tomentosa, of Travancore, yields a dye from the interior of the wood in old trees. H. umbellata, of Travancore, Malabar, and Cochin China, contains a permanent yellow colouring matter in its roots ; added to sapan-wood, it produces brilliant and permanent reds, rivalling madder.
Indigo (FR. and GEE., indigo).—The well-known blue colouring matter termed "indigo" is produced by a great number and variety of plants, distributed throughout all the tropical countries of the globe. Commercially, it is obtained chiefly from species of Indigofera, as I. tinctoria, the cultivated species of India, furnishing the chief article of commerce, found also in Madagascar, St. Domingo, &c.; and L Anil, in the Punjab, W. Indies, and on the Gambia river. Some is also obtained from I. argentea, in Africa and America; I. Carolinian ; I. disperma, the cultivated plant of Spain, America, and some of the E. Indies ; I. cwrulea, the " black indigo " of India ; I. glauca, in Egypt and Arabia ; I. pseudo-tinctoria, cultivated in some parts of the E. Indies, and said to yield the best dye ; I. cinerea, I. erecta, I. hirsuta, and I. glabra, in Guinea. Considerable local supplies are obtained from the following plants :—Isatis tinctoria, in Europe and China (see Woad); I. indigotica, cultivated in some parts of China ; Amorpha fruticosa, iu Carolina ; Baptisia tinctoria, wild, in the United States ; Gymnemia (Asclepias) tingens, in Burmah ; Polygala tinctoria, in Arabia; Polygonum Chincnse, P. tinctorium, P. perfoliatum, P. barbatum, P. aviculare, in China and Japan, and introduced into Belgium ; Ruellia indigotica, largely cultivated in Assam, as well as in India, and at Che-king, in China ; Tephrosia tinctoria, and T. apollinea, in India and Egypt ; Wrightia tinctoria (Nerium tinctoriam), the Palos indigo of the Carnatic.
The cultivation of indigo (chiefly Indigofera tinctoria) is very extensively carried on in India, especially in the district included between 20° and 30° N. lat. The soil best suited for the culture is a rich loam, with a subsoil which is neither too sandy nor too stiff ; alluvial soils give the best returns, but good crops are sometimes raised on higher grounds. The land is ploughed in October November, after the rains ; the seed, about 12 lb. to the acre, is sown in February-April. Too rapid
growth diminishes the yield of dye. In July-September, the plants are in full blossom, and the harvest takes place. The preparations of the dye-stuff may be performed in either of two ways which are distinguished as the "dry-leaf," and the "green-leaf" process. The latter is considered the better, and is the more general; it is conducted as follows flowering plants are cut down at about 6 in. from the ground, and immediately taken to the steeping vats, within which they are spread out, and pressed down by beams fitted to the side posts of the tanks. Enough water is then admitted to cover the plants ; if this be delayed, fermentation may set in and spoil the product. Tho duration of the steeping ie liable to considerable modification, and needs much judgment and experience ; with a temperature of 35i° (96° F.) in the shade, 11-12 hours may suffice ; in cooler weather, 15-16 hours may be necessary. Moreover, very ripe plants require lees time than young and unripe ones. The following general conditions indicate the time for suspend ing the maceration :—(1) The sinking of the water in the vat ; (2) the immediate bursting of the bubbles that arise ; (3) an orange tint mingling with the green, when the surface water ie disturbed ; (4) the omission of a sweetish, pungent odour, quite distinct from the raw odour of the unripe liquor. At this point, men enter the vat, and stir up its contents, either by hand or by a wooden paddle. The agitation is at first gentle, but increases as the fecula begins to separate ; this is known by the disappearance of the froth, aud by the colour of the liquor changing from green to blue. The " beating," as it is called, is continued for 11-3 hours, the following conditions being a guide as to its sufficiency :—(1) The ready precipitation of the fecula from a sample of the liquor, and the Madeira-wine colour of the latter ; (2) a brownish colour observed on dipping a cloth into the liquor, and wringing it out ; (3) the appearance of a glassy surface on the liquor, and the subsidence of the froth with sparkling and effervescence.