indigo plant: A genus of the decandra order, belonging to the diadelphia class of plants; and in the natural method ranking under the 32d order, Papilionaceae. The calyx is patent; the carina of the corolla furnished with a subulate patulous spur on each side; the legumen is linear.—There are five species; the most remarkable of which is the tinctoria, a native of the warm parts of Asia, Africa, and America, and from which the indigo dye is made. The root of this plant is three or four lines thick, and more than a foot long, of a faint smell something like parsley. From this root issues a single stem nearly of the same thickness, about two feet high, straight, hard, almost woody, covered with a bark slightly split, of a grey ash-colour towards the bottom, green in the middle, reddish at the extremity, and without appearance of pith in the inside. The leaves, ranged in pairs around the stalk, are of an oval form, smooth, soft to the touch, furrowed Indigofera above, of a deep green on the under-side, and connected by a very short peduncle. From about one third of the stem to the extremity there are ears that are loaded with very small flowers from a dozen to fifteen, but destitute of smell. The pistil, which is in the midst of each flower, changes into a pod, in which the seeds are inclosed.

This plant requires a smooth rich soil, well tilled, and not too dry. The seed of it, which, as to figure and colour, resembles gun-powder, is sown in little furrows that are about the breadth of the hoe, two or three inches deep, at a foot's distance from each other, and in as straight a line as possible. Continual attention is required to pluck up the weeds, which would soon choke the plant. Though it may be sown in all seasons, the spring is commonly preferred. Moisture causes this plant to shoot above the surface in three or four days. It is ripe at the end of two months. When it begins to flower, it is cut with pruning-knives; and cut again at the end of every six weeks, if the weather is a little rainy. It lasts about two years, after which term it degenerates; it is then plucked up, and planted afresh. As this plant soon exhausts the soil, because it does not absorb a sufficient quantity of air and dew to moisten the earth, it is of advantage to the planter to have a vast space which may remain covered with trees, till it becomes necessary to fell them in order to make room for the indigo.

Indigo is distinguished into two kinds, the true and the bastard. Though the first is sold at a higher price on account of its superiority, it is usually advantageous to cultivate the other, because it is heavier. The first will grow in many different soils; the second succeeds best in those which are most exposed to the rain. Both are liable to great accidents. Sometimes the plant becomes dry, and is destroyed by an insect frequently found on it; at other times, the leaves, which are the valuable part of the plant, are devoured in the space of 24 hours by caterpillars. This last misfortune, which is but too common, has given occasion to the saying, "that the planters of indigo go to bed rich, and rise in the morning totally ruined."

This production ought to be gathered in with great precaution, for fear of making the farina that lies on the leaves, and is very valuable, fall off by shaking it. When gathered, it is thrown into the steeping-vat, which is a large tub filled with water. Here it undergoes a fermentation, which in 24 hours at furthest is completed. A cock is then turned, to let the water run into the second tub, called the mortar or pounding tub. The steeping-vat is then cleaned out, that fresh plants may be thrown in; and thus the work is continued without interruption.

The water which has run into the pounding-tub is found impregnated with a very subtile earth, which alone constitutes the dregs or blue substance that is the object of this process, and which must be separated from the nitre salt of the plant, because this makes the dregs swim on the surface. To effect this, the water is forcibly agitated with wooden buckets, that are full of holes and fixed to a long handle. This part of the process requires the greatest precautions. If the agitation be discontinued too soon, the part that is used in dying, not being sufficiently separated from the salt, would be lost. If, on the other hand, the dye were to be agitated too long after the complete separation, the parts would be brought together again, and form a new combination; and the salt reacting on the dregs would excite a second fermentation, that would alter the dye, spoil its colour, and make what is called burnt indigo. These accidents are prevented by a close attention to the least alterations that the dye undergoes, and by the precaution which the workmen take to draw out a little of it from time to time in a clean vessel. When they perceive that the coloured particles collect by separating from the rest of the liquor, they leave off shaking the buckets, in order to allow time to the blue dregs to precipitate to the bottom of the tub, where they are left to settle till the water is quite clear.—Holes made in the tub, at different heights, are then opened one after another, and this useless water is let out.

The blue dregs remaining at the bottom having acquired the consistence of a thick muddy liquid, cocks are then opened, which draw it off into the settler. After it is still more cleared of much superfluous water in this third and last tub, it is drained into sacks; from whence, when water no longer filters through the cloth, this matter, now become of a thicker consistence, is put into chests, where it entirely loses its moisture. At the end of three months the indigo is fit for sale.

It is used, in washing, to give a bluish colour to linen; painters also employ it in their water colours; and dyers cannot make fine blue without indigo. The ancients procured it from the East-Indies; in modern times, it has been transplanted into America. The cultivation of it, successively attempted at different places, appears to be fixed at Carolina, St Domingo, and Mexico. That which is known under the name of Guatemala indigo, from whence it comes, is the most perfect of all.

There are two kinds of indigo prepared in the East-Indies, particularly on the coast of Coromandel, at Pondicherry, &c. Of these the worst kind is used for giving the body of colour to the dyed substance, the other being employed only to give it a glost afterwards. The finest is prepared on the coast of Agra, Mafulpataam, and Ayanoa, but especially in the island of Java; but this last, being extremely dear, is very little used by the dyers. The best ought to float on the surface of water; its colour ought to be a very dark blue inclining to violet, bright and sparkling, especially when broken. It may be tried by dissolving a little in a glass of water: if pure, it will mix equably with the liquor; but if otherwise, will separate and fall to the bottom. Another method of trying the goodness of this substance is by fire; for the pure indigo will be entirely consumed, while the extraneous particles will remain. The pounded indigo is much more subject to adulteration than such as is sold in cakes or tablets; as the ashes or dirt with which it is mixed are very apt to separate from the pure colouring substance when standing in a liquid state, as it must always do before the moisture is evaporated: whence, on breaking a bit of indigo so adulterated, the extraneous matter will be perceived in strata of a different colour.