INDIGO. 917 



C 16 H 6 NO 2 Blue indigo C 14 H 5 O 2 Benzoyl 



C ]6 H 6 NO 2 ,H White indigo C 14 H 5 O 2 ,H Essence of bitter almonds 



C 16 H 5 NO 4 Isatin of Laurent C 14 H 5 O 4 Salicyl 



C 14 H 6 O 4 ,H Hydruret of salicyl. 



Action of fused potash on indigo. M. Gerhardt has made 

 the curious observation that when blue indigo is thrown in small 

 portions into fused hydrate of potash, that body dissolves losing 

 its colour, disengaging abundance of hydrogen and ammoniacal 

 gas, and leaving as a fixed residue a mixture of valerate and 

 carbonate of potash. The reaction takes place at the expense 

 of the elements of water ; 1 atom of indigo with 14 atoms of 

 water giving 1 atom of valerianic acid, 6 atoms of carbonic acid, 

 1 atom of ammonia and 6 atoms of hydrogen. On heating the 

 saline residue slightly with sulphuric acid, valerianic acid is 

 obtained in large quantity. This indeed appears to be the most 

 eligible process for preparing that acid. 



Indigo is much used in dyeing, being principally applied in 

 the deoxidised state, and forms one of the most permanent 

 colours, resisting every thing except chlorine and nitric acid. 

 In the form of sulphate of indigo, it is used for Saxon blue, 

 which is much less permanent. 



Anilic or indigotic acid, IiO + C 14 H 4 NO 9 (Dumas); is 

 formed when indigo is dissolved in nitric acid considerably 

 diluted, and may be crystallized by concentration of the liquid. 

 It forms colourless prisms, of a sourish bitter taste, fusible and 

 volatile, sparingly soluble in water. It forms crystallizable salts, 

 which detonate feebly when heated. The formula of the 

 salt of silver is AgO + C 14 H 4 NO 9 (Dumas) . The indigo therefore 

 in forming anilic acid with nitric acid, loses 2 atoms of carbon 

 and 1 of hydrogen. 



Picric acid, carbazotic acid, HO + C ]2 H 2 N 3 O 13 (Dumas). 

 This acid, which was first known as the bitter of Welter, 

 is produced by the action of nitric acid on the preceding com- 

 pound, and by the solution of indigo or any other azotised 

 organic substance in concentrated nitric acid. It crystallizes 

 in yellow brilliant prisms, of a very bitter taste, which are fusible 

 and volatile, and burn with flame when sharply heated ; they 

 are sparingly soluble in water, which they colour yellow. Picric 

 acid is not decomposed by other acids nor by chlorine. Its 

 salts are yellow and generally crystalline ; they detonate strongly 

 when sharply heated, or sometimes by a blow, particularly the 



