454

DYEING AND CALICO PRINTING.

To obtain this interesting decomposition with acids, it is simply necessaryto heat the indican with strong sulphuric or hydrochloric acid, when theindigo precipitates, while the sugar remains in solution. But indican is soliable to undergo modification that, if the aCtion of the acids be continued,besides the indigo-blue an indigo-purple is formed, called by Dr. SchunckIndirubin. This author’s researches have a very important bearing upon themanufacture of indigo, since they show that under the influence of a fermentthe indican is converted into sugar and white indigo; and they also explainthat if the manufacturer is not very careful he may experience great loss inthe amount of indigo obtained. Dr. Schunck has observed that indican, whendissolved in water, is liable to undergo rapid modifications, and that instead ofyielding, in contact with acids, indigo-blue and indirubin, it gives indiretin,indihumin, and other compounds of decomposition.

Indican is met with as an ordinary constituent of human urine and blood,and also the blood of oxen. Its discovery in blood is due, we believe,to Carter. The plant botanically known as the Isatis indigotica y inChina Tein-hoa, Tem-ching, is a native of that country, and is largely culti-vated for the purpose of manufacturing a kind of indigo, which is sold in apasty, viscous state, since when once dry it only yields a blackish tint ondyeing. It is used as a fast blue dye at Canton, Emoui, and other places.The Polygonum tinctorium t a herbaceous plant of the natural order of thePolygonese, is also used in China for the purpose of indigo manufacture,yielding a material of good quality; but according to M. Natalis Rondot, theplant, called lau in Chinese language, is not the Polygonum above mentioned,but a kind of asclepiad. However this may be, the Polygonum tinctoriumhas been acclimatised in Europe from seed obtained from China. The smallquantity of indigo it yields, only about \ per cent on an average, was not foundto make the cultivation a paying business* in those parts of Europe whereindigo is readily obtainable.

M. E. Schwartz states that the indigo obtained from the Polygonum is notso good as that obtained from India, in so far as the colouring value of theformer is less. The yield also of the plant varies according to its age, theclimate, the nature of the soil, &c. The manufacture of indigo from thePolygonum is performed precisely in the same manner as is above describedfor the preparation of indigo.

The indigo obtained by the process above described from the differentspecies of the Indigofera may vary considerably as regards its richness intinctorial matter, and the quality also of the latter differs sometimes con-siderably. This is due to various causes, among which are the following:—In the first place, too frequently, sufficient care is not taken in carrying outthe manufacturing process, the workmen, and even the planters, being oftenentirely ignorant of the principles upon which the manufacture is based.There are also employed different varieties of plants, gathered under atmo-spheric conditions, which are not the same year by year; while, moreover, theripeness of the plants and the nature of the soil on which they are grown areof considerable importance.

The following kinds of indigo are distinguished in commerce, according to

* “Bulletin de la Societe Industrielle,” vol, xi., p. 186; xii,, 216; xiv., 212. “Journal dePharmacie,” No. 5, May, 1840.