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PYROCATECHIN, MORITANNIC, AND GALLIC ACIDS.

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it becomes first yellow, then red, and finally black. Catechin precipitates ferric chloride of a dark green colour. It is soluble in the dilute acids: when treated with oil of vitriol it gives a purple solution: nitric acid converts it into oxalic acid. Catechin is fusible at 422° (217° C.); at a higher temperature it is decomposed, and amongst other products yields a crystallizable substance termed pyrocatechin, or oxyphenic acid (HO). Lautemann has obtained pyrocatechin artificially by decomposing moniodo-salicylic acid by means of an alkali (Lieb. Annal. cxx. 312):

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Moritannic acid (18H1610) (the tannin of the Morus tinctoria, the yellow dye called fustic) yields the same product when submitted to distillation.

It has been suggested that the different varieties of tannic acid may be homologous terms of the same series; but the facility with which they undergo change renders their investigation a matter of great difficulty; and the accuracy of the formulæ assigned to these different compounds, with the exception of that of gallotannic acid, must be regarded as very doubtful.

6. Gallic Acid and its Derivatives.

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(1333) GALLIC ACID (H ̧¤,н ̧Ð ̧,н,0=170+18).—This acid exists ready formed in the gall nut, in sumach, in valonia, and in a large number of other astringent vegetables, although the quantity in each is but small. It may readily be obtained by allowing an infusion of gall nuts, or the powdered galls moistened freely with water, to stand in a warm place for some weeks, exposed to the air: it quickly becomes mouldy, absorbs oxygen, and emits carbonic anhydride, owing to the occurrence of a species of fermentation, during which abundance of gallic acid is deposited in the form of crystals. The acid must be purified by digestion with animal charcoal and re-crystallization from boiling water. Gallic acid is also deposited from infusion of gall nuts, if kept in vessels from which air is excluded; but in this case no evolution of gas occurs; the exact stages of these processes of fermentation have not been clearly made out. Gallic acid may likewise be obtained by boiling tannic acid with dilute

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GALLIC ACID-GALLATES.

sulphuric or hydrochloric acid, in which case water is assimilated and glucose is set at liberty (1331). Lautemann, by converting salicylic acid into di-iodo-salicylic acid, and treating this compound with potassic carbonate, has succeeded in obtaining gallic acid; when caustic potash was used a good deal of the acid was decomposed and pyrogallin was formed:

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+ 2 KHO = C,H,O + K,€g + H,O.

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The acid crystallizes either in the form of delicate silky needles, or in brilliant oblique rhomboidal prisms, which require about 100 parts of cold water for their solution, but only 3 parts of boiling water: they are freely soluble in alcohol, but only sparingly so in ether. When dried at 212° they lose 95 per

cent., or one atom of water.

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Gallates. According to Strecker, gallic acid is tribasic, and it forms three classes of salts, the general formulæ of which are MH,E,HO; M2HЄ,H,,; and M,E,H,,; besides these salts there are salts with an excess of acid and with excess of base; thus an acid salt of potassium [2 (KH,Є,H,,),H ̧¤‚н ̧Ð ̧‚н ̧Ð] may be obtained by adding an alcoholic solution of potash to an alcoholic solution of gallic acid. The gallates of the alkalies speedily absorb oxygen, and become brown when their solutions are exposed to the air, especially if an excess of base be present. Nearly all the gallates of the heavy metals are insoluble. If an excess of gallic acid be added to a salt of lead, the precipitate is at first white and amorphous, but it gradually becomes crystalline if left in contact with the liquid. But the most characteristic reaction of this acid is the formation of a deep bluish black solution, when added to a mixture of ferrous and ferric salts: if the solutions be free from acid, and particularly if a solution of calcic carbonate in carbonic acid be added, the reaction is one of extreme delicacy. The salts of gallic acid do not cause a precipitate in solutions of gelatin.

When gallic acid is exposed to a temperature of from 410° to 419° (210° to 215° C.) it is wholly volatilized, and is converted into pyrogallin and carbonic anhydride:

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PYROGALLIC ACID OR PYROGALLIN.

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but if the temperature be allowed to reach 482° (250° C.) the pyrogallin in its turn suffers decomposition; water is expelled, and a dark ulmin-like body is formed, termed metagallic acid, which is insoluble in water, but soluble in alkaline solutions :

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(1334) Pyrogallin or Pyrogallic Acid (HO).-This substance can scarcely be said to possess acid properties. It may be obtained by the sublimation of gallic acid, or of any vegetable extract which contains gallotannic acid. The extract may be placed in a shallow iron pan, which is covered with a sheet of bibulous paper, over which a cone of writing-paper is fastened; on applying a regulated sand heat, the acid is decomposed and the pyrogallin is converted into vapour; it passes through the bibulous paper, and rises into the chamber formed by the paper cone, where it is condensed, and is prevented from falling back into the pan beneath, by the interposed sheet of filtering paper. This method of sublimation, contrived by Mohr, is applicable in many other cases of a similar kind. Liebig mixes well-dried gallic acid with twice its weight of coarsely powdered pumicestone, and heats the mixture in a retort, through which a current of dry carbonic anhydride is maintained :-31 or 32 parts of pyrogallin are thus obtained for every 100 parts of gallic acid employed.

