INDIGO.

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chlorine : a gentle heat is applied, whereby not only the iron and lime andother mineral substances are dissolved, but the amylaceous matters presentare converted into dextrin or sugar, which is removed by subsequent washing.The indigo is next treated with a weak solution of caustic soda, whichdissolves the chlorophyll and other organic impurities. A still purer qualityof indigotin (refined indigo of commerce) may be obtained by adding to a largequantity of water 1 part of finely-ground indigo, 2 parts of sulphate ofprotoxide of iron (green copperas), and a quantity of caustic soda slightly inexcess of that necessary to neutralise the sulphuric acid of the copperas.The mixture is boiled and allowed to settle, when the supernatant liquor isrun off from the sediment and well agitated, in order to convert the whitesoluble indigo into the blue insoluble form, which is collected and dried. Theinsoluble sediment is treated several times with a weak solution of causticalkali, in order to remove the whole of the indigotin which it contains.

In order that indigo may be used for the purposes of dyeing, printing, and forthe preparation of extracts, See., the ware has always to be pulverised in the firstplace, and this pulverisation must be so effedted as to produce an almost im-palpable powder. If this is negledted, a loss of colouring matter is the result.Very frequently this dye ware is ground in a wet state, whereby the lossensuing by the flying off of dust is prevented ; but where the indigo isrequired for conversion into extradl the addition of water is inadmissible. Inalmost all other cases the indigo is first steeped in water for some time, inorder to soften it, as in its ordinary state it is a peculiarly tough, and evensomewhat fibrous, substance. It is put in a suitable tub or trough, hot wateris poured over it sufficient to cover it, and the tub is then closed with a lidand left standing for a day or two, until the water has been absorbed andpenetrated into the entire mass ; 10 kilos, of indigo require 15 litres of water.

In this country indigo is frequently pulverised on the large scale by dry-salters, but in many cases dyers prefer to carry out this operation at their ownworks. Among the various simple apparatus in use for that purpose wemention briefly the following, which consists of a nearly hemispheric coppervessel, of 07 metre diameter, and furnished at its upper rim with two handles.The operator puts this vessel on a wooden bench, and seats himself thereoncrosswise ; he next puts three large cannon-balls, 12- to 18-pounders, into thevessel, and also a quantity of previously softened indigo and some water: bymeans of the handles a circular oscillatory motion is imparted to the vessel,and the balls, partaking that movement, reduce the indigo to the state of animpalpable pulp: after a certain time the liquid, containing finely dividedindigo in a state of suspension, is poured into a tub, and a fresh quantity ofwater is added to the residue in the copper vessel; the agitation is resumed,and continued until all the dye ware is properly divided. On the large scalevarious machines are used for this purpose, the construdtion of which differsvery greatly : some of these contrivances are arranged upon the same prin-ciple as the hand-mill just described. One of the best machines consists ofan iron or copper cylinder, placed horizontally, and made to turn on a hori-zontal axis; inside of this cylinder are placed two rows of loose, movable,cast-iron cylinders, consisting of loose pieces, discs as it were, and set in thelonger axis of the larger hollow cylinder. Since the tangential lines of thecast-iron cylinders and of the internal surface of the hollow cylinder differ,

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