mineralogy. i55 
lead could be melted from it by 
means of the blowpipe, as can be 
done opt of other lead fpars ^ but 
it muft be performed in a crucible^ 
and then that part of the arfenic 
which did not fly off in fmoke^ 
during the experiment, was like- 
wife reduced, and found in form of 
grains difperfed, and forced into the 
lead* Another ore of this kind, 
which likewife was not eafily re- 
duced by means of the blowpipe, 
did always after being melted, and 
during the cooling, hafl:ily flioot 
into polygonal, but moftly hexago- 
nal criftals, with Ihining furfaces. 
Can this criftallifation be owing to 
falts, which are faid not to in 
this manner, but when they are dif- 
folved in water ? 
X With a calcareous earth See 
XXX vii. 
* The abovementioned lead ores are very rich in lead, and 
fafy to be tried i becaufe moil of them, being llowly heated, 
may be reduced to lead by means of the blowpipe on a piece 
of charcoal. The calx of the lead in thefeores has, perhaps, 
iirft been diffolved by fulphur and arfenic, and has afterwards, 
when thefe two have weathered away or decayed^ and parted 
from it, affumed this form ; in the fame manner as we fee it 
really happens during the calcinadon, with rich lead ores, or 
fuch regiiles as contain lead. The fame, very likely, is the 
cafe ^ith other m,etals ; for which reafon their ores, when they 
occur in form of a calx, often contain a little fulphur, and 
more efpecially arfenic. 
SECT, 
