Great attention must be paid, in all the stages of this process, to adjust the best degree of each change. If the fermentation is too soon stopped, a part of the indigo remains unextracted. This is ascertained by the colour of the twigs, when a few are taken out and inspected. They are of a pale yellow, and tender, when they have yielded the whole. The degree of evolution of gas is also to be regulated : when the quantity is deficient, the liquor is of a yellowish green. When the fermentation is carried too far, the liquor becomes turbid, from an admixture with grosser parts separated from the plant, and the condensa tion and precipitation of the indigo are prevented from taking place with the due facility, and from yielding an article of genuine purity. This is more dangerous than a deficient fermentation, as not being so easily remedied. It appears from the experiments of Dr. Roxburgh, that the absorption of oxygen is essential, both during the fermenta tion and the precipitation.
The only further process which the indigo undergoes is that of drying, which sometimes is effected solely by dripping ; at other times it is subjected to a pressure, by which the water is squeezed out. It is exposed to a free current of air, but shaded from the light of the sun.
Indigo differs greatly in its excellence, as well as in its accidental properties. Some specimens of it have muchless specific gravity than others, and the lightest is the purest. Some of it will even float in water. if the air is excluded while it is in any degree moist, it undergoes a chemical change, by which white specks are formed in its sub stance. When too much lime has been used in precipita tion, it subsides mixed with the indigo, and throws down other adulterating matters. Indigo differs greatly in co lour. Of the Guatimala indigo, the most esteemed sort which comes from America, there are three varieties ; one, called flora, has a fine blue colour; a second, called sobre saliente, is violet ; and the third, called corti color, is copper coloured. When the first sells at 9s. per lib., the second is 7s. and the third 5s. 6d. Of the East India in digo, the Javanese was formerly preferred to all others ; but the manufacture is now cultivated to a considerable extent in the British possessions, and an article is pro duced which is often superior even to that of Guatimala.
The comparative value of indigo may be ascertained by dissolving equal portions of different specimens in sul phuric acid, and afterwards comparing the quantities of oxymuriatic acid requisite to destroy the colour of each. Dr. Bancroft proposes to compare merely the depth of the colours, which the same proportion of each communicate to water—a test which must be less accurate. Al. Cite vreuil analysed the best Guatimala indigo, and found that it contained other ingredients besides the true colouring matter. He obtained from it, by hot water, an unoxygen ated base of indigo, with a green matter united to gum, some ammonia, and a little yellow extractive, amounting in all to 12 parts in 100. From the remaining 83 he ob
tained, by digesting with alcohol, 30 parts, consisting of a green matter, a reddish resin, and a little indigo. By di gesting the residue with muriatic acid, he obtained two parts of red resin, two of carbonate of iron, two of red oxide or iron combined with aluminous earth, and there remained three parts of silex, and 45 of pure indigo. This last alone gave out the beautiful purple smoke on burning, which characterises indigo, and which is the substance itself in a state of sublimation. When thus purified, it is of a purple colour, a circumstance which is also observed to take place with Prussian blue, when most condensed.
Bergman obtained 47 parts of pure indigo from the article of commerce, which, when distilled, yielded 2 parts of carbo nic acid, 8 of an alkalilie liquor, 9 of empyreumatic oil, 23 of a charcoal, containing 2 parts of oxide of iron, and 2 of silex.
Some have attributed the blue colour of the indigo to iron, but this is improbable. Chaptal ascribes it to char coal ; but the mode of combination in which the colouring matter exists is so peculiar, that we ought not to ascribe it i exclusively to oiw principle. One thing in it remarkable is, that the compound base she ws a stronger affinity among its component parts, than that which takes place between it and oxygen, as the base may be obtained in a limpid state, in which it requires only the addition of oxygen to impart to it the colouring property. It is by being de prived of a part of its oxygen that it is rendered soluble. This was formerly done by mixing it with woad which had been ferre.ented. A boiling decoction of weld, madder, and broom, was first poured on the woad, and allowed to stand in a proper temperature, till blue veins were formed on its surface. Quicklime, and the indigo in powder, are then added, and the fermentation conducted secundunz anon; the liquor becomes green and fit for dyeing. This process is employed in dyeing woollen stuffs ; but the pro portion of lime requires to be carefully adjusted. When in excess, it imparts to it a brown colour. For the same purpose of disengaging the oxygen, or a part of it, urine is employed, also metallic salts, such as green sulphate of iron. in which the metal is at an inferior degree of oxyda tion : arsenic and oxide of tin are used for the same pur pose, or metallic tin assisted by the muriatic or other acids. The substance chitfly employed as a solvent of in digo, without depriving it of its oxygen, or imparting any temporary change of colour, is sulphuric acid. These pro cesses form part of the art of DYEING, and to that article we refer for the details. See Dr. Bancroft's Researches concerning the Philosophy of Permanent Colours. CHEM ISTRY, and INDIA, p. 262. (H. D.)