Indigo. Indigo is obtained from an Asiatic and American plant, which is bruised and fermented in vats of water. During the process, a blue powder is deposited, which is collected and dried so as to form the cubic cakes in which it occurs in commerce. Indigo is quite insoluble in water; when heated it yields a purple vapour, which condenses in the form of deep blue acicular crystals When it 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 white by the acids; but it becomes blue on exposure to air. This white indigo has been termed indigogene, and indigo appears to be its oxide. When indigo is dissolved in concentrated sulphuric acid it forms a deepblue liquid, known to the dyers under the name of Saxon blue. Bengal is the great mart for this drug.