Obfervations on Lead Ore. 865 
dreffings, to feparate all the fpar from an ore, or equal 
portions of it from equal portions of ore. 
There is a diverfity, however, in the weights of equal 
meafures of lead ore, which probably does not arife from 
fparry or other heterogeneous accretions, but from the 
nature of the ore itfelf. I have carefully taken the fpe- 
cific gravities of many of the Derby fhire lead ores ; the 
weight of a cubic foot of the lighted: which I met with 
was 7051 ounces, and the weight of a cubic foot of the 
heavieft was 7786 ounces; the difference amounting to 
between a ninth and a tenth part of the weight of the 
lighteft. There are probably other ores of lead which 
differ more in their fpecific gravities than thefe here 
mentioned ; but the difference between thefe is fufficient 
to fhew the great uncertainty of purchafing lead ore by 
the meafure, fince ten difhes of one fort of ore may not 
weigh more than nine difhes of another fort, though 
both the forts be equally well deeded. 
Lead ore is not always of the fame goodnefs in the 
fame mine, nor even in the fame part of the fame mine ; 
and, what is more remarkable, the different parts of the 
fame lump of ore have different fpecific gravities. I could 
not eafily have believed this, unlefs a variety of experi- 
ments had convinced me of the fadt. 
Vol. LXVIII. 5 P They 
