581 A SYSTEM OF 
1 
timany to advantage i and the antimony 
likewife in a great meafure hinders the 
extraSing of the filver. 
SECT* CXCL 
Observations on Leap Ores. 
I know of no native lead •, and all which has 
been faid on that fubjed is liable to remarkable 
reftriftions* 
Such of the potters ores as do not contain any 
lilver are very fcarce •, yet they are often found 
fo poor in filver, that it does not anfwer the ex- 
pences of extrafling^ Thefe, when they are free 
fropn mixtures of the rock, are, without any pre- 
vious fufion, employed to glaze earthen -ware ; 
and a great trade is carried on in the Mediterra- 
nean with fuch ores, from the lead-mines of Sar- 
dinia and France, 
SECT. CXCIL 
Copper, Cuprum^ Vmus^ As. 
This metal is. 
Of a red colour. 
h. The fpecific gravity of the Japan copper 
is 9000, and of the Swedifh 8784 or 8843, 
to 1000. 
€. It is pretty foft and tough. 
i. The calx of copper being diffolved by 
acids becomes green, and by alcalies blue. 
It is eafily calcined in the fire into a 
blackilh blue fubftance, which, when 
rubbed to a fine powder, is red *, when 
melted together with glafs, it tinges it firft 
reddifli 
