EXPERIMENTS ON INDIGO. 215 



this colouring matter itself volatilized; and that the sub- 

 stance crystallized in silky tufts, obtained by distilling 

 indigo, is indigo in a state of purity. These crystals dis- 

 solve in concentrated sulphuric acid, imparting to it a fine 

 blue colour ; and are volatilized anew in a purple smoke, 

 when thrown on a hot body. 



Indigo, therefore, is volatile, and capable of crystalliza- Pur' lfi ed in- 

 tion ; and may be purified either in the dry or in the wet e 

 way. The indigo obtained in both ways is perfectly similar, 

 except that the latter always retains some earthy matter : 

 and it is remarkable, that the indigo purified in the wet way 

 is not so blue as it was before, and has a perceptible violet 

 tinge; while indigo not purified, if placed by its side, ap- 

 pears of a dull blue. 



When pure indigo is thrown into concentrated sulphuric Action of sub- 

 acid, it first produces a yellow, which afterward becomes P h * uc acW 

 green, and at length of a fine blue. In this process the in- 

 digo undergoes some change of composition, that merits 

 examination. This is shown, by its being soluble in a 

 number of menstruums, after it has been precipitated from 

 this solution, which before had no action on it : and, which 

 is more strange, by its no longer producing the purple 

 smoke, at least in the same circumstances, and appearing 

 to have lost its volatility. 



Hot alcohol dissolves a small portion of indigo, which Action of 

 gives it a fine blue colour ; but as it cools the colouring alcoh& * 

 matter fulls down, and after some time scarcely any is re- 

 tained in solution. If however the indigo contain a certain 

 quantity of the red resin, the solution will remain coloured 

 for some months. 



From the facts adduced it follows : 

 I 1, That pure indigo is purple: General P ro- 



2, That it is volatilized in the form of a purple smoke, P. erties of ln " 

 crystai Sizable in needles of the same colour : 



3, This vo'atization of a highly carbonated substance is 

 remarkable, as it demonstrates, that the volatility of com- 

 pounds does not depend simply on the volatility of their ele- 

 ments, but also on the affinity, with which the most dilata- 

 ble are united to the most fixed : 



4 f Indigo in a little soluble in alcohol. 



A very 



