INDIGO.

465

the application of heat is continued until a small quantity taken out and putin water dissolves completely, with a violet colour. The produdt of this re-aftion is then dissolved in from seventy to eighty times its weight of water, towhich are next added 2 parts of common salt for every 1 part of the mixtureof indigo and bisulphate ; the precipitate thus produced is washed with watercontaining salt in solution, and next dried.

Indigo may also be converted into indigo carmine by mixing it with anhy-drous sulphuric acid (the solid substance), dissolving the mixture in water,neutralising with carbonate of soda, and precipitating with chloride ofsodium.

The Boiley blue is a crystalline, light purplish mass, yielding, when dis-solved in water, a beautiful blue-violet solution : it is soluble in concentratedboiling acetic acid, depositing, on cooling, large prismatic coppery-huedcrystals ; it is insoluble in alcohol and ether; soluble in water, but morereadily in hot than in cold : this solution is red when viewed by transmittedlight. The substance in question is probably chiefly composed of sulpho-purpurate of soda, and is remarkable on account of its ready crystallisation :its chemical properties, however, in many respedts resemble those of the sul-phuric acid compounds above named. It yields, with the chlorides of bariumand strontium, violet precipitates.

The tindtorial value of indigo-carmine can be ascertained by a dyeingprocess, viz., a piece of woollen tissue is taken, and placed in an acidulatedsolution of the carmine to which cream of tartar has been added.

The following are the principal sulphuric preparations of indigo used atpresent in dyeing and printing :—

“ Chemic,” “ chemic-blue,” or “ sour extradt,” is the solution of indigo insulphuric acid, without any neutralisation or subsequent treatment. Thefollowing receipt may be used for its manufadture:—Take 1 lb. refined indigo,perfedtly dry and in very fine powder, and stir it gradually into 15 lbs. of thestrongest sulphuric acid. Let the mixture stand for three days, at a tempera-ture of from 120 0 to 140“ F. On dilution with water no sediment shouldappear. Some makers vary the quantity of acid, using only 12, 10, or g lbs.per lb. of indigo. Sometimes the acid solution is diluted with water to suchan extent that it can be filtered through flannel, and is afterwards re-concen-trated. If unrefined indigo is used the quality is inferior, as there is no meansof getting rid of the brown and green colouring matters.

The paste, or “ sweet ” extradt, varies very much in its degree of acidity.The following are formulas for its preparation, beginning with the most acid :—Stir 1 lb. of indigo, dry and in fine powder,—conditions which are imperativein all cases,—into 5 lbs. of sulphuric acid. The mixture is kept for ten totwelve hours at a temperature of ioo° F. It is diluted with 4 gallons of hotwater, filtered through flannel, and washed with hot water till the liquid runsthrough nearly colourless. The liquid is then concentrated to 3 gallons,4 lbs. of common salt are added, and the mass thrown upon a filter to drain.The filter consists of a stout wooden frame, about 3 feet square, supported ona stand, either fixed or movable. No iron must be used in the construdtion ofthe frame. It is “ dressed,” as it is technically called, with a stout, compadt,white woollen cloth. Over this is placed a layer of stout fine canvas, and