73. According to the experiments of Bergman, one part in nine of indigo is soluble in water by boiling; and this part appeared to consist of mucilaginous, astringent, and saponacecus particles. The astringent particles are precipitated by solutions of alum, sulphate of iron, and sulphate of copper. M. Quat•amere also has separated the soluble parts by means of water. Ile mentions, that their quantity is greater in proportion as the indigo is inferior in quality, and that the residuum, after this operation, is equal to the finest indigo. He accordingly proposes to purify indigo of inferior quality by boiling it in a bag, renewing the water till it acquires no more co lour. For the chemical properties and composition of indigo, see CHEMISTRY, VOL vi.
74. About the year 1790, Mr Alderman Prinsep, who had then lately returned from India, presented Dr Ban croft with a specimen of indigo whiRh he had procured in that country, and to which he gave the name of green indigo. Dr Bancroft at first entertained the hope of its proving the same kind of colouring matter as that ob taMed by the inhabitants of Cochin china from a plant called tsai, which, when macerated and fermented like indigo, yields a green fecula capable of dyeing a beau tiful and lasting emerald green. The specimen was too small, however, to enable Dr Bancroft to examine its properties with sufficient precision; but he ascertained, that it differed in several respects from common indigo, and particularly that it did not produce, like that sub stance, a fixed colour on linen or cotton by topical appli cation. See Bancroft, i. 264.
75. About three years afterwards, considerable atten tion was excited, among those who took an interest in the improvements of dyeing, to another species of in digo which had been sent from Calcutta, under the name of barasalverte. It was described to be a simple substance, and to have been prepared with water and fire only, "from an indigoferous plant, an ever green, with leaves somewhat resembling those of the laurel, bearing large clusters of small yellow'flowers, and pro ducing seeds in large pods pointed at the end ;" and it was added, "that the seed did not vegetate in Bengal." It was also represented as giving a durable light green colour, without any mordant or basis, to silk and wool. Dr Bancroft soon procured a specimen of this colouring matter, and, with his usual zeal, set about an examina tion of its properties. After making a few expeilments upon it, he discovered that it was a species of impure indigo, and that it owed its green colour to the presence of an olive-coloured matter, which, on being burnt, ap peared to consist chiefly of carbon. The manufacture
of this dye was therefore abandoned.
76. Pastel, or woad, also affords a blue colouring matter, which possesses properties somewhat analogous to those of indigo. Two species of plants are distin guished under this name, the isatis tinctoria, and the isatis Lusitanica. The former is chiefly cultivated in Languedoc, Provence, and Normandy; and also in some parts of England. The plant, after being cut down, is washed in a running stream, and then dried in the sun as speedily as possible. When it is dried slowly, it is in danger of being spoiled by the colour becoming black. It is next carried to the mill to be ground, and reduced to a paste. In this state it is formed into heaps, which are covered to secure them from rain. At the end of a fortnight these heaps are opened, and beaten, to produce a thorough intermixture of the crust formed on the surface with the internal part. It is then made into round balls, and placed in a situation where it may be freed from the moisture which still adheres to it. The balls are afterwards heaped above one another, to un dergo the process of fermentation, and evolve the blue colour yielded by the plant. As the process advances, the smell of ammonia is exhaled, and the balls are gra dually reduced to a coarse powder; the state in which woad occurs in commerce.
77. Pastel affords, without the assistance of indigo, a blue colour of considerable permanency, but little lus tre. As it yields a small quantity of colouring matter in comparison of indigo, and of inferior beauty, the use of woad, as a dye, is now almost totally abandoned. Astruc affirms, that pastel treated like indigofera yields a colouring matter which greatly resembles indigo; and Chaptal states, that, from experiments which he made in 1795, with the view of adding some vegetables to the list of those which furnish blue colours, he disco vered that goats rue, sant foin, chick peas, and lucern, yielded a blue colour when treated like indigo, which he did not, however, succeed in precipitating. D'Am bourney also attempted to procure indigo from pastel. He succeeded by letting fresh leaves of pastel ferment in a certain quantity of water; taking out the leaves, and pouring solution of caustic alkali into the liquor, and afterwards filtering. The fecula which remained on the filter, he says, resembles Carolina indigo. Thirty five pounds of fresh ripe pastel leaves yielded eight oun ces of fecula.