clear he is not, but so far as I conceive his meaning, the first and the third 

 preparations, which he analyzed, contain no indican at all, yet he calls them the 

 purest; the second he considers as less pure, and he seems to have subjected it 

 to the analysis after having convinced himself that by precipitating it with alcohol, 

 lead acetate and ammonia it contained no longer unchanged indican, which 

 consequently means, that he had before him the said matter turning yellow by 

 alkalies and thus containing no more indigo chromogene. 



Word for word he says the following, first concerning his analyses in general 

 (1. c. Part I, pag. 89) : I have hitherto been unable, I regret to say, to ascertain 

 the exact composition of indican by direct experiment. On account of the deli- 

 quescent nature, and its so readily undergoing change when heated, it was im- 

 possible to subject it to analysis in a free state and I was therefore obliged to 

 have recourse to the lead-compound.* Then follows the description of the three 

 analyses themselves. Of the first he says (1. c. pag. 90) : Notwithstanding the care, 

 however, which I took in the preparation of the specimen, I found that it did 

 not contain unchanged indican, as a little of it, when tested with sulphuric acid, 

 gave no indigo-blue. It is nevertheless the purest specimen of the lead-compound 

 which I have analysed. Then he says of the second and third: The next 

 analysis which I shall give, places in a striking light the effect which alkalies 

 exert on indican. I took some of the same solution of indican which I had 

 employed for the preceding analysis, and which I found to give, when a little 

 of it was boiled with acid, very pure indigo-blue; but instead of evaporating it, 

 I added a large quantity of alcohol to it, and then precipitated with acetate of 

 lead and ammonia. The precipitate no longer contained unchanged indican . . . 

 The third analysis was performed with a lead-compound made in the same way 

 as that of the first analysis 1 ).* 



All this is not quite clear, but I read from it that these analyses have 

 nothing to do with the indigo chromogene itself, that is to say, with isatan, and 

 I think that they relate to a mixture of the chromogene from the woad, which 

 colours yellow by alkalies, and plantslime (indiglucine). The explanation of this 

 enormous fact should, I think, be sought in the following circumstances. Schunck 

 prepared the indican by alcohol extraction from carefully dried woad-leaves, 

 which in itself is quite rational, because in this way relatively concentrated and 

 rather pure solutions are obtained. But if the dried leaves are kept a little too 

 long, for instance two days at 28 to 30 C., or if they grow a little moisty, the 

 isatan vanishes completely from them. Though Schunck evidently knew that the 

 chromogene can easily disappear from the dry leaves, he does not mention the 



') Three analyses of such doubtful substances are the sole foundation upon which 

 the well-known indican formula of Schunck 



C 29 H 81 NO 17 + 2H-O == C 8 H 5 NO-t-3(C 6 H 1 <>O 6 ) 

 Indican Indigo-blue Indiglucine 



is based and which, since 1855, has been accepted, without criticism, in all great 

 chemical manuals. Formerly I was inclined to write the formula thus: 



C 26 H 27 NO 11 -|-2H 2 O = C 8 H 7 NO-}-3(C a H 8 O 6 ) 



Indoxyl Glukoron 



but now, having carefully studied Schunck's essay, I think this interpretation also 

 worthless. 



