DYEING. 195 



or wood is to be added. Wlien the wool or cloth 

 is to be put into the indigo vat to be dyed, it 

 should be wrung out of tepid water, and then in- 

 troduced into the vat, where it should be kept for a 

 longer or shorter time according to the strength of 

 shade required. After being taken out, it is ex- 

 posed to air, when the green colour which it had 

 imbibed in the vat is changed to a blue by the 

 absorption of the oxygen of the atmosphere. It is 

 then to be carefully washed. 



Woad itself contains a colouring matter exactly 

 similar to indigo, and indigo may be extracted from 

 it, but the quantity is small. 



Cotton and linen are dyed blue by putting the 

 indigo into a solution of some substance that has 

 a stronger affinity for oxygen than the green bases 

 of indigo. Green sulphate of iron, and metallic 

 sulphurets answer this purpose, the green sulphate 

 attracts the oxygen from the indigo, and reduces 

 it to the green state, in which it is dissolved by 

 lime added to the solution. The cloth is then put 

 into the bath. 



Silk is dyed blue by indigo fermented by bran 

 and madder, and the indigo dissolved by potash. 

 If the shade required be dark, it is dyed first with 

 archill, which is called giving it a ground colour.n 



Of dyeing Black. 



The substance that produces the black dye is the 

 ta nno'gallate oi'h'on. Decoctions of many vege- 

 tables strike a black with a solution of the red 

 oxide of iron. Of these nut-galls give the most 

 copious precipitate. 



Logwood is generally employed as an auxiliary, 

 because it communicates lustre, and adds consider- 



VOL. II. O 



