DYES. 147 



dants. It is frequently difficult to say which is the color 

 and which the mordant. 



Among substances employed as mordants are included 

 numerous oxides and salts ; the principal are the acetate of 

 alumina, the sulphate or acetate of iron, and the muriate of 

 tin. The material to be dyed is first impregnated with the 

 mordant, before being dipped in the solution of the coloring- 

 matter. Mordants, besides fixing the color, often brighten 

 the tint. 



Substances used for dyeing are exceedingly numerous ; 

 the mineral, animal, and vegetable kingdoms all pay tribute 

 to this exquisite art. 



BLUE DYES. 



The Indigo of commerce is the chief blue dye. The best 

 kind is the Indigofera tinctoria. The green parts of the 

 plant are cut before flowering, put into large vats with water, 

 when fermentation takes place, and the indigo settles into 

 powdery, pulpy matter ; its color is at first green, but by ex- 

 posure to air it absorbs oxygen, and assumes a blue color. 

 This plant is cultivated only in warm climates. 



Indigo is also found in Woad, Isatis tinctoria, and some 

 other vegetables. "Woad, before the introduction of Indigo- 

 fera, was very extensively cultivated in the North of Europe. 

 The coloring-matter of this plant also is obtained from the 

 leaves ; the processes for obtaining it are generally less artifi- 

 cial than those used for the indigo-plant. 



Indigo is capable of distillation or sublimation by a mod- 

 crate heat, and, on being burnt, emits a fine purple smoke. 

 Indigo is insoluble in water and alcohol, and alkalies have 

 only partial effect on it ; it is rendered soluble by being put 

 into the dyer's vat with various deoxidizing agents, where, 

 after the fermentation has continued some time, the surface of 

 the liquor will be blue, and that not offered to the atmos- 



