Dr. Schunck on the Formation of Indigo-blue. 183 



mineral iu its present form is so small, that some of the original 

 fibrous mineral must have been wholly removed before the re- 

 mainder began to be encrustatcd. 



The organic matter noted in the analysis appeared to have 

 been derived from infiltrated watei", and followed the direction 

 of the fibres. When a fragment of the mineral was heated in a 

 small glass tube, the junction of the fibres was well marked by 

 black lines from the charred matter. 



The peculiar character of the pseudomorph, especially if we 

 assume that it was tremolite, which is so frequently found in 

 calcareous rocks, suggests the idea that many of the fibrous 

 varieties of dolomite may have been formed in a similar way. It 

 would be worth while to examine some specimens of these dolo- 

 mites from this point of view. 



XXIV. On the Formation of Indigo-blue. — Part II, 

 By Edward Schunck, Ph.D., F.R.S. 



[Concluded from p. 133.] 



Action of Alkalies on Indican. 



IN the first part of this paper I have described in general 

 terms the efi"ect produced on indican by alkalies. I shall 

 now proceed to give a more detailed account of this process of 

 decomposition, and of the products to which it gives rise. 



When a watery solution of indican is mixed with caustic soda 

 it turns of a dark yellow colour, but no further apparent change 

 takes place. If, however, after the mixture has been left to 

 stand for several days, a portion of it be mixed with an excess of 

 sulphuric acid and boiled, it deposits dark flocks, which after 

 being collected on a filter and washed, are found to contain no 

 indigo-blue, and to be entirely soluble in boiling alcohol. The 

 alcoholic solution has a fine purple colour, and gives only a 

 slight precipitate with acetate of lead. Hence it follows, that 

 by the action of the alkali indican is converted into a body which 

 by decomposition with acids yields indirubine. This body may 

 be prepared in the follov>iug manner. A watery solution of in- 

 dican having been mixed with baryta-water is left to stand until 

 a portion of it, on being boiled with an excess of muriatic acid, 

 no longer yields indigo-blue, but only indirubine. The baryta 

 is then precipitated with sulphuric acid, the excess of the latter 

 is removed by means of carbonate of lead, the liquid is filtered, 

 and after sulphuretted hydrogen has been passed through it, it 

 is filtered again from the precipitated sulphuret of lead and then 

 evaporated by means of a current of air in the apparatus above 

 described. The dark yellow syrup left after evaporation is 



