INDIGO.

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percentage of indigotin contained in the indigo under examination. In orderto obtain reliable results by this method it is, in the first place, necessary totake a fair average sample of the indigo, by breaking off lumps from a numberof the cakes or squares contained in a chest; the next point to be attended tois the pulverising of this substance so as to be really an impalpable powder,to be sifted through a very fine silk-gauze sieve, every morsel passing throughthe sieve. After the indigo has been adted upon by the fuming sulphuric acid,and this fluid has been diluted with water, that solution must be carefullyfiltered, in order thereby to remove the brown-coloured particles of extractivematter which float about, and which, by their presence, might give rise toerrors of more than 15 per cent in the estimation of the indigotin, especiallyfor the lower sorts of indigo. When the indigo is dissolved in the fuming sul-phuric acid,[it may happen that sulphurous acid is given off ;* this is sometimespresent in the fuming acid itself, but it is completely removed before water isadded : if, however, the indigo happens to contain iron in the state of per-oxide, that body would, by the action of the sulphurous acid, be reduced toprotoxide, and would of course in that case vitiate the adtion of the bichromateby taking up some of that for its own oxidation. In case, therefore, indigocontains peroxide of iron, it is necessary to treat it, previous to estimating theindigotin, with boiling dilute hydrochloric acid—a precaution also requiredwhen the indigo contains a large amount of carbonate or other salt of lime.

Indigo as met with in commerce rarely, if ever, contains so much iron as tomake the use of hydrochloric acid necessary, but it sometimes happens thatin dye-works indigo residues from the vats have to be analysed which containa large proportion of iron, and in that case the treatment with hydrochloricacid becomes necessary.

Some chemists propose to dissolve the indigo in flasks, aiding the admixtureof the ware with the acid by the addition of some few grains of lead shot;but by adopting this method the escape of the sulphurous acid gas is prevented,and it is preferable to conduct the operation in porcelain capsules or basins.

M. H. Schlumberger has published! a comparative review, in tabulatedform, of the value of different kinds of indigo: the highest proportion ofindigotin was found by him in a superfine purplish-coloured Java indigo, at96 per cent, a figure which M. E. Schlumberger considers by far too high : theresearches on this subject of Dr. G. J. Mulder, who has been for nearly fortyyears the analytical chemist and scientific referee to the Dutch Society ofTrade and the Dutch Indian Department, prove that occasionally a few samplesof indigo, some few boxes, have been sent from Java containing indigo ofgreat purity, but the average varies from 16 to 82 per cent for differentqualities, and the superior is a rare article, not daily met with. M. E. Schlum-berger states that by using the bichromate process, and testing by it variouskinds of indigo, he found that the quantity of indigotin averaged from 12 to72 per cent.

* The production of sulphurous acid by the action of the sulphuric acid upon the impurities°f the indigo is a very grave objection to the use of all decolourising methods. The errorsthus produced are considerable, and become greater the lower the quality of the sample.t.Schunck—certainly no mean authority—considers that no rapid and trustworthy means ofe erm ining the amount of indigo-blue in commercial samples is known,f ‘ Bulletin de la Societe Industrielle de Mulhouse,” xv., 277.