short time after which this occurs already, so that I think it very well possible 

 that the chromogene has disappeared during his preparation without his having 

 observed it. For it is to be kept in view that his method of demonstrating the 

 indigo-blue qualitatively is highly deficient and consisted in decomposing the 

 chromogene by strong mineral acids, the very worst method to be followed, 

 as strong acids are pernicious as well to isatan as to indoxyl. 



My opinion that Schunck at the moments when it was particularly important, 

 had not to do with the indigo chromogene itself, but with another substance, 

 is also based on several observations which he makes about the properties of 

 the pure indican. So we read on pag. 85 (I.e. Parti): With caustic alkalies, 

 baryta and lime-water the watery solution turns of a bright yellow. This reaction 

 holds only good for the impurity which remains in the dried leaves after the 

 isatan is destroyed in them. If in the preparations any isatan had been pre- 

 sent the yellow colouring would have been immediately followed by the form- 

 ation of indigo-blue, which then becomes much more distinctly visible than if 

 the same preparation is decomposed by acids. Evidently he has examined different 

 samples with acids and alkalies, and samples, free from isatan, only with the 

 latter, else he would certainly have found that those preparations, which by acids 

 produce indigo-blue, yield much more indigo if they are treated with an alkali. 

 Likewise the following statement of his preliminary researches is for the greater 

 part unintelligible if it is admitted that Schunck speaks of isatan. He says (I.e. 

 pag. 81): I was enabled to infer, with positive certainty, that the Isalis tinctoria 

 contains a substance easily soluble in heat and cold water, alcohol and ether, 

 which, by the action of strong mineral acids, yields indigo-blue; that the formation 

 of the colouring matter from it can be effected without the intervention of oxygen 

 or of alkalies; and that the latter, indeed, if allowed to act on it before the 

 application of acid, entirely prevent the formation of colouring matter. In opposi- 

 tion to this, the fact must be stated, that the best method for demonstrating 

 with certainty and quickness isatan or indoxyl in woad-sap, just consists in adding 

 alkali to it, by which the isatan is decomposed and the indoxyl is quickly oxi- 

 dized to indigo at the air; after this, the addition of acid may be desirable to 

 decolour the yellow pigment formed by the alkali, by which the indigo-blue 

 appears with greater purity. 



The uncertainty of the whole research explains hpw it is possible, that 

 Schunck, when later becoming acquainted 1 ) with Polygonum tinctorium. could think 

 that the indican therein occurring, the composition of which, C u H' 7 NCT -\- 3 H" O, 

 has recently been determined by Messrs. Hoogewerff and ter Meulen 2 ), and 

 which is entirely different from isatan, could be identic with his woad-indican. 



Consequently I believe that Schunck cannot be considered as the dis- 

 coverer of the isatan, though it is not to be doubted, that in his experiments, 

 he has sometimes had this substance before him, and, basing on the above expo- 

 sition I take his indican formula for not appliable to isatan. 



M On Indigo-blue from Polygonian tinctorium. The Chemical News. Vol. 39, pag. 

 119, 1879. 



2 ) Kon. Akacl. van Wetensch. te Amsterdam, 31 Manrt 1900, pa^. 508. 



