Dr. Ure on tndigo, 1B3 



Besides impurities accidentally pfesent, from a bad season, 

 want of skill or care, the purest commercial indigo consists of 

 no less than five constituents— 1. Indigo-hlue, a very singular 

 vegetable compound of carbon, hydrogen, and oxygen, with 

 fully 10 per cent, of azote ; 2. Indigo-gluten, a yellow, or 

 brownish-yellow varnish, which differs from wheat-gluten by 

 its solubility in water. It has the taste of osmazome, or of 

 beef-soup, melts when heated, burns with flame, and affords an 

 empyreumatic oil along with ammonia by distillation.*— *3. Indu- 

 go^brown. This constituent is more abundant than the preced- 

 ing^' Jt \% extracted by a concentrated water of potash, made 

 to act on powdered indigo, previously digested in dilute sul- 

 phuric acid. Chevreuil's indigo-green seems to have consisted 

 of this substance, mixed with some alkaline matter, and indigo- 

 blue. — 4. Indigo-red, This is readily dissolved by boiling 

 alcohol out of indigo previously subjected to the action of an 

 acid or alkaline menstruum. The alcohol acquires a beautiful 

 red tinge, and leaves by its evaporation the red principle in the 

 form of a blackish-brown varnish. — 5. Phosphate of littie. I 

 have found the bone phosphate in notable quantity in some 

 fine indigo, constituting another feature of resemblance between 

 this vegetable and animal products. Hence, also, the charcoal 

 of indigo is most difficult of incineration, and requires, for per- 

 fect combustion in some cases, the deflagratory powers of nitric 

 acid. 



Pure indigo^blue is most easily obtained from the blue vat 

 of the indigo-dyer; the yellow liquid of which being acidulated 

 faintly with muriatic acid, and exposed, with occasional agita^ 

 tion, in a shallow basin, soon deposits the blue precipitate, 

 mixed, however, with a considerable quantity (more or less 

 according to the quality of indigo used) of indigo-red. This 

 must be removed from the dried blue powder, by the solvent 

 action of boiling alcohol, applied in successive quantities. 



In my paper on the *' Ultimate Analysis of Vegetable and 

 Animal Substances," which the Royal Society did me the 

 honour to read at their Meeting in June, 1822, and to publish 

 in the volume of their Transactions for that year, I gave an 

 analysis of indigo-blue, to which I appended the following re- 

 marks : — •• I had intended to pursue at considerable detail 



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