the Nature of fome Mineral Suhjlances . ' 609 
four grains;, but if acid of fait, or a folution of fea-falt, 
be added to the walhmgs of this precipitate, the remain- 
der of the diver is preciptated, and forms a luna cornea. 
This fhews, that there is a difference between tartar of 
vitriol and fal polychrejl , notwith Handing what chemilts 
think to the contrary ; and indeed, upon trial, I found the 
fal polychrejl to contain a fmall portion of liver of fulphur. 
Silver dipped into a warm folution of this fait becomes 
mftantly black; thus the fal polychrejl may be readily dif- 
tinguifhed from tartar of vitriol, which does not tarnifli 
iilver. 
The precipitate of filver, by tartar of vitriol put on a 
red-hot iron, melts and grows liquid like luna cornea. 
The precipitate of filver by fal polychrejl , treated in the 
fame manner, does not grow fo liquid, but boils up and 
at laft dries. 
[c] Half an ounce of lead, dilfolved in aqua fortis , and 
precipitated by a folution of tartar of vitriol, edulcorated 
and dried, weighed five drams and a half.. 
[d ] The like quantity of lead, dilfolved as at [c], pre- 
cipitated with a folution of fea-falt, edulcorated and dried,, 
weighed only half an ounce and eighteen grains. A lo- 
lution of tartar of vitriol, added to the walkings of this 
precipitate, caufed a frefh precipitation, which, after 
edulcoration and exliccation, weighed forty-two grains. 
Hence we fee, that lead united to acid of fait is foluble in. 
water, and on that account fo fmall a quantity of preci- 
pitate was obtained ; but the tartar of vitriol precipitating 
its walkings (for the acid of vitriol does the fame) fhews, 
that 
