in 

 alkalies and is precipitated by acids. According to Pelouze it 

 possesses the same composition as dried gallic acid, C 7 H 3 O 5 ; 

 between 212 and 248, it loses an atom of water and becomes 

 C 7 H 2 4 . 



Catechu, the brown dried extract of the Acacia or Mimosa 

 catechu contains a large quantity of tannic acid differing 

 little from the tannic acid of gallnuts, which may be extracted 

 by cold water. The portion insoluble in cold water, contains 

 a particular principle catechin, C 15 H 6 O 6 (Swanberg). Two 

 acids are produced when catechin with alkalies or alkaline 

 carbonates absorbs oxygen from the air, and forms black solu- 

 tions with the former and red with the latter ; which have 

 been named japonic acid, HO + C 12 H 4 O 4 , and rubinic acid, 

 HO,C 18 H 6 O 9 (Swanberg.) 



