ISOPURPURIC ACID.

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which are in general soluble, crystallisable, yellow, and which if heated,especially in a closed vessel, explode with great violence. The picrate ofsoda is sometimes sold to dyers under the name of aniline-yellow, but itsuse requires care. Picric acid dyes upon silk and wool shades varying indepth from a straw-yellow to a maize. Wool is easily dyed either hot or cold,and is often previously mordanted with alum and tartar to render the colourfaster. Picric acid is also used along with extradt of indigo, and withPrussian blue, for producing greens, and in modifying the tone of the anilinegreens.

The intensity and purity of a sample of picric acid may be known bydyeing known weights of wool or silk with a given quantity of the dye, andcomparing the shades produced with those furnished by a standard sample.It is frequently adulterated with a variety of articles, such as sulphate of soda,borax, oxalic acid, alum. These impurities are easily detedled by dissolvingthe sample in benzol, warming the tube gently by plunging it into lukewarmwater. The impurities remain insoluble.

Picramic acid maybe prepared by dissolving 1 part of picric acid and 7 partsof protosulphate of iron in hot water, and adding a boiling solution of causticbaryta. A deep red liquid is obtained, containing the picramate of baryta.Girard saturates with ammonia picric acid dissolved in alcohol, and treats thesolution with an excess of sulphuretted hydrogen. The liquid takes a darkred colour, and deposits a mass of deep red crystals, the picramate of ammonia.This salt is dissolved in boiling water, mixed with an excess of acetic acid,and allowed to cool. Picramic acid crystallises after a time in the form oftables or needles of a brilliant garnet red. It is easily soluble in alcohol andether, sparingly in water, even when hot. Both picramic acid and the solublepicramates are true dyes, capable of fixing themselves upon animal fibres, butthey have not yet found their way into commerce.

Isopurpuric Acid.

(Picric Red, Picrocyanic Acid, Homorceine, Soluble Garnet.)

This colour is prepared by dissolving 2 parts cyanide of potassium in 4 parts ofwater at 60°. A solution of 1 part of picric acid in 9 parts of boiling water is thengradually added with constant stirring. On cooling there is formed a crystallinemass. This is thrown on a table, pressed, stirred up in a little water, filtered anew,and washed with cold water. The residue is purified by re-solution in boilingwater, and re-crystallisation. The potash salt thus obtained is dissolved ina small quantity of hot water, and a solution of sal-ammoniac is added. Itis deposited in wedge-like crystals, of a reddish brown colour, with metallicgreen refledtion. The isopurpurate of ammonia was at one time consideredidentical with the purpurate of ammonia (murexide). A careful comparisonof the shades produced by these two dyes under identical conditions provesa decided difference. Isopurpurate of ammonia has been prepared on a com-mercial scale by Messrs. Roberts and Dale, of Manchester, and yields beautifuland peculiar reddish purples on wool, leather, and silk. Its use, however, isstill very limited. When dry the isopurpurates explode with a fearful violenceon the least shock. Hence they should be preserved in a paste with waterand a little glycerine, which prevents drying up.