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DYEING AND CALICO PRINTING.

The impurities which are found in indigo along with indigotin are eithernatural or have been artificially added. Among the first-named we find—

1. Water of hydration, which varies on an average in quantity from 3 to

6 per cent, and is readily estimated by drying at ioo°.

2. Various saline matters ; among them phosphates and carbonates of lime

and magnesia, sulphate and chloride of potassium ; also silica, oxide ofiron, in variable quantity: the lime salts are, however, the principalconstituents of the ash, and are present in quantities varying from 3 to7, and even 20, per cent. The quantitative determination of the ash ofindigo is best executed by the careful incineration of about 5 grms. ofthe sample placed in a platinum crucible: the residue, after ignition,may—of course after having been weighed—be analysed or tested quali-tatively, as suits the operator’s objedt,

3. Indigo contains some brown and red organic matters, insoluble in water,

but soluble in other liquids, among which are boiling acetic acid, boilingalcohol, ether, and caustic alkalies : these organic substances more orless modify the dyeing operation executed in the vat, and affedt the bluecolour imparted to a greater or less extent: if, for instance, the redcolouring matter predominates, that substance becomes reduced, in thesame manner as indigotin, by the adtion of the alkali, and in the dyeingprocess becomes precipitated upon the fabric along with the last-namedprinciple, and causes thereby a purplish colour different from that pro-duced by pure indigotin : even if this latter substance is prepared inthe crystalline state by the glucose vat process, and therefore apparentlyquite pure, it yet frequently yields, when treated with boiling aceticacid, a certain quantity of red matter, which appears to belong to thesame class of bodies as isatopurpurin, one of the produdts of thereduction of isatin by hydriodic acid.

We may here mention that indigo-blue has been artificially prepared byEmmerling and Engler (“ Berichte der Chemischen Gesellschaft zu Berlin,”No. 17, 1870), and that it has been obtained by Dr. Schunck by the adtion ofstrong acids upon normal healthy urine.

According to Berzelius, indigo contains gluten, soluble in dilute acids. Thesubstances which are added to indigo for the purpose of adulteration are rathernumerous, and vary in kind with the more or less refined knowledge of thepersons who commit this fraud, either wilfully or from sheer ignorance, beinginduced to believe that some foreign substances promote the adtion of fer-mentation and precipitation of the indigo while being prepared as abovedescribed. Among the adulterants are starch, resins, logwood and its extra#,Prussian blue, earths artificially blue-coloured, and it is said sometimes,though rarely, German ultramarine: the detedtion of these bodies offers nodifficulty when the operator bears in mind what are the natural constituentsof indigo, as above laid down :—Starch is converted into sugar by boiling witha dilute acid; resin is soluble in alcohol; logwood dissolves in water, givingrise to a solution which turns red on the addition of oxalic acid, and yields ablue precipitate with aluminate of soda ; Prussian blue gives, on being treatedwith an alkaline solution, ferrocyanide of potassium ; mineral substances canbe recognised after incineration, and if any ultramarine is present this will bereadily detedted by becoming decomposed by dilute mineral acids, with escapeof sulphuretted hydrogen.