478 Experimenis on Indigo. 



,-it,W3& ill confequencejof a converfation with an eminent chemift here, that I was le^ 

 to make the followhig experiments on indigo ; and I beg leave to take this opportunity of 

 making my acknowledgements to him, for the trouble he has taken In repeating nioft of 

 them, and adding others, which will be particularly noticed here^ and which very much 

 illuftrate the general opinion I have formed of the changes that indigo undergoes in 

 affuming the blue colour. 



That gentleman, coftfidering Carbon as a ccmpounrf, froth-fome tnalagous reafoning, had 

 been led to think indigo the bafe of that fubftance, and which, by its union with oxigen, 

 formed charcoal, and fome accounts given of the mode of procuring indigo, ftrongly fa- 

 vored this fuppofition ; I, on the contrary, had conceived an opinion, that black was owing 

 to an abfence of 'oxigen ; and that purple and blue held the next degree in oxigenation, of 

 Which we have many examples ih the mineral kingdom. 



I therefore fought, by means of inflammable bodies, to divert indigo of that portion of 

 oxigen I fuppofed it to retain, and to difcover, if poffible, its bafe. My firft experiment 

 was more aftonifliing than fatisfaclory ; I had mixed a quantity of indigo with fulphur of 

 pot-afh, phofphorus and water, and was boiling them in a glafs tube by the flame of a 

 candle, when I was furprifed with the appearance of a leaf of a metallic afpe£t, (in fome 

 places of a golden, and in others of a filvery hue) precipitated on the fides of the tube ; 

 (I mud here obferve that there feemed no appearances of folution j that the indigo never 

 came to the green, and that the golden leaf took firft a purple appearance, and aflumed 

 the metallic one immediately, when the phofphorus came ofi^ in white vapour) this leaf I 

 long confidcred as a compound of phofphorus, or at leaft the phofphoric acid, or of fome 

 change wrought on them-; I afterwards 'procured the fame fubftance in needles, by heat- 

 ing the fame meters, a little moiftened with water in a crucible, and by phofphorus and 

 Indigo only, which confirmed me in the above opinion, till I found that the needles could 

 be obtained by means of a lens concentrating the rays of the fun on indigo, under ajar 

 of atmofphcrical air ; it however ftiould be noted that the lens was of a moderate fize, ■ 

 and nttt powerful enough to inflame the indigo cfimpletely, I have fince I think got jfuf- 

 ficient data to prove, that thtfe needles are a fublimation of a fubftance fui generis, and 

 that they are probably difperfed in a ftate of combination through the whole animal and 

 vegetable world, and in fuch foflile bodies as have once been organized. 



I will here divide this account into three parts; I will firft take notice of the properties 

 of the needles, and leaf, then detail a fingular one which I fuppofe to belong to the refi- 

 nous matter, and finally obferve the adlion of certain bodies on indigo in fubftance, 

 chiefly With a view to fhbw, that irfdigo acquires a green 'colour by abforbing oxigen, not 

 by giving it out, according to Berthoilet, Bancroft, and others ; and that in HaulTeman's 

 famous experiments, an acid was moft probably prefent to abforb the alkali, which was 

 the true reafon of the indigo's being precipitated. 



Needles apparently metallic are to be obtained by fublimating indigo, either with phof- 

 phorus and fulphure of pot-afh, with phofphorus alone, in fonie«al"e6 from indigo per fe, 



: ' (if 



