Besides the formula of anthraquinon, a compound of two molecules of gallic acid, with separation of 2 H2O, is conceivable, according to the following formula :

which explains the formation of anthracen. Independently of the consideration that this formula of Schiff's is rightly attributed to ellagic acid, it is inadmissible, as otherwise tannic acid,

would yield rufigallic acid more readily than gallic acid, whilst in fact the former gives mere traces. According to this formula also, tannic acid with zinc-dust should also yield anthracen, which is not the case. The anthraquinon formula is further supported by the reduction of rufigallic acid with hydriodic acid. If rufigallic acid is heated with hydriodic acid and phosphorus, anthracen is not indeed formed directly; but there appears a substance, which crystallises in microscopic yellow needles, and which, if distilled with zinc-dust, yields anthracen, but differs in its properties from all known derivatives of anthracen. (Probably this compound is a substance similar to purpuroxanthin, and formed from purpurin and pseudopurpurin by reduction.)

According to Wagner, rufigallic acid is obtained as

follows: 1 part of gallic acid is slowly heated to 140° C. with 4 parts of concentrated sulphuric acid. Löwe, however, has shown that the conversion of gallic acid into the rufigallic is complete even at the temperature of the water-bath. The red solution, when cold, is poured into water, when a redbrown precipitate is formed, partly flocculent, and partly granular crystalline, and is freed from adherent sulphuric acid by decantation, and finally by washing on the filter.

The small shining crystals obtained by elutriation lose their crystalline water at 120°. If more strongly heated, rufigallic acid sublimes in vermilion prisms. It is insoluble in cold water, sparingly soluble at 100°, sparingly soluble in boiling alcohol and ether, insoluble in oil of turpentine and chloroform, but in aceton it dissolves with a brown colour. Alkalis dissolve it with a brown colour if air is excluded, but if the air has access, humus-like bodies are formed. If covered with ammonia, it takes a red colour. Baryta converts it into an indigo-blue mass, without solution. If introduced into melting potassa, it becomes a splendid green, and then turns indigo-blue in places. On the addition of water it turns indigo-blue, and is decomposed with the formation of oxyquinon, CH,O,. On treatment with zincpowder, anthracen is formed, just as is the case with alizarin. If treated with reducing agents, reduction-products are formed; but alizarin has not yet been obtained by this process. Tissues mordanted with salts of alumina and iron are dyed by rufigallic acids, in shades resembling those of alizarin, but duller and dirty. They resist soaping, however, perfectly.

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The first experiments on the tinctorial properties of rufigallic acid were made by Robiquet.

Chrysophanic Acid, C15H10O4.

This acid, though not a direct derivative of anthracen, is interesting, on account of its derivation from methylanthra

cen, and may therefore be appropriately considered here, along with its congener, emodin.

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In their earlier papers, Graebe and Liebermann ascribed to chrysophanic acid the formula C1HO4, and regarded it as an isomer of alizarin. They were led to this view because chrysophanic acid, on reduction with zinc-powder, produces a hydrocarbon very similar to anthracen, and because, on analysing the acid, they obtained figures closely approximating on those required by alizarin. In their subsequent researches on emodin, Liebermann and Fischer ascertained the connection of the two compounds, and subjected chrysophanic acid to a re-examination. The result was that chrysophanic acid is a derivative of methylanthracen, and bears to emodin the same relation as does purpurin to alizarin. They proved that the hydrocarbon obtained, on its reduction with zinc-powder, was methylanthracen, by converting it into anthraquinoncarbonic acid and methylanthraquinon.

The acid occurs in the roots of various rhubarbs and docks, as Rumex obtusifolius, patientia, palustris, &c., in the lichen Parmelia paricina, in senna-leaves, &c. It was formerly obtained in an impure state from these sources; and, according to its origin, was named rhein, rhubarb yellow, phaeoretin, erythrosetin, rhaponticin, rumicin, lapathin, parietin, &c. For its preparation various methods have been proposed. Rochleder extracts the rhubarb or the lichens with very weak alcohol, to which a little potassa-lye has been added, filters, presses the residue, and passes a current of carbonic acid through the liquid. The precipitate is filtered, dissolved in alcohol at 50° per cent., mixed with a little potassa-lye, filtered, and the filtrate precipitated with acetic acid. The precipitate is collected, dissolved in boiling alcohol, and filtered whilst hot. The filtrate is mixed with water, and the acid thus separated is obtained pure on recrystallisation from alcohol.

