A text-book on chemistryFor the use of schools and colleges . d in the presence ofsulphuric acid in vacuo. It yields tannicacid, or tannin, in the form of an uncrystal-lized mass. This acid is soluble in water,but much less so in ether, has an astringenttaste, and reddens litmus paper. With the persalts ofiron, it gives a characteristic and valuable precipitateof a black color, the basis of common writing ink. Thefollowing furnishes a good waiting ink. Digest threequarters of a pound of bruised galls in a gallon of coldwater, then add six ounces of sulphate of iron, with anequal weight of gum
A text-book on chemistryFor the use of schools and colleges . d in the presence ofsulphuric acid in vacuo. It yields tannicacid, or tannin, in the form of an uncrystal-lized mass. This acid is soluble in water,but much less so in ether, has an astringenttaste, and reddens litmus paper. With the persalts ofiron, it gives a characteristic and valuable precipitateof a black color, the basis of common writing ink. Thefollowing furnishes a good waiting ink. Digest threequarters of a pound of bruised galls in a gallon of coldwater, then add six ounces of sulphate of iron, with anequal weight of gum arabic, and a few drops of crea-sote. Let the mixture digest for two or three weeks,with occasional agitation, then decant. Tannic acidforms insoluble compounds with starch, gelatine, andother organic bodies, that with gelatine being of con-siderable interest: it is the basis of leather. From the Where does malic acid occur? What effect has heat on malicacid ? Where is tannic acid found ? How is it prepared ? Givethe process for making ink. What is leather?. GALLIC ACID. 457 characteristic precipitate it gives with iron, it is usedas a test for that metal, which must, however, be in thestate of peroxide, as the protosalts are unacted gradual darkening of pale writing inks is due tothe slow oxidation of the iron they contain. Tannin isvery valuable as a preservative in the dry collodion pro-cess ; a solution of it, being allowed to dry upon the sen-sitized collodion, will keep it in a condition impressibleto light for many months. Tannigenic Acid— Catechine (C15H6 06)—is extract-ed by hot water from catechu. It forms white, silkycrystals, not giving an insoluble precipitate with gela-tine, but producing a green color with persalts of the action of caustic potassa in excess it yields ablack insoluble substance, Japonic Acid (Cl2H4c04:+HO). Carbonate of potassa converts it into a red acid,JRubinic Acid (OlQH6 0A). In coffee and tea there exist similar acids, t
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