502

DYEING AND CALICO PRINTING .

We have now to turn our attention to the chemical composition and otherproperties of the substances enumerated. Tannin, or tannic acid, is the onlyadtive principle of nut-galls, and, according to M. Guibourt’s analysis, Aleppogalls contain in ioo parts—Tannin, 65*0; water, 11*5; woody fibre, io'5;starch, 2*0 ; gum, 2*5 ; gallic acid, 2*0 ; ellagic acid, 2*0 ; brown extradivematter, 2*5 ; chlorophyll, sugar, albumen, and inorganic matter, 2*0.

As shown by the following figures, galls differ largely in their richness oftannin:—Aleppo galls contain, on an average, from 60 to 77^03 per cent;China galls, 58 to 77 per cent; Smyrna galls, 33 to 60 per cent.

Tannin or tannic acid is readily prepared, in an almost pure state, fromgalls previously coarsely powdered, by exhausting them either with aqueousether (g parts of ether and 1 part of water), or with a mixture of equal partsof alcohol and ether in the first instance, in a displacement apparatus. In thelower vessel will colled two layers, the upper being ether, and the lower asyrupy solution of tannin ; but if the mixture of alcohol and ether is employedas direded by M. Mohr, the fluid, after having served to macerate the galls,is evaporated to dryness.

Tannin is a whitish-yellow, amorphous, inodorous powder, possessed of anastringent taste ; it is very soluble in water, less so in alcohol, and almost in-soluble in ether, especially if the latter is anhydrous. When submitted toheat, tannin fuses, and begins to decompose at about 210°, giving rise to the for-mation of carbonic acid and pyrogallic acid, which sublimes, while a brownishresidue of gallohumic acid is left. The aqueous solutions of this substanceare precipitated by a great many salts, as well as acids, in consequence of thesmaller solubility of tannin in such liquids.

Solution of gelatin is precipitated by tannin solution, and from the latter, ifa piece of fresh skin be therein immersed, the tannin is gradually withdrawn,because the skin (the derma proper) combines with the tannin, forming whatwe term leather.

Solution of tartar emetic yields with tannin a white, and per-salts of iron adeep blue, precipitate ; ferrous salts do not yield a precipitate with tannin unlessafter lengthy exposure to air as a consequence of oxidation which has set in.With acetate of lead tannin yields a white, and with silver salts a brown, pre-cipitate. The solutions of the salts of the alkaloids are also precipitated bytannin. Alkaline solutions of tannin become rapidly brown when exposed to air.

According to Dr. Strecker tannin is a glucoside, which under various influ-ences is split up into gallic acid and glucose, the formula of tannin beingC 27 H 22 0 j 7 > its splitting up into glucose is exhibited by—

C 2 7H 22 0 I7 + 4 H 2 0=:3(C 7 H 6 0 5 )-fC 6 H I2 06.

Gallic acid. Sugar.

This reaction takes place— (1) under the influence of a ferment, when coarselypulverised and moistened nut-galls are left to become mouldy, or also whenthe deco&ions are kept for a length of time; (2) by boiling tannin with verydilute sulphuric acid, or with a dilute caustic alkaline solution.

Gallic acid is always present to some extent in galls, and in the otherastringent substances above enumerated. It can be artificially obtained bydecomposing di-iodosalycilic acid by means of caustic potassa. Gallic acidforms silky acicular crystals, soluble in 100 parts of cold and in 3. of warmwater, and also in alcohol and ether : its formula is C 7 H60 5 H 2 0.