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DYEING AND CALICO PRINTING.

possess, by themselves alone, the property of decolourising sulphate of indigo,and, moreover, this zinc solution only retains that property for a few hours.M. Schcenbein* states that sulphurous acid and neutral sulphites barely adlupon the solution of indigo in sulphuric acid, but that acid sulphites (bisul-phites) decolourise this solution very energetically; the original blue colour isrestored by the adtion of strong acids, alkalies, water in excess, lowering aswell as raising the temperature, and other influences. The zinc solution alsodecolourises powdered indigotin if, simultaneously, ebullition be applied. Theinstance is the only exception to the rule that the redudtionf of indigotinrequires, and is effedted by, the intervention of alkalies or alkaline earths, thelatter adting as solvents for every particle of hydrogenised or white indigo.The resulting solution is a clear, transparent, golden-yellow liquid, which incontadt with air becomes covered with a coppery-looking thin film of regene-rated (re-oxidised) indigotin.

Dyers apply the term indigo-vats to solutions of reduced indigo prepared onthe large scale, by means which we shall presently explain.

The following substances have the power of converting indigotin (includingof course indigo as well) into what is termed white or hydrogenised indigo.The substances and methods applied in pradtice on the large scale will beafterwards fully explained.

1. Alkaline metals : as might be expedted, these bodies—which, in contadt

with water, yield hydrogen as well as an alkaline solution—are highlyreducing agents, and their mode of adlion is greatly regularised bybeing made into amalgam with mercury. If sodium amalgam isbrought into contadl with finely pulverised indigotin and water, reduc-tion rapidly ensues.

2. The metals and non-metallic elements which decompose water in the

presence of an alkaline base, viz., tin, antimony, aluminium, phos-phorus : these, bodies, however, only adl when an alkali is present, andrequire a higher temperature.

3. Such metallic oxides as are readily capable of higher degrees of oxidation,

as, for instance, the protoxides of iron and tin.

4. The acids which, as phosphorous and hypophosphorous, are readily per-

oxidised ; but these acids only adt as reducing agents when bases arepresent.

5. Some sulphurets, phosphurets, and arseniurets; for instance, sulphuret of

arsenic (realgar).

6. Such organic matters as are oxidisable in the presence of alkalies, as,

for instance, glucose and gallic acid.

7. Reducing and alkaline fermentation, butyric fermentation.

Sulphur and the oxides thereof, of a higher degree than sulphuric acid, notonly do not adt as reducing agents upon indigotin, but are without any adtionwhatever, even in the presence of alkalies, and it appears, even, that—as re-gards all the polysulphurets of the latter—they abstradt from already-reducedwhite indigo its combined hydrogen, and precipitate, consequently, blueindigotin.

* “ Poggendorff’s Annalen,” civ., 300.

t Although, as already stated above, this word is not precisely a corredt expression for thereal process hereby alluded to, we preserve its use here, since it is a generally accepted term.