on the Terra Ponderofa, &c. 307 
Profeflor BERGMAN, in order to obtain the earth from the 
terra ponderofa vitriolata, directs the latter to be roafted with 
fixed alkaly, and the duft of charcoal; but I have always done 
it by charcoal duft alone, though probably this method may 
require a greater degree. of heat. 
It has been remarked, that terra ponderofa and calx of lead 
refemble each other in many refpeéts; and I muft add, that the 
vitriol of lead diffolves in the highly concentrated vitriolic acid 
much in the fame manner that the marmor metallicum does, 
and like this too feparates upon the affufion of water; but I” 
never obferved it difpofed to cryftallize. 
The marmor metallicum may probably be ufeful in fome 
cafes where a powerful flux is wanted; for I once mixed fome 
of it with the black flux, and expofing it toa pretty fharp heat, 
it entirely ran through the crucible. May not, therefore, fome 
of the more common varieties of it be ufed advantageoully as 
a flux to fome of the more refractory metallic ores ? 
Sh we oh ol OeN oa 
Terra ponderofa Vitriolata. 
Variety, Calk or Cauk. 
Marmor Metallicum, Cronstept Min. § 18. B? 
Plentiful in the Mines in Derbythire. 
More obvious Properties. 
Of a white or reddifh colour; cryftallized in rhombotdal 
jaminz, but thefe very much intermixed and confufed. Lofes 
a {2 little 
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