A Manual of the Chemistry of the Carbon Compounds

THE CARBON COMPOUNDS. 373
Gallic Acid, or Trioxybemoic Add C6H2 \ $Pi* occurs in the free
state, or as glucoside (tannin) in nutgalls, iu the leaves of the bearberry,
in tea, in the fruits of Camlpinia eoriaria (Divi-Divi), iu
the root-hark of the pomegranate, and in other plants. It is
obtained by boiling tannin with dilute acids, or exposing it in the
moist state to the air at a temperature of 20° to 30°.
Gallic acid is also formed by heating di-iodosalicylic acid with
potassium carbonate and by fusing bromocatechuic or j8 bromodioxybenzoic
acid with caustic potash.
The formation of gallic acid from salicylic acid and pyrocatechuic
acid proves that the hydroxyls have the position 2. 3. 4 (or 3. 4 6).
Gallic acid dissolves in 100° parts of cold, and three parts of
boiling water, and crystallizes in fine silky needles, containing one
molecule of water. It has a slightly acid and astringent taste, and
gives a deep-blue colour with ferric salts. It melts at 200°, and at
210° it is resolved into carbon dioxide and pyrogallol.
It forms four series of salts, which are stable in the dry state, or if
the solution is acid; but in an alkaline solution they rapidly absorb
oxygen, and colour brown and black. From a solution of silver or
gold it precipitates the metals.
Mthyl Gallate C6H2 j QQ H ^ TJ is formed by passing hydrochloric
acid gas into a solution of the acid in absolute alcohol. It crystallizes
from water in rhombic prisms, possessing a bitter taste and acid reaction.
On heating it carefully, it sublimes in brilliant needles. It
colours ferric salts blue; forms, as phenol, metallic compounds; and
reduces the solutions of the noble metals.
JDigalliQ Add c% a (C0 H)(OH) 1 ° ia fonned *>y heating gallio
acid with phosphorus oxyehloride, or by boiling its aqueous solution
with arsenic acid. Dignllic acid is an amorphous yellowish powder,
having a strong astringent taste, and giving with ferric salts a darkblue
colour. By boiling it with hydrochloric acid, it is reconverted
into gallic acid. Commercial tannin, or taiinic acid (see Glucosides),
appears to contain digallio acid in considerable quantity.
Bttagio Add. CMH6O8 is formed, together with gallio acid, by
exposing an aqueous solution of tannic acid to the air, and by
heating gallio acid with dry arsenic acid, the latter being reduced
to arsenic trioxide, Ellagic acid also occurs in certain " Oriental
bezoaT-stones," or calcnli found in the intestines of wild goats
iu Peraia. It is a yellowish crystalline powder, and insoluble
in water,
Qwiwfe or Kinic Add C0H7(OH)4COSH. occurs in the oiuchonabarks,
in the leaves of bilberry, in coffee, and in other plants, and is
obtained as a by-product hi the manufacture of quinine. It is
readily soluble iu water, and crystallizes in thombic prisms, melting
at 162°. By dry distillation it is decomposed, yielding hydioqnionne