SECT. VIII. COLOURING MATTER. 405 



sorts : extractive colours, oxygenated colours, car- 

 bonated colours, and bydrogenated colours : the 

 first being soluble in water and requiring the aid of 

 saline or metallic mordants, to fix them upon cloth ; 

 the second being insoluble in water, as altered by 

 the absorption of oxygene, and requiring no mordant 

 to fix them upon cloth ; the third containing in 

 their composition a great proportion of carbon, but 

 soluble in alkalies ; and the fourth containing a 

 great proportion of resin, but soluble in oils and 

 alcohol.* 



The foregoing views of the subject are, no doubt, 

 useful to the chemist. But the simplest mode 

 of arrangement, which shall accordingly be here 

 adopted, is that by which the different species of 

 colouring matter are classed according to their 

 effect in the art of dyeing. Now the principal and 

 fundamental colours in the art of dyeing are the 

 blue, the red, the yellow, and the brown. 



The finest of all vegetable blues is that which Blue or 

 is known by the name of Indigo. It is the pro- md ' s ' 

 duce of the Indigofera tinctoria of Linnaeus, a 

 shrub which is cultivated, for the sake of the dye 

 it affords, in Mexico and the West Indies. The 

 plant reaches maturity in about six months, when 

 its leaves are gathered and immersed in vessels filled 

 with water till fermentation takes place. The water 

 then becomes opaque and green, exhaling an odour 

 like that of volatile alkali, and evolving bubbles 

 * Connais. de Chem. vol. viii. p. 67, 



