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DYEING AND CALICO PRINTING .

madder difficult and tedious; though it would be an improvement to washthe madder first with water. Purpurin, as obtained by this process, still con-tains alizarin, but it is distinguished from that substance by its greater solu-bility in water, to which it imparts a fine rose-colour. Kcechlin obtained from90 parts of madder 4*5 of purpurin. Claubry and Persoz mix the madderwith water to a thin paste, and add, for every kilo, of dry madder, go grms. ofsulphuric acid. The mixture is next boiled by means of steam blown into itwith leaden pipes. The material is thrown on a filter, washed with water, andthe residue treated first with carbonate of soda, which dissolves a red colouringmatter (alizarin ?), and afterwards with a boiling solution of alum. To thislatter solution sulphuric acid is added. A flocculent orange matter is throwndown, which, after treatment with alcohol and ether, yields crystalline needles,called by the above-named chemists “ madder-rose.” It is distinguished fromalizarin by its greater solubility in alum, and its insolubility in protochlorideof tin.

Runge and Debus boil the madder, previously washed, with alum-water,filter, and let the filtrate stand for some time, to permit of the precipitation ofthe alizarin. This is removed by a second filtration, after which the rose-coloured liquid is precipitated with sulphuric acid. The flocculent matterthus thrown down is washed with water, boiled in dilute hydrochloric acid,and dissolved in boiling alcohol (85 per cent). On cooling, a crystallinematter is precipitated, which is purified by repeated crystallisation from alcoholand ether. Wolff and Strecker employ a similar mode of preparation, butmix the madder previously with water, and allow it to undergo fermentations,after which they wash again. Whilst thus experimenting they did not obtaina trace of alizarin. Their inference that the alizarin was converted by fer-mentation into purpurin has been proved to be incorreCt. They propose, as amethod for separating alizarin from purpurin, to combine both with hydrate ofalumina, and to boil this lake with carbonate of soda, which dissolves pur-purin. This operation may be conducted as follows:—The aqueous decoCtion-of madder is precipitated with sulphuric acid, the resulting precipitate ex-tracted with boiling chloride of aluminium, and the colours thus dissolved re-precipitated with hydrochloric acid. The precipitate is washed, boiled withhydrate of alumina, and then boiled with a solution of carbonate of soda.

The most elegant, and at the same time most practicable, method of preparingpure purpurin, free from alizarin, has been devised by Kopp, and is carried outon the large scale by Schseraff and Lauth:—600 lbs. of ground madder areallowed to steep for ten hours, in a vat containing 800 to 1000 gallons of asolution of sulphurous acid, and, after running off this liquid, the madder isagain treated with 200 to 250 gallons of the same acid solution. These liquorsare then mixed with 3 per cent of sulphuric acid, at sp. gr. 1*60, and the wholeis treated as described under the preparation of “green alizarin .”

Schiffer and Schutzenberger have investigated purpurin thus prepared, andpronounce it a well-defined chemical substance and a natural constituent ofmadder, and not alizarin soiled by some foreign matter. Having operatedupon some 500 grms. of the commercial purpurin, they have found it to containtwo red colouring substances, viz., purpurin properly so-called, and pseudo-purpurin, besides an orange and a yellow tinctorial body. Each of thesesubstances was obtained in a crystalline state, exhibiting distinct characters,