Dr. Hofmann on the Metamorphoses of Indigo. 391 



itory combination of isatine with potash. By warming the 

 potash-ley, a solution of the dark red crystals is made of the 

 same colour, but so soon as the fluid comes to the boiling-point 

 the colour changes to orange-yellow, and contains now isati- 

 nate of potash. At the beginning of the distillation pure water 

 passes over, but at a certain concentration of the fluid, which 

 can be hastened by the addition of pieces of solid hydrate of 

 potash, a lively reaction takes place, and there distils over with 

 the water a body in the form of oily drops, which possesses 

 all the properties of aniline. During the operation, and par- 

 ticularly at the end, free hydrogen gas is evolved, and carbo- 

 nate of potash remains in the retort. The following formula 

 exhibits the decomposition: — 

 C,gH5N04 + 4(KO,HO) = C,2H7N + 4(C02,KO) + 2H. 



<«- V- ' V ^ ; 



Isatine. Aniline. 



After this experiment, the preparation of the desired com- 

 pounds was no longer a matter of difficulty. They must be 

 obtained by the action of alkaline bases on chlorisatine and 

 bichlorisatine. 



The experiments about to be detailed will show how far 

 this conclusion was correct, supported as it was by the com- 

 plete analogy existing between isatine, chlorisatine and bro- 

 ni isatine. 



Action of fused Hydrate of Potash on Chlorisatine. 



On distilling chlorisatine* with a mixture of potash-ley and 

 solid hydrate of potash, exactly the same phaenomena present 

 themselves as in the case of isatine, with the single difference, 



tration of the fluid, and the mass flows over the largest vessels. The por- 

 tions remaining contain generally only indigotic acid. 



Isatine may be purified very easily in the following manner : — The crude 

 product, still containing a quantity of resinous matter, is dissolved in pot- 

 ash-ley, and then caretully decomposed by the addition of hydrochloric 

 acid so long as there is formed a black or brown precipitate. Should a 

 small filtered portion possess a pure yellow colour, or the precipitate arising 

 from the further addition of hydrochloric acid he of a pure lively red, the 

 whole fluid is then to be separated by filtration from the resin and com- 

 pletely decomposed by hydrochloric acid. After washing with water, isa- 

 tine obtained in this way is chemically pure. 



* For the preparation of large quantities of chlorisatine I have always 

 employed isatine, from the fact that chlorine in its action on indigo pro- 

 duces a mixture of chlorisatine and bichlorisatine, which can with difficulty 

 be separated. Isatine is therefore much more convenient for this purpose. 

 The change can be very n)uch hastened by suspemling isatine in boiling 

 water, and directing a stream of chlorine through the fluid under the direct 

 influence of the sun's rays. The transition is instantly recognised by the 

 orange-yellow colour assumed by the crystals. As already stated by Erd- 

 manu {Journal fur Prakt. Chem., Bd. xxii. S. 272), not a trace of bichlori- 

 satine is formed in this reaction, a fact also confirmed by my experiments. 



