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would poifon and render dangerous for animalsto ufe. The thick dark-blue pulp is then letoff into the lowed: vat, out of which it is ladedinto common facks, which when full are hungup that the water may drain off, the indigoitfelf being too thick to pafs through. Theindigo is then transferred to fmali woodenboxes where it is further dried by alternate ex-pofure to fun and {hade, and as it becomes folidis cut into fquare cakes. It is not yet perfeddhowever, for if exported in this date it wouldmould and fpoil, fo that a fecond fermentationis neceffary. For this, the cakes are heaped ina cafk, and {imply differed to remain for aboutthree weeks. During this time it undergoes akind of fermentation, heats, fweats at the fur-face, gives out a difagreeable fmell, and iscovered with a line white meal. It is then takenout and dried in the {hade for five or fix dayswhen it is quite compleat.

This, with fome flight variations in differentplantations, is the general way of preparingindigo, which therefore may be defined to be afecula or pulverulent pulp feparated from thefibre and juices of the entire plant by fermen-tation, and materially changed by the procefs,and doubtlefs by expofure to the atmofphere, foas to pafs from green to a deep blue, and froma foluble to an infoluble flate.

Indigo is generally packed in chefts of abouttwo hundred pounds weight each. The veryfine kind that comes from Guatimala is ufually-wrapped up in goats’ fkins.

Very Angular chemical properties have beendifcovered in indigo by different experimenters,among whom may be particularly mentionedBergman, Hauffman, c and Bancroft. The mollfingular circumftance concerning indigo, is theinfolubility of the blue colouring part in everyftmple menftruum hitherto known (except thefulphuric acid) without fuch an alteration asentirely deftroys the colour for which it is fomuch valued.

Water boiled long upon indigo (the fineftand pureft kind being always fuppofed, as theordinary forts are largely adulterated) diffolvesabout a ninth according to Bergman, or a ,twelfth according to Quatremere, of the weight. 11The folution is of a reddifh brown colour, andcontains what may be called the extractivepart, but the colouring portion remains abfo-lutely unaltered, and fomewhat of a brighterhue. The watery folution is aftringent andmucilaginous.

Alcohol diffoves a {till fmaller portion than* Jour. Phyf. for 1788. 1 Jour. Phyf, tom. x & xi.

water, and the colouring part remains equallyuntouched. Ether has nearly the fame effect.Neither the fixed nor the volatile oils have anyeffedd.on indigo.

The iulphuric acid is the only fingle agentthat diffolves indigo without deltroying its co-lour, and its application as a dye, iri whichcircumftance it is called Saxon blue, has beenmentioned under the article Dyeing .

The acid {hould be tolerably concentrated todifi'olve the indigo, and it may be ufed quiteconcentrated without detriment, at leaft infmali quantities. A moderate heat much affiftsits addion, but if too high, the indigo becomespartially burnt or charred, fulphuric acid gas isgiven out, and all the portion thus altered re-mains infoluble and is l'poilt. The followingrecipe for the fulphat of indigo, or Saxon blue,is given by Mr. Woulfe. e Mix one ounce efthe beft powdered indigo with four ounces ofoil of vitriol, in a glafs matrafs, and digeft itfor one hour in the heat of boiling water,{baking the mixture feveral times, then addtwelve ounces of water to it, ftir the wholewell, and when cold filter it. This producesa very rich deep blue colour, of a much brighterhue than any of the other folutions of this pig-ment, but not a fall colour upon animal orvegetable fibre. The colouring power is verygreat, a few drops of this folution giving avery fenfible bluenefs to a confiderable bulk ofwater.

The nitric acid adds upon indigo with greatvehemence, and in a very fingular manner. Ifon a dram or two of finely powdered indigo bepoured about an ounce of fuming nitrous acid, ina fliort time the mixture heats violently, fendsforth a copious flow of nitrous gas, with aftream of {parks, and the whole ends withburfting into flame. When diluted, the acidadds much more mildly, but always with theentire deftruddion of the blue colour, and in itsftead produces a yellow aftringent liquor, whichis pretty fall upon cotton or cloth dipped in it.This folution however is ufelefs in manufaddure.Mr. Hauffman has examined with fome mi-nutenefs the addion of nitric acid on indigo. fOn adding at intervals four ounces of powderedindigo to 16 ounces of common aqua fortis heateda little, a great fwelling and difengagement ofnitrous gas took place, and a coagulum wasleft, which when waftied with cold water,formed a brown, vifcous, and very brittle mafs,in appearance like a gum-refin, diffolving inalcohol with eafe, but not ealily in water, except• Phil. Tranf. vol. lxi. f Journ. Phyf. for 1788.