DYEING. 191 



To dye nankeen yellow, boil the cotton in a so- 

 lution of carbonate of potash, and then dip it in a 

 solution of the red sulphate of iron. 



Of Dyeing Blue. 



There are but few substances capable of furnish- 

 ing blue dyes. The only vegetable products are 

 indigo and wood. Indigo is a rich blue colour 

 procured from the fecula of a species of plant that 

 is cultivated in America, and also in the East Indies. 

 The colouring matter is extracted by water, and is 

 at first green, but immediately absorbs oxygen, 

 and then assumes a blue colour. It becomes at 

 the same time insoluble in water, but is soluble in 

 sulphuric acid. 



As indigo has a very strong affinity for wool, silk, 

 cotton, and linen, a mordant is unnecessary in dye- 

 ing with it. The colour is very permanent, because 

 the indigo being already saturated with oxygen, to 

 which it owes its blue colour, is little liable to be 

 decomposed. But it is essential that the indigo 

 be applied in a state of solution in order to attach 

 itself to the cloth. 



A solution of indigo in the sulphuric acid is used 

 for dyeing wool ; this is called Saxon blue, and it 

 gives a very beautiful colour. But it will not do 

 for dying cotton or wool, because their affinity for 

 indigo is not sufficiently great to enable them to 

 decompose the sulphate of indigo. 



To dye by the sulphate of indigo, dissolve one 

 part of indigo in four parts of concentrated sul- 

 phuric acid 5 add to the solution one part of dry car- 

 bonate of potash, and dilute the whole with eight 

 times its weight of water. Boil the cloth for an 



