328 VEGETABLE SUBSTANCES. 



and the other advantages attending its employment, 

 indigo has nearly superseded the European woad as 

 a first colour ; woad being now rarely used except as 

 an auxiliary. Indigo can only be applied as a dye in a 

 state of solution, and must consequently be deprived 

 of its oxygen, to be rendered again soluble in water. 

 Ingredients therefore, having a strong affinity for 

 oxygen, are mixed in the vat together with the indigo, 

 whereby it is again held in a state of solution. To 

 produce this effect, the dyers usually employ protoxide 

 of iron, to deoxidize it, and lime-water to render it 

 soluble in its yellow green state*. Bancroft con- 

 siders that its colouring matter is somewhat injured 

 by this process, and supposes that the very durable 

 blue dyes of some nations, in different parts of Asia 

 and Africa, are derived from the indigo plant em- 

 ployed when the colour is first extracted by steeping 

 and fermentation. The Chinese are said thus to 

 apply this dye, and the Africans use it in a way 

 nearly similar. Mr. Clarkson has remarked that the 

 dyes of Africa are superior to those of any other part 

 of the globe. The blue produced there is so much 

 more beautiful and permanent than that which is ex- 

 tracted from the same plant in other countries, that 

 many have been led to doubt whether the African 

 cloths brought into this country t were dyed with 

 indigo. It was believed that this vivid and perma- 

 nent African colour, which obtained more lustre by 

 repeated washings, must have been derived from 

 some other plant, or extracted from some of the 

 woods of the country celebrated for imparting beau- 

 tiful colours. It has, however, been clearly ascer- 

 tained, that the balls of indigo, prepared by the 

 Africans, are simply the leaves rolled up. Two or 

 three of these balls have been procured, and subjected 

 to chemical examination. 



M. Adanson, in noticing the indigo cultivated by 

 * Ure's Chemistry, 



