IN A LEAD MINE. MINERS ABOUT TO ASCEND. FROM A TRANSPARENCY IN THE DIVISION OF MINERAL TECHNOLOGY, UNITED STATES NATIONAL MUSEUM. THE CONCENTRATION OF LEAD ORES. SEPARATING THE LEAD MINERALS FROM THEIR NATURAL ASSOCIATES. FROM A TRANS- Smithsonian Report, 1921.-Mitman. PLATE 4. THE SMELTING OF LEAD ORES. THE SCOTCH HEARTH, WHERE THE LEAD IS EXTRACTED FROM THE ORE. FROM A TRANSPARENCY |