90G INDIGO 



to Mr. Crum, first to crimson and then to or;mgo. By the action of dilute nitric and 

 chromic acids indigo-blue is decomposed and converted into isatine, a body soluble in 

 water and crystallising in red needles. Chlorine also decomposes indigo-blue, 

 changing it into cJilorisatinc. a substance, having properties very similar to those of 

 isatino. Both isatine and chlorisutine afford with different reagents a great number 

 of products of decomposition, none of which have, however, as yet found any applica- 

 tion in the arts. By the long-continued action of boiling nitric acid indigo-blue is 

 converted, first into indigotic acid, a white crystalline acid, and then into wV >/-/ //'< 

 acid, which is yellow and crystallised. The latter is sometimes employed for impart- 

 ing a yellow colour to silk and wool, but it is generally prepared from cheaper 

 materials than indigo-blue. The action of concentrated sulphuric acid on indigo-bluo 

 is very remarkable. When the acid is poured on the pure substance and 

 heated it acquires in the first instance a green colour, which changes after some time 

 to blue. No gas of any kind is evolved. "When however crude indigo is employed, 

 there is a perceptible disengagement of sulphurous acid, resulting from the action of 

 the sulphuric acid on the impurities of the indigo, such as the indigo-gluten, &c. 

 On adding water, a solution of a beautiful deep blue colour is obtained. The filtered 

 liquid contains a peculiar acid, to which the names of indigo-sulphuric, sulphindigotic, 

 sulphindylic, or c&ndeo-sulpkuric acid have been applied. 



This acid is a so-called double acid. It contains indigo-blue and sulphuric acid, but 

 in such a peculiar state of combination, that neither of the two constituents can bo 

 detected by ordinary re-agents, nor again eliminated as such from the compound. 

 It combines with bases, without either of the two constituents separating. The com- 

 pounds are called indigo-sulphates, and are, like the acid, of a dark blue colour. 

 "When the solution of indigo-blue in concentrated sulphuric acid is diluted with wal<T, 

 there is usually formed a small quantity of a dark -blue flocculent precipitate, which is 

 the phenicine of Mr. Crum, or the indigo-purple of Berzelius. It is a compound of 

 indigo-blue with sulphuric acid, containing less of the latter than indigo-sulphuric acid. 

 It is always formed when the quantity of sulphuric acid employed is not more than 

 eight times that of the indigo-blue, or when the action of the acid on the latter has con- 

 tinued for only a short time. By heating it with an excess of acid it is changed into 

 indigo-sulphuric acid. Though soluble in concentrated sulphuric acid, it is insoluble 

 in the dilute acid, and hence is precipitated on the addition of water. On filtering 

 and washing, however, it begins to dissolve, as soon as the free sulphuric acid has 

 been removed, and may then be completely dissolved by pure water. The solution 

 has a blue colour just like that of indigo-sulphuric acid. Its compounds with bases 

 have a blue colour with a purplish tinge. The blue acid liquid filtered from the in- 

 digo-purple on being supersaturated with carbonate of potash or soda, deposits a dark 

 blue powder, which consists of the indigo-sulphate of potash or soda. These compounds 

 are insoluble in water containing a large quantity of neutral salts, and are therefore 

 precipitated when the excess of sulphuric acid is neutralised by carbonate of potash 

 or soda. As soon, however, as the sulphate of potash or soda has been removed by 

 washing, the indigo-sulphate may be dissolved in pure water, yielding a dark-bluo 

 solution. The indigo-sulphates of the alkalis may also be prepared by steeping 

 wool, previously well cleaned, into the solution in sulphuric acid. The wool takes up 

 the colour, becoming of a dark blue colour, and after having been well washed with 

 water, in order to remove the excess of acid as well as the impurities whieh an; 

 always present in the solution when crude indigo has been employed, is treated with 

 carbonate of potash, soda, or ammonia, which separate the acid from the wool, and 

 produce blue solutions containing the salts of the respective bases. The indigo- 

 sulphates of the earths and metallic oxides, which are mostly insoluble blue po\v>!< TS, 

 may be obtained from the alkaline salts by double decomposition. \\\ . n excess of 

 caustic alkali, indigo-sulphuric acid is immediately decomposed, iri v j n g a yellow 

 solution, from which it is impossible to obtain the acid again. By means of reducing 

 agents, such as sulphuretted hydrogen, nascent hydrogen, protosalts of tin and iron, 

 &c., indigo-sulphuric acid is decolorised, but the colour is restored by the oxygen 

 of the atmosphere. Indigo-sulphuric acid, in a free state or in combination with 

 alkalis, is employed in the arts for the purpose of imparting a Muo colour to silk 

 and wool. It has very little affinity for cotton fibre, but is nevertheless employed 

 occasionally for blueing white cotton -yarn and other bleached goods. 



By treatment with strong boiling caustic potash- or soda-lye, indigo-blue is gradually 

 decomposed and converted into a colourless crystallised acid. anthranttic acid. By 

 weak solutions of caustic alkalis it is not in the least afiirlcd. If, however, it bo 

 subjected to the combined action of an alkali or alkaline earth and some bdy 

 having fl strong affinity for oxygen, such as protoxide of iron and tin, sulphur, sul- 

 phurous or phosphorous acid, or organic matters, such as grape-sugar. iVc.. it 

 disappears by degrees, yielding a yellow solution, containing in the place of indigo- 



