489 Experimtnts ox Indigo, 



pofed by the muriatic acid, nor to the Due D'Ayen's fublimatc 5 which, befides requiring 

 a great heat, was attrafted by the magnet. 



Neither does fulphuric acid adit on tliis Angular product of indigo, though concentratech" 

 and boiling. Nitrous acid only diflblves it, which it does rapidly; and on on that folution 

 my experiments, further to afcertain its quality, were performed. 



The muriatic acid being waflied off, the carbonaceous refiduum, (the laft of which gave 

 3 faint blue with prufliates) the leaves were diflblved in nitrous acid, which gave no fymp- 

 tom of containing iron ; and I found alkalis invariably precipitate this folution white j 

 unlefs, as will afterwards be fliown, that the metallic matter be converted into an acid, in 

 which cafe ammoniac diflblves with it, and feems to have the- ftrongeft attra£tion for it 

 of any of them. When the muriatic acid has not been fufficiently wafhed from it, and it 

 ftill retains a portion of iron, very pure prufliate of lime forms with it a deep green 

 colour, which no acid whatever has the efFefl of changing to blue ; but the air and light 

 make it yield a blue precipitate. I once confidered this to be the colouring matter of in- 

 digo regenerated, without tha fixing refinous fubftance, and nearly in the fame ftate with 

 what Bergman calls precipitated indigoy as a cauftic alkali turns it yellow ; but I have fince 

 difcovered that when fufficiently digefted in muriatic acid, the nitrous folution yields no 

 colour whatever with prufliate of lime. Galls, either in a watery or fpiritous infufion, give 

 a white precipitate, and the abforbant earths the fame. By diftilling the nitrous acid from 

 indigo, I procured Haufleman's acid ; which I found produced the fame effects, and is un- 

 doubtedly this volatile matter converted into an acid by oxigenation, in a manner analogous 

 to many other inflammable bodies; this acid forms an infoluble precipitate with pot- 

 afh, but has a Itronger attraction for cauftic ammoniac, which combination cannot be de- 

 compofed but by muriate of pot-afli. I found alfo that this acid formed a green with pruf- 

 fiates, when not fufficiently purified from iron, which, when fpread on paper, the rays of 

 the fun (to which it was expofed for above a day) changed to yellow, and not to blue. I 

 may here remark, that indigo in fubftance, heated with wax or tallow, produces a pink 

 colour, which ftained the ftick with which I ftirred the mixture, and that the nitrous 

 acid changes it to a yellow, which I have obferved return to the green and the blue by , 

 expofure to the light. Thefe faCts have caufed me to form an opinion, that this volatile 

 matter is a more general colouring principle of organic bodies than might be fuppofed} 

 and though at prefent I will not enter further on the fubjeft, I think I have traced it in other 

 fubftances as well as indigo. Though I tried various other acids, both animal and vege- 

 table, they had no peculiar effedts on the above nitrous folution. 



I now come to the Angular adlion, that nitrous acid, repeatedly diftilled from indigo, 

 produces (as I fuppofe) on the refinous fixing matter, and which will be a teft to difcover 

 that matter in any other body. 



To twenty grains of indigo I added about two drachms and two fcruples of nitrous acid 

 with two waters ; after repeated diftillations, the refinous matter was nearly deftroyed, 

 and, the cryftals of the acid of Haufleman were vifible on the fides of the retort ; the liquor 



in 



4 



