in the Pyrites. 173 



orange colour. Yet, the kind of the fire and fur- 

 nace, where the ore comes to be treated, is, in this 

 cafe, certainly, of great confequence, not only as 

 to the production of the feveral colours of the 

 arfenic-meal (the footy matter either burning away 

 and difiipating, or remaining therewith ; the ful- 

 phur either foftly retiring apart, or being hurried 

 or laid hold of by the arfenic) but alfo as to the 

 corporal form, in which it appears : for inftance, 

 the poifon-pyrites yields, indeed, in the open air, a 

 mealy, commonly a grey powdery arfenic •, but in 

 clofe veffels appears of a flakey, reguline form. 

 Thus, it is one thing to drhe or force it off in 

 retorts, fubliming veffels, and the like, where the 

 ore is out of the contact of the fire, and where the 

 arfenic acquires its proper reguline form ; another 

 thing, to force it off in roafting furnaces, as is done 

 at the cobald and tin huts, on occafion of roafting 

 the cobald and tin ores, where, by means of the 

 adjoining wind- furnace, the actual flame, and fat- 

 ty particles of the fire, play over and through the 

 ore. The cafe proves yet different . in fmelting 

 furnaces, where not only the flame of the coals, 

 but alfo the fatty, and at laft the alcalifed coals 

 themfelves, exert their efficacy on the ore^ be- 

 fides, the force of the blaft volatilifing and carry- 

 ing off along with the arfenic, not only crude earthy 

 particles out of the ore, brought to the highefi 

 degree of activity by the fcrong glow, but alfd v |»;, 

 metallic particles themfelves, as coppery, leady, 

 and filvery. Laftly, there is a further difference 

 in open roafling hearths, where the ore is not only 

 expofed to the immediate contact of the flame and 

 coals, but to the actual influx of the air, weather, 

 and rain ; whence we are enabled to obferve in 

 fume-powders, that variety of colours, as, white, 

 grey, and yellow, which, under covered and clofe 

 furnaces, does not fo eafily happen, 



Now 



