104 Oeheimrath Professor Otto N. Witt [March 21, 



all the products at onr disposal which fulfil that condition, ortho- 

 nitrotoluene is the most easily accessible. Now, taking it for granted 

 that indigo could be prepared regularly and with good yields from 

 orthonitrotoluene, there still remained that difficulty, that all the 

 toluene produced in the world, even if we suppose that all the other 

 uses to which this hydrocarbon is put at present could be sup- 

 pressed, would not suffice for the production of the world's consump- 

 tion of indigo. 



If, under such circumstances, the industry of artificial dye-stufifs 

 continued to work at the indigo problem, it did so more for the 

 general interest attached to it, and with a view to securing some of 

 the finer applications of indigo in printing, than in the hope of being 

 able to compete with the natural product in the great consumption 

 of vat dyeing. If, on the other hand, the indigo planters in the far 

 East showed but small apprehension of the danger of which they 

 were occasionally warned, we cannot blame them for it ; they had, 

 no doubt, taken the advice of competent people, and these had told 

 them what was correct according to the knowledge of the time. 



The final result has shown all the calculations of experts to be 

 wrong, but in such a way that they, too, can surely not be blamed for 

 the error they committed. 



The process by which indigo is at present manufactured on a 

 colossal scale by the Badische Anilin- und Soda- Fabrik in Ludwigs- 

 hafen on the Rhine, is based on Heumann's synthesis of this most 

 important dye-stuflf, which consists in submitting phenylglycine to a 

 fusion with caustic alkali. Phenylglycine is prepared by the action 

 of monochloracetic acid upon aniline. The yield of indigo obtained 

 is a poor one, but it can be very much improved if, instead of phenyl- 

 glycine, we take its orthocarbonic acid. In this we have again the 

 presence of a nitrogenous and a carbonic side-chain in ortho-posi- 

 tion. To prepare this acid we should have to start, according to the 

 ordinary rules, from toluene, transforming it by a succession of opera- 

 tions. Thus we come again to toluene as a starting-point, and to the 

 difficulty already explained. 



There is, however, one somewhat abnormal process of preparing 

 the same compound from phtalic acid. It consists in converting 

 this into phtalimide, and treating the latter with sodium hypo- 

 chlorite. By a somewhat complicated reaction, the nature of which 

 need not be explained, one of the carboxyl groups of the phtalic acid 

 is replaced by the amido group, anthranilic acid is formed, and this, 

 if treated with monochloracetic acid, yields phenylglycine-carbonic 

 acid, which has proved so important for the manufacture of indigo. 

 Now phtalic acid is prepared by a powerful oxidation of naphtha- 

 lene, and naphthalene again is that constituent of coal-tar which is 

 present in by far the largest quantity. 



It is true that the process for transforming naphthalene into phtalic 

 acid, which was the only one known at the time when all these facts 

 were first recognised, gave very bad yields, and was at the same time 



