Indigo comes into commerce in a great variety of qualities, all imported from the tropics, in which alone the plant grows. The principal varieties are Indian, Java, Egyptian, American and African, but in these merchants make numerous subdivisions which are often to be distinguished from one another only by very small differences. Good indigo, whatever its source, has the following characteristics: It appears in almost cubical lumps, but may also form irregular masses. The lumps must show a small specific gravity, a higher might be caused by intentional additions of sand or earth. It is without taste and smell, and the colour of the finest varieties is not particularly bright, but when rubbed with a hard substance, or even by the pressure of the finger nail, it acquires a peculiar coppery lustre, which is the more intense the purer the indigo, and is thus regarded as a common test of quality. Indigo is completely insoluble in almost all solvents; in hot oils a small quantity dissolves, which again separates on cooling. The following are the principal signs by which the quality of indigo is recognised in commerce: It forms a dull-coloured, loose, light mass of earthy nature, a fresh fracture of which has a completely uniform appearance. It must be so porous that it at once adheres to the tongue. Uniformity of colour, light weight and fine fracture are the most important of the practical tests for good indigo. The composition of indigo is well known; we shall glance rapidly over the most important substances which occur in indigo in varying quantity. =The Constituents of Indigo.=—Indigo contains the following compounds: Indigo blue, indigo red, indigo brown, indigo gluten, water and salts. The single important constituent for the purposes of the colour maker is indigo blue, or indigotin. Pure indigo blue can be obtained from indigo only by a lengthy process. The indigo is boiled in turn with dilute sulphuric acid, caustic potash, and strong alcohol. The gluten dissolves in the acid, the indigo brown in the potash solution, and the indigo red in the alcohol. By these operations indigo blue is finally obtained in a state of almost complete purity, it now contains only a small quantity of inorganic substances. The quantity of pure indigo blue obtained in this manner varies, according to the purity of the indigo employed, from 20 to 75 per cent. Indigo blue can also be obtained by direct sublimation from indigo, but the yield is very small, since a large proportion is decomposed by dry distillation.