The Penny Cyclopædia of The Society for the Diffusion of Useful Knowledge.
Fuego, Tierra Del - Haddingtonshire.
London: Charles Knight and Co., 22, Ludgate Street.
GALLIC ACID was the last discovery of Scheele. It exists in most astringent vegetables, but principally in the gall-nut, and hence its name. In this it is mixed with ellagic acid, and with tannic acid, or the tanning principle. Many processes have been proposed for obtaining gallic acid; the simplest is that of reducing galls to coarse powder, and digesting them in about four times their weight of water for several days. The strained solution, when exposed to the air, deposits the gallic acid in small crystals in a short time, and continues to do so for a considerable period. A large portion of the acid exists ready formed in the gall-nut, and by some substance which appears to act as a kind of ferment a further portion of the acid is also formed by the action of the air. It has indeed been supposed that the tannic acid is by oxidation converted into the gallic, and from this source more of the gallic acid may probably be formed. The crystals are sometimes perfect though small, and of a brown colour ; they are rendered perfectly colourless by digesting and dissolving in hot water with animal charcoal, which has been treated with hydrochloric acid to dissolve its phosphate of lime ; when the colourless solution is sufficiently evaporated, slender prismatic colour-less crystals are obtained, the primary form of which is an oblique rhombic prism. The taste of gallic acid is slightly sour and astringent : the solution reddens litmus paper. When exposed to a temperature of above 250° the crystals lose their water and effloresce. Gallic acid is soluble in about 100 times its weight of cold water, and very soluble when it is boiling ; it dissolves also in alcohol and in æther.
Gallic acid combines with bases to form salts, which are termed gallates ; these will be considered, when important, under their respective bases ; we merely add that on proto-sulphate of iron it has scarcely any action, but with the persulphate of iron it produces i deep blue-black precipitate, which is the principal colouring matter of writing-ink. The gallates of ammonia, potash, and soda are soluble in water; but most of the metallic gallates, strictly speaking, are insoluble : gallic acid in the form of tincture of galls is much employed as a chemical re-agent ; but part of its effect is owing to the presence of tannic acid, which is well known to precipitate solutions of albumen and gelatin, which gallic acid, when pure, does not.
Anhydrous gallic acid, according to Pelouze, is composed of
Three equivalents of Hydrogen . . . 3
Seven " Carbon . . . 42
Five " Oxygen . . . 40
In the crystalline state it contains water, and is com-composed of
One equivalent of Anhydrous Gallic Acid . . . 85
One " Water . . . . . 9
When gallic acid is heated it is, as already noticed, rendered anhydrous, but at higher temperatures it is converted into Pyrogallic ACID and MELOGALLIC ACID. At a red heat it is decomposed and totally dissipated.