INDIGO. 915 



White or reduced indigo is produced by the action upon blue in- 

 digo of deoxidating bodies of all kinds, such as the protoxides of 

 iron and tin, sulphites and phosphites and many sulphurets, 

 particularly the sulphuret of arsenic or orpiment, but only with 

 the concurrence of an alkali or alkaline earth, which may 

 combine with the reduced indigo. On neutralising a solution 

 of the alkaline compound with hydrochloric acid, carefully 

 excluding air, the reduced indigo is thrown down as a white 

 precipitate, flocculent, and composed of minute crystalline 

 plates. Carefully dried in vacuo, it is coherent, of a greyish 

 white colour and silky lustre ; in the dry state it soon becomes 

 blue superficially in the air, but requires several days to become 

 entirely blue. When humid or dry, it is tasteless, inodorous, 

 insoluble in water, soluble in alcohol and ether, which it 

 colours yellow; but it is soon deposited from the alcoholic 

 solution as blue indigo, when exposed to air. White indigo 

 does not affect litmus, dissolves in alkalies without neutral- 

 ising them, and has not marked acid characters, although 

 it combines with bases. According to the observations of 

 Dumas, the conversion of white into blue indigo occurs in air 

 without any change of weight, or there is, at the utmost a slight 

 but sensible loss. 



White indigo was named indigogen by Liebig, and blue 

 indigo considered the oxide of that radical. M. Dumas 

 takes another view of the relation between these bodies, con- 

 sidering white indigo not as deoxidised blue indigo, but blue 

 indigo combined with an atom of hydrogen, and forming a 

 hydruret, analogoiis to the hydruret of benzoyl, thus : 



White indigo . C 16 H 5 NO 2 . 

 Blue indigo . . C 16 H 5 NO 



In the oxidation of indigo, on this view, water is formed and 

 liberated. M. Dumas still adheres to this view in his late 

 Memoir on indigo. The combustion of white indigo he found 

 to be easily affected, but that of blue indigo is attended with 

 difficulty, so as to leave some uncertainty as to its composition. 

 According to M. Erdmann, the formula of blue indigo is 

 C 32 H 10 N 2 3 . 



Action of sulphuric acid. Indigo combines with fuming 



2 o o 2 



