The Action of Acetic Acid on Indigo Blue. 167 



body depends upon the size and perhaps hardness of 

 the crystals. It commences immediately, and may be 

 completed in twenty minutes, or it may take several hours 

 boiling or steaming ; it is effected more rapidly and com- 

 pletely in sealed tubes heated to i5o°C. 



Decomposition by dry heat. Heated in a tube retort the 

 aceto-product suffers little change until the temperature 

 rises to 130° or 140° C. ; it then gives off acetic acid, and 

 continues to do so until a temperature of i8o°C. is reached, 

 the acetic acid when collected is found to be the mono- 

 hydrate, and crystallises readily upon cooling. The 

 residuum in the retort is a complex substance, yielding no 

 isatin to water or alkalies, contains a strong dark red 

 colouring matter, but is mostly a pitchy matter, very 

 soluble in chloroform, which leaves indigo insoluble, usually, 

 in a crystalline state, but in less quantity than indicated by 

 theory. In the hot air bath at 180 C. acetic acid is expelled 

 quickly, partial fusion takes place, and chloroform dissolves 

 out only a red pitchy matter, leaving indigo ; but if the heat 

 be lower, say at 140 to i5o°C, and the operation stopped 

 when the loss of acid is about 18 or 20°/ instead of 31 %, 

 and then treated with cold chloroform, there is dissolved 

 with the pitchy matter a notable quantity of the oxy-aceto 

 body. This chloroform solution soon decomposes upon 

 standing, depositing indigo blue, and is found to contain 

 isatin. If calico be dipped in the chloroform solution, it is 

 found after standing, or steaming and washing, to be stained 

 of a light blue colour, which is permanent, and, in fact, is an 

 indigo dye. 



Decomposition with soda hydrate. If to a mixture of 

 the oxy-aceto-indigotin with water there be added a 

 sufficient quantity of soda hydrate, and the mixture left in 

 the cold for a time, complete decomposition takes place. 

 There is formation of indigo blue, no isatine, or only a trace, 

 is produced, two molecules of acetate of soda and the 



