406 COMPOUND PRODUCTS. CHAP. I. 



of carbonic acid gas. When the fermentation has 

 been continued long enough, the liquid is decanted 

 and put into other vessels where it is agitated till 

 blue flakes begin to appear. Water is now poured 

 in, and the flakes are precipitated in the form of a 

 blue powdery sediment, which is obtained by de- 

 cantation ; and which, after being made up into 

 small lumps and dried in the shade, is the Indigo of 

 the shops. 



It is insoluble in water, though slightly soluble 

 in alcohol. But its true solvent is sulphuric acid, 

 with which it forms a fine blue dye, known by the 

 name of liquid blue. It affords by distillation car- 

 bonic acid gas, water, ammonia, some oily and acid 

 matter, and much charcoal ; whence its consti- 

 tuent principles are most probably carbon, hy- 

 drogene, oxygene, and nitrogene. 



Indigo may be procured also from several other 

 plants besides Indigo/era tinctoria, and particu- 

 larly from Isatis tinctoria or Woad, a plant indi- 

 genous to Britain, and thought to be the plant with 

 the juice of which the ancient Britons stained their 

 naked bodies to make them look terrible to their 

 enemies. If this plant is digested in alcohol, and 

 the solution evaporated, white crystalline grains, 

 somewhat resembling starch, will be left behind ; 

 which grains are Indigo, becoming gradually blue 

 by the action of the atmosphere. The blue colour 

 of Indigo therefore is owing to its combination with 

 oxygen. 



