INI>IGO. 247 



in seven years, and two crops are obtained in each year. The 

 indigo, which seems to be nothing else than the chromule of the 

 plant, is usually extracted by fermentation. The plants are laid 

 in a vat, and covered with water ; and in about 18 hours they 

 begin to swell, and to give off a large quantity of gas, the 

 water at the same time acquiring a green tinge. This process 

 is allowed to go on, until the colouring-matter of the vegetable 

 tissue has been entirely yielded to the water ; but if it continue 

 too long, so that any putrefaction take place, the dye is destroyed 

 The fluid is then drawn off into another vat, where it is violently 

 agitated, for the purpose of separating the pulp from the water. 

 The former consists of little grains, which, during the process, 

 turn from green to blue by attracting oxygen from the air ; and, 

 by further processes, it is dried into a solid mass, constituting 

 the indigo of commerce. Nearly all the indigo imported into 

 Britain, is produced in the East Indies ; its amount averages 

 about seven millions of pounds every year, of which, however, 

 more than half is exported again, chiefly to the North of Europe 

 and Italy. Owing to the great variation in the productiveness 

 of the crops, the price of Indigo is almost constantly changing. 

 In the season 1824-5, it was nearly 11s. 6d. a pound; whilst in 

 the season 1829-30, owing to an over-abundant supply, it was 

 only 4s. 4d. At the former rate, the value of the average quan- 

 tity annually imported would be about four millions sterling ; 

 and at the latter, scarcely above one and a half. 



389. This valuable dye has so strong an attraction for almost 

 every kind of fibrous texture, whether animal (as woollen or 

 silk) or vegetable (as linen or cotton), that it will impart to it a 

 permanent colour, without the assistance of a mordant*. In 

 order to apply it, however, it must be dissolved in water ; and 

 this can only be accomplished by a change in its chemical 

 nature, which restores it to its original yellow-green colour ; the 

 stuffs, after being dyed, change again to blue, by exposure to the 



* Mordants are substances used in dyeing and calico-printing, to hold together 

 the particles of the texture dyed, and those of the dyeing material, when these have 

 iiot a sufficient attraction for each other. If not so united, many colours would 

 be washed off, as readily as they are laid ou. 



