INDIGO

A blue substance much used as a dye-stuff The best indigo is obtained from an Asiatic and American plant, the Indigofrra. The plant is bruised and fermented in vats of water, during which it deposits indigo in the form of a blue powder, which is collected and dried, so as to form the cubic cakes in which it usually occurs In commerce. Indigo is quite insoluble in water; when heated it yields a purple vapour, which condenses in the form of deep blue or purple acicular crystals. When indigo Is exposed to the action of certain deoxidizing agents, it becomes soluble in alkaline solutions, losing its blue colour and forming a green solution, from which it is precipitated by the acids white; bflt it instantly becomes blue by exposure to air. This white indiirn has been termed indigo gene, and indigo appears to be its oxide. It is best obtained by mixing 3 pans of finely powdered and pure indigo with 4 of green vitriol, 5 of slaked quicklime, and 100 of water, repeatedly shaking the mixture. In about twenty-four hours the supernatant liquor, which is transparent, and of a green colour, is to br decanted off, and poured into dilute muriatic acid, when thi [s. 613]