Indigo may be obtained from the me rium tinctorium, and the isatis tinctoria or woad ; a plant cultivated and even found wild in England. When arrived at maturity, this plant is cut down, wash ed, dried hastily in the sun, ground in a mill, placed in heaps, and allowed to fer ment for a fortnight. It is then well mix ed, and made up into balls, which are piled upon each other, and exposed to the wind and sun. In this state they be come hot, and exhale a putrid ammonia cal smell. The fermentation is promoted, if necessary, by sprinkling the balls with water. When it has continued for a suf ficient time, the woad is allowed to fall to a coarse powder ; in which state it is sold as a dye-stuff. By treating woad nearly in the same manner with the indi gofera, indigo has been obtained from it by different chemists.
Indigo is a soft powder of a deep blue, without either taste or smell. It under goes no change, though kept exposed to the air. Water, unless kept long upon it, does not dissolve any part of it, nor pro duce any change. When heat is applied to indigo, it emits a bluish red smoke, and at last burns away with a very faint white flame, leaving behind it the earthy parts in the state of ashes. Neither oxy gen nor the simple combustibles have any effect upon indigo, except it is in a state of solution ; and the same remark applies to the metallic bodies. The fixed alka line solutions have no action on indigo, except it is newly precipitated from a state of solution. In that case, they dis solve it with facility. The solution has at first a green colour, which gradually disappears, and the natural colour of the indigo cannot be again restored. Hence we see that the alkalies, when concen trated, decompose indigo. Pure liqkrid ammonia acts in the same way. Even carbonate of ammonia dissolves precipi tated indigo, and destroys its colour ; but the fixed alkaline carbonates have no such effect. Lime-water has scarcely any effect upon indigo in its usual state • bet it readily dissolves precipitated indi go. The solution is at first green, but becomes grs}clually yellow. When the solution is exposed to the air, a slight green colour returns, as happens to the solution of indigo in ammonia, but it soon disappears.
The action of the acids upon indigo has been examined with most attention ; it certainly exhibits the most important phenomena. When diluted sulphuric acid is digested over indigo, it produces no effect, except that of dissolving the impurities ; but concentrated sulphuric acid dissolves it readily. One part of indigo, when mixed with eight parts of sulphuric acid, evolves heat, and is dis solved in about twenty-four hours. Ac cording to Haussman, some sulphurous acid and hydrogen gas are evolved during the solution. If so, we are to ascribe them to the mucilage and resin which are doubtless destroyed by the action of the concentrated acid.
The solution of indigo is well known in this country by the name of liquid blue, or sulphate of indigo. While concen trated it is opaque and black r. but when diluted it assumes a fine deep blue co. lour ; and its intensity is such, that a sin gle drop of the concentrated sulphate is sufficient to give a blue colour to many pounds of water. Bergman ascertained the effect of different re-agents on this solution with great precision. Dropt into sulphurous acid, the colour was at first blue, then green, and very speedily de stroyed. In vinegar it becomes green, and in a few weeks the colour disap pears. In weak potash it becomes green, and then colourless. In weak carbonate of potash, there are the same changes, but more slowly. In ammonia, and its carbonate, the colour becomes green, and then disappears. In a solution of sugar, it became green, and at last yellowish. In sulphate of iron, the colour became green, and in three weeks disappeared. In the sulphurets the colour was destroy ed in a few hours Realgar, white oxide of arsenic, and orpiment, produced no change. Black oxide of manganese de stroyed the colour completely. From these and many other experiments it was inferred, that all those substances which have a very strong affinity for oxygen give a green colour to indigo, and at last destroy it. Hence it is imagined, that indigo becomes green, by giving out oxy. gen. Of course it owes its blue colour to that principle.