Warren De la Rue extracts rhubarb with benzol, distils off the benzol, and, when most of it has gone over, allows the residue to cool. Impure chrysophanic acid crystallises out, and is redissolved in boiling benzol, filtered, crystallised on cooling, and purified by solution in glacial acetic acid.

Batka treats rhubarb or senna-leaves with causticpotassa, filters, and precipitates chrysophanic acid from the filtrate by the addition of hydrochloric acid. The precipitate is washed on the filter, well dried, extracted with chloroform, and evaporated. Chrysophanic acid remains in the capsule as fine-yellow granular crystals.

Chrysophanic acid either forms gold-coloured crystals, or orange-red masses of a golden lustre, like crystalline iodide of lead. From benzol it crystallises in hexagonal monoclinar tables, of a yellow or orange colour; from glacial acetic acid, alcohol, and fusel oil, it is deposited in mossy aggregation. It is inodorous and almost tasteless, scarcely soluble in cold, and very slightly in hot water. In cold ether and alcohol it dissolves with a yellow colour. One part of chrysophanic acid dissolves in 224 parts of boiling alcohol at 86 per cent., or in 1125 of alcohol at 30 per cent. In aqueous alkalis it is sparingly soluble with a red colour. Acids turn the red solutions yellow. The alkaline solution can be evaporated to dryness without decomposition, but at a certain stage of concentration the soda-salt is separated out in blue flocks, with a violet cast.

Chrysophanic acid melts, without decomposition, at 162°, and crystallises on cooling. At higher temperatures a portion of the acid is decomposed, but another portion sublimes in golden-yellow needles. It dissolves, without decomposition, in concentrated sulphuric acid and dilute nitric acid; concentrated nitric acid at a boiling heat converts it into chrysammic acid. Potassa-lye has scarcely any decomposing action. According to Warren De la Rue, the red alkaline solution of chrysophanic acid turns blue on con

centration. The blue compound turns red on solution in water, and on mixture with hydrochloric acid chrysophanic acid is deposited undecomposed. On prolonged melting with hydrate of potassa, chrysophanic acid is decomposed, and a strongly fluorescent compound is formed. The ammoniacompound is decomposed on boiling in water, and undecomposed chrysophanic acid is left behind. Phosphorus perchloride forms a chlorine-compound, which, on heating with water, forms chrysophanic acid again.

It is a very feeble acid. The alkaline-salts are blue or violet, and are deposited in blue flocks on concentrating the aqueous solutions. The lime- and baryta-salts are obtained by mixing an alkaline solution of chrysophanic acid with chloride of calcium or of barium. They are insoluble in water, and are readily decomposed in the air by absorbing carbonic acid. If a solution of chrysophanic acid is mixed with an alcoholic solution of basic acetate of lead, there is formed a slight whitish precipitate, which disappears on boiling. Subsequently carmine-red flocks are deposited, which are insoluble in water, and are decomposed by alcohol.

Acetylchrysophanic Acid, CH,O,(O.C2H2O)2.

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This compound is obtained by heating chrysophanic acid and acetyl chloride in sealed tubes in the water-bath. It separates out in pale yellow crystals, which are soluble in dilute alcohol, but soon decomposed into chrysophanic and acetic acids.

Benzoylchrysophanic Acid, C1HO2(O.C,H ̧O)2.

Obtained by introducing chrysophanic acid into chlorbenzyl heated almost to a boil. The resulting crystalline mass is pressed, and forms coloured crystals of a silky lustre. In alcohol it is sparingly soluble, more freely in benzol, from which it crystallises in long irregular