a well known and very valuable vegetable dye, originally brought to Europe from India, as the name (a modification of Indicum) implies.

The indigo of commerce occurs in powder or in cubic cakes of a deep blue colour, which acquires a copper tint and lustre on the substance being rubbed. If pure, it is tasteless and inodorous, insoluble in water, and very slightly soluble in alcohol. By boiling it with very dilute sulphuric acid, a viscous matter is separated. Dilute alkaline solutions remove from it a brown colouring matter, and boiling alcohol a red one.

Chemists recognize two kinds of indigo, the blue and the white, which are mutually convertible; and a consideration of the processes by which this conversion is effected furnishes the rationale of the methods by which the dye is procured from the plants which yield it.

When purified commercial indigo, reduced to powder, is digested in water, with excess of lime, and a moderate amount of green sulphate of iron, so as to expose it to the deoxidizing action of the precipitated protoxide of the metal, it loses its blue colour and its insolubility, and forms a soluble compound with the lime of a yellow colour. If diluted hydrochloric acid be added to this solution, whilst air is excluded, a dirty white crystalline powder precipitates, which may be dried in vacuo, and constitutes the white indigo. It consists of carbon, hydrogen, oxygen, and nitrogen; and is most satisfactorily represented as a chemical compound by the formula $C_{16}H_{10}N_2O_2$.

If the addition of hydrochloric acid to the yellow lime-solution be made, with free exposure to the air, then the precipitate rapidly changes from white to blue, and returns to the condition of blue indigo. The formula for this is $C_{16}H_8N_2O_2$, so that it differs from white indigo by containing one equivalent less of hydrogen. An alkaline solution of white indigo yields the same blue precipitate, by simple exposure to the air, without addition of an acid; and one of the best methods of obtaining chemically pure indigo is to digest the powdered commercial dye in an alcoholic solution of weak caustic soda, along with honey or grape-sugar as the deoxidizer. On exposing the filtered liquid to the air, blue indigo separates in crystals. It may be sublimed without decomposition.

It is probable that indigo exists in the plants which yield it, as a soluble compound of its white modification with potash, soda, or lime. The indigo plants grow chiefly in the East and West Indies, in Central America, Africa, and Europe. They belong to different genera, but the most important is the genus Indigofera, of which the species tinctoria yields the greater part of the indigo of commerce, and the species pseudo-tinctoria the best. Besides these, the I. dispersa, argentea, anil, glauca, cinerea, are largely cultivated. The Wrightia tinctoria, an evergreen native of the East Indies; the Isatis tinctoria, or wood of Europe; the Polygonum tinctorium of China; the Baptisia tinctoria of the United States; and other plants of the genera Nerium, Isatis, Pergularia, Tephrosia, Amorpha, &c., also yield indigo.

The Indigofera tinctoria grows about 2 feet high, with pinnate leaves and papilionaceous flowers, followed by the pods characteristic of the leguminous order of plants to which it belongs. The colouring matter is most abundant when the flower-blossoms are expanding, and occurs in nearly the whole plant, but chiefly in the leaves. Two methods are followed in preparing the dye,—the one by fermenting a watery infusion of the fresh plant or leaves, the other by simply macerating in water the fully dried leaves. The fermentation process is followed in the East and West Indies; the maceration process in America. In conducting the former, cuttings of the entire plant are stratified in layers in a large cistern, and covered with a kind of frame-work of wood and bamboo twigs to prevent the plants rising out of the vat after they swell. Water is then pumped upon them, and fermentation soon commences, accompanied by the production of ammonia and the evolution of much carbonic acid. Gradually, also, the liquid changes from nearly white to grey, blue, and deep purple, and becomes covered with a copper-coloured scum. When the fermentation begins to abate, the liquid,—now of a yellowish-green colour,—is drawn off into a second open cistern, and beaten with oars; in short, exposed to a process like that of churning for hours. During this process carbonic acid is expelled; the oxygen of the air allowed to act freely on the soluble indigo, so as to change it into the insoluble blue dye, and the particles of the latter gathered into flocks or grains, which fall to the bottom. This pulpy precipitate is then passed through a strainer, raised to the boiling point, drained by a woollen filter, squeezed in a press, cut by a wire into cubes, and dried.

In the maceration process, the leaves, threshed from the stems, are dried and set aside. At first they yield, on maceration with water, no indigo; but, after some weeks' keeping, they change from a fine green into a blue-grey; and, if then soaked in water, yield a green solution, which, on exposure to the air, gradually deposits blue indigo.

Indigo dissolves in strong sulphuric acid; and the solution, after dilution with water, is largely used in dyeing. Cloth is also dyed by soaking it in the solution obtained by digesting in water indigo, sulphate of iron, and lime. White indigo adheres to the fabric, and changes into the insoluble blue modification on exposure to the air.

(g.w.)