Its peculiarity is that in the form in which it comes to the dyer it is insoluble in water, and to enable it to be dissolved and therefore to be used as a dye, the indigo has to go under a special treatment. The colouring principle of indigo is a body named indigotin, to which the formula C{16}H{16}N{2}O{2} has been given. When indigo is mixed with substances like lime and copperas, lime and zinc, zinc and bisulphite of soda, which cause the evolution of nascent hydrogen, it takes up this body and passes into another substance which is called indigo white that has the formula C{16}H{12}N{2}O{2}, leuco, or white indigo; this substance is soluble in water, and so when it is formed the indigo passes into solution and can then be used for dyeing. But indigo white is an unstable substance on exposure to air, the oxygen of the latter attacks the hydrogen which it has taken up, and indigotin is reformed, the indigo white changing again into indigo blue. Indigo dyeing consists of three operations:-- (1) Preparation of the indigo solution, or, as it is called, setting the dye vat. (2) Steeping the cotton in this vat. (3) Exposing to the air. [Illustration: FIG. 30.--Indigo Dye-vat for Cloth.] There are several methods of preparing, or setting the dye vat, and of each of these modifications are in use in every indigo dye-house:-- (1) With lime and copperas. (2) Zinc and lime vat. (3) Zinc and bisulphite of soda. In all cases it is necessary for the indigo to be ground to the form of a fine paste with water; this is usually done in what is known as the ball-grinding mill. The finer it is ground the more easy is it to make the dye-vats. The dye-vats may be either round tubs or square wooden tanks; when cloths or warps are being dyed these may be fitted with winces and guide rollers, so as to draw the materials through the liquor. In the case of yarns in hanks these appliances are not necessary. Fig. 30 is a sketch of an indigo dye-vat for cloth or warps.