390 Dr. Hofmann on the Metamorphoses of Indigo. 



wise be transformed into a similar body, C^^ < Cl^" f ^^4» 



and this body would require only to be treated with sulphu- 

 retted hydrogen to be changed into the desired compound. 



Whilst occupied with the preparation of anthranilic acid, 

 for the purpose of these experiments, another expedient of- 

 fered itself, which, by changing completely the direction of 

 my researches, led me quickly to the desired object. 



It is well known that it is not necessary to employ pure an- 

 thranilic acid for the preparation of aniline, which may be 

 obtained directly as a true product of oxidation by distilling 

 indigo and hydrate of potash, as is shown in the following 



manner 



Ci6H5NO,-|-4HO = C,2H7N + 4C02 + 2H. 



Indigo. Aniline. 



We may here take a short review of the substances obtained 

 from indigo by other oxidizing means. By the action of ni- 

 tric acid there is formed as the first product of oxidation a 

 body named isatine, which was discovered by Laurent and 

 Erdmann about the same time. This compound contains the 

 elements of indigo, together with 2 equivalents of oxygen, 



C,,H,NO, + 20 = Ci6H,N04 



' , . ^ , 



Indigo. Isatine. 



Aniline being, as was already remarked, a product of oxida- 

 tion of indigo, it was in the highest degree probable, not to say 

 certain, that it could also be obtained from isatine by fusing 

 the latter with hydrate of potash. This question remained to 

 be decided by experiment. 



Action of Hydrate of Potash in fusion on Isatine*. 



On pouring a solution of caustic potash on isatine in a tu- 

 bulated retort, its colour becomes dark red, almost amounting 

 to black, which, according to Laurent, is owing to a trans- 



* All the isatine employed in the following experiments was prepared 

 from commercial indigo, according to the method of Laurent, with common 

 nitric acid; a very considerable quantity is thus obtained. In its prepara- 

 tion care must be taken not to employ too large a quantity of nitric acid, 

 otherwise there is formed the indigotic acid discovered by Buff" (also called 

 aniline, and nitrosalicylic acid), which is at once recognised by the yellow 

 colour of its crystals and the blood-red reaction with perchloride of iron. 

 Neither must the indigo be mixed with too much water. After each addi- 

 tion of nitric acid effervescence should take place, which must be allowed 

 to finish before adding a fresh quantity. Should this effervescence not take 

 place, and a large quantity of nitric acid be added gradually to the boiling 

 mixture, a most powerful reaction takes place at a certain point of concen- 



