Ohfervations on Lead Ore . 867 
the heavieft would have weighed 7442 ounces. The 
mean fpecific gravity of the fix piec^ywas 7.342. 
Other lumps of ore, from different mines, were re- 
fpeCtively broken into different pieces, and fcarcely any 
two pieces of the fame lump were obferved to agree in 
their fpecific gravities. This diverfity in the fpecific gra- 
vities of the fever al pieces of the fame lump of ore may 
be owing, either to the different proportions in which 
the conflituent parts of the ore are combined in the fe- 
veral pieces ; or to the different quantities of extraneous 
fubftances imperceptibly mixed with them, or, which 
feems molt probable, to a diverfity in the fize or configu- 
ration of thefe pores. 
But be the caufe of the diverfity in the fpecific gravi- 
ties of different pieces of the fame lump of ore what it 
may, the fa6t, I believe, is certain, and by no means lin- 
gular; for not to mention the varieties obfervable in the 
fpecific gravities of different pieces of roll brimftone, of 
corrofive fublimate, of caff Heel, and other factitious fub- 
fiances, the natural fpars generally found along with 
lead ore are fubjedt to a fimilar diverfity, though not 
perhaps in an equal degree. 
A piece of rhomboidal, otherways called refracting or 
lantern fpar, was broken into four fmaller pieces, the fpe- 
cific gravities of which were 2.675, 2.687, 2.715, 
5 P 2 2.723; 
