Singular P-rcduS of Indigo. 479 



(if the temp^ratur? is. WtiU regulated) expofed to heat in a crucible, and by the rays of the 

 fun concentrated by means of a middle fized lens on indigo } and the metallic leaf may be 

 procured by fulphure of pot-aflj, phofphorus and water, or volatile alkali and phofphorus 

 boiled with indigo j and in the dry way by diftilling indigo per fe in a reverberatory 

 furnace *. 



As nsy moft accurate experiments were tried pr^ the refiduum obtained in tlie latter 

 mode, I will begin by giving the account of that experiment, and the means I ufed to in- 

 (ure ag^inft deception -, though 1 had already drawn conclufioiis from former cues, of 

 which this tnight be cpnGdered only as a pprr«^ repetition. I took half an ounce of indigo, 

 and expofed it in a wedgewood retort, to the heat of a reverberatory furnace for feveral 

 hours, and joined it to a pneumatic ap'paratus for receiving gafes ; I got none, though the 

 heat was ye?y intenfe, but in the receiver there was ?n alkaline liquor (mixed with a hrown 

 oil or tarry matter) which was evidently carbenate of ammoniac, as I afccruiacdi hy the 

 muriatic acid. I once confidered this as taking its origin from the pruflic acid, but was 

 afterwards undeceived, as will be obferved in the fcquel. The refidue in the retort ap- 

 parently confifted of a charred matter, which other chemifts would have calcined in the 

 open air ; but as I knew previoufly that it contained the volatile matter (which, e»cept,io 

 rapid combuflion, generally falls down by its own gravity) and the properties of which J 

 was already pretty well acquainted with, I fubmitted the whole to digeftlon in muriatic 

 acid, after examining it minutely; when I obferved various apparently metallic leaves in- 

 terfperfed through it, fome of which I have preferved, and, along with the needles, have 

 put into the obje£l: Aiders of a microfcope. 



The muriatic acid did not a£l on the leaf o.therwife than by brightening its appearance, 

 which was before blackifh, and now took the golden hue, that was obferved on that pro- 

 cured by phofphorus, &c. It jvas iiot at all afted on by the magnet ; (the muriatic acid 

 muft have taken up both the iron and lime that is ufually found in indigo) neither does 

 alcohol a£t on the needles, gr leaf; confequemly, they can have nothing in common with 

 rcfinous or bituminous fubftances, neither does cauftic alkali adt on them, though boiling, 

 which precludes the idea of their being a concrete acid ; no combination of iron with 

 phofphorus, or muriate of ammoniac can refemble them ; as fiderite has neither their 

 volatile properties, nor has phofphorus ever been found in indigo ; befides, that its com- 

 bination with iron gives a purple with priiffiates; a, very different Ciffeft from that which 

 the folution of the needles prefent with thofe falts. 



Neither have they any refemblance to martial flowefs, which indeed would be decom- 



* As I 4p pot wifli to take the credit of any thing that ?nay be conftrued to belong to another, I will 

 here mention, that I have read in O'Brien's calico printing fince I made thefe experiments, that " the cu- 

 rious may fublime indigo, and thereby procure flowers, as with zinc, fulphur, &c." Whetlier this gen- 

 tleman ever prpcured the needles I have jnentioned I know not, or what meaniiig he annexes to flowers 

 of Indigo, though I know indigo may be fublimed in the blue ftate by a gentle heat. ' ' 



3 Qw.2 ' pofed 