Sublimed pyrogallin forms voluminous brilliant plates, which are freely soluble in water, alcohol, and ether: the solution has a bitterish taste and does not redden litmus paper. When heated to about 239° (115° C.) it melts; and when further heated it emits a colourless irritating vapour. Pyrogallin may be preserved unchanged whilst dry, but its solution quickly becomes brown, and absorbs oxygen; and if a free alkali be added, the absorption of oxygen is so rapid, as to afford a valuable method of removing free oxygen from gaseous mixtures (p. 349, note).

The alkaline solutions of pyrogallin rapidly assume an intense reddish brown colour: this brown solution, according to Stenhouse (Liebig's Ann. xlv. 6), contains acetate and carbonate of the basyl. When pyrogallin is dropped into milk of lime it produces a beautiful purplish red colour, which soon becomes of a dingy brown: this reaction is very characteristic. Pyrogallin, on account of the facility with which it absorbs oxygen, decomposes most of the salts of silver, of gold, and of platinum; but it forms a definite compound with oxide of lead, and dissolves freshly

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ELLAGIC AND RUFIGALLIC ACIDS.

precipitated alumina. If a solution of pyrogallin be added to a boiling solution of tartar emetic, sparingly soluble pearly crystals (Sbe,,H,) are deposited. With ferrous sulphate, free from ferric salt, pyrogallin gives a deep indigo-blue solution, which becomes green by the action of air: with ferric salts it forms a bright red solution.

Pyrogallin is now extensively employed in photographic operations for the purpose of developing the latent image upon the argentiferous collodion film after it has been exposed to the action of light (1018).

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(1335) Ellagic Acid (H¤‚H24,H2O; or if dried at 239° (115 C.) HЄ,H2O).-During the preparation of gallic acid from gall nuts. a quantity of a grey crystalline powder, termed ellagic acid, is formed. This substance is insoluble in water, but soluble in the alkalies; with potassium it forms a flaky salt which is but sparingly soluble the solution of this salt speedily absorbs oxygen from the air, and becomes of a blood-red colour. When treated with a solution of ferric chloride, ellagic acid is gradually dissolved and forms a deep blue liquid. The most singular circumstance in connexion with this body is the discovery by Wöhler and Merklein that it is a constituent of certain kinds of oriental bezoars. These bezoars are intestinal concretions which occur in the wild goats, antelopes, and deer of the countries of Central Asia: they are brittle, ovoid, or reniform masses of a dark olive-green colour, polished upon the surface, and in size seldom exceeding a pigeon's egg; when broken, they are usually found to have been deposited in concentric layers upon some foreign nucleus: they are not fusible like the lithofellic bezoars.

Concentrated sulphuric acid dissolves ellagic acid, and precipitates it, unaltered, on dilution.

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Rufigallic or Parellagic Acid (E,HO,H,O).—If the crystals of gallic acid be dissolved in five times their weight of oil of vitriol, a crimson solution is formed, which, if allowed to fall drop by drop into water, deposits a reddish-brown substance, partly in flocculi and partly in crystals. The crystalline portion, or rufigallic acid, is insoluble in water, but soluble in solutions of the alkalies. This acid, when heated, yields a sublimate consisting of fine red prisms, resembling those of alizarin. If a piece of calico which has been moistened with alum, or with a salt of iron, be boiled with rufigallic acid, it becomes dyed with the same tints as if madder had been used; but the colours are less brilliant than those of madder.

7. Quinic Acid and its Derivatives.

(1336) QUINIC or KINIC ACID (H‚¤‚H1,).—The bark of the cinchonas contains a peculiar modification of tannic acid, termed the quinotannic, and besides this, it furnishes the remarkable acid to which the name of quinic acid has been given. Quinic acid appears to exist in the bark in combination with calcium and with the organic bases of the bark. If an aqueous decoction of cinchona bark be mixed with milk of lime until it assumes a feebly alkaline reaction, the vegetable bases and the tannic acid are precipitated, and calcic quinate remains in the liquid this salt may be crystallized from the mother-liquor by evaporation, and decomposed by the cautious addition of oxalic or of sulphuric acid. The quinic acid may then be obtained in crystals from the solution.

Quinic acid crystallizes in colourless, oblique rhombic prisms, which have a strongly acid taste; they are freely soluble in boiling water, less so in cold water, still less so in alcohol, and are very sparingly soluble in ether. When heated to 311° (155 C.) the acid fuses, and at a higher temperature yields a complicated mixture containing amongst other matters, benzol, benzoic acid, carbonic anhydride, salicylous acid, and a colourless substance termed hydroquinone. Lautemann (Liebig's Annal. cxxv. 9) has shown that by the action of hydriodic acid quinic may be converted into benzoic acid :

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When swallowed, quinic acid is converted into hippuric acid, and escapes as such by the kidneys.

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Quinates. Nearly all these salts are soluble in water, with the exception of the basic quinate of lead (E,Pb",H ̧ ̧), which is obtained by adding a solution of basic acetate of lead to a neutral quinate. It is remarkable that in this salt 2 atoms of the hydrogen of the acid radicle appear to have been displaced by lead. Quinic acid also forms a basic salt with copper (E,Eu”Н10¤ ̧, 2 H2O), which may be obtained in brilliant green, sparingly soluble crystals.

(1337) Quinone or Kinone (CHO).-When one part of quinic acid is heated with four parts of finely powdered peroxide of manganese and one part of sulphuric acid diluted with an equal weight of water, a beautiful yellow crystalline compound is sublimed, and becomes condensed in the neck of the retort in

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