87 
perma , or Gautimala indigo, and the Indigofera 
tinctoria , or French indigo. It is prepared by 
fermenting the leaves of those trees in water. 
Indigo, in its common form, appears as a fine deep 
blue powder. It is insoluble in water, and but 
slightly soluble in alcohol : its true solvent is 
sulphuric acid : 8 parts of sulphuric acid dissolve 
1 part of indigo ; and the solution diluted with 
water forms a very fine blue dye. 
Indigo, by its distillation, affords carbonic acid 
gas, water, charcoal, ammonia, and some oily 
and acid matter : the charcoal is in very large pro- 
portion. Pure indigo, therefore, most probably 
consists of carbon, hydrogene, oxygene, and azote. 
Indigo owes its blue colour to combination wdth 
oxygene. For the uses of the dyers, it is partly 
deprived of oxygene, by digesting it with orpi- 
ment and lime-water, when it becomes soluble 
in the lime-water, and of a greenish colour. 
Cloths steeped in this solution, combine with the 
indigo ; they are green when taken out of the 
liquor, but become blue by absorbing oxygene 
when exposed to air. 
Indigo is one of the most valuable and most 
extensively used of the dyeing materials. 
10. There are a number of colouring principles 
found in different vegetable productions, the pro- 
perties of which are less marked than those of 
indigo, and the separation more difficult. The 
colouring matters of carthamus and madder are 
the most fixed amongst the red vegetable colours. 
A number of vegetable substances are rendered 
red by the action of acids, and green by that of 
g 4 
