LEAD, a metallic chemical element known to the ancients, and mentioned in the Old Testament. (Symbol Pb, atomic num ber 82, atomic weight 207.2, chief isotopes 206, 207, 208.) The Romans used it largely, as it is still used, for the making of water pipes, and soldered these with an alloy of lead and tin. Pliny treats of these two metals as plumbum nigrurn and plum bum album respectively, which seems to show that at his time they were looked upon as being only two varieties of the same species. In regard to the ancients' knowledge of lead compounds, we may state that the substance described by Dioscorides as poXv136aLva was undoubtedly litharge, that Pliny uses the word minium in its present sense of red lead, and that white lead was well known to Geber in the 8th century. The alchemists desig nated it by the sign of Saturn T.
The mineral galena is found in many parts of the world and some lead is produced in nearly every country. Galena occurs in ore bodies of many different kinds varying in genesis, size, richness and general characteristics. Galena sometimes occurs in genuine fissure veins where the crystallized mineral has been deposited from aqueous solutions in veins or fractures in the earth's surface. These vein or fissure deposits vary in size from a fraction of an inch to many feet in thickness, and in richness from a trace of lead to practically pure galena mineral. Galena also occurs in very large deposits, disseminated through horizontal beds of limestone in the form of small galena crystals.
The surface of the earth contains permanent ground water at some depth which in some places is practically at the surface and in other places, such as the great deserts of the world, may be many feet below the surface. The portions of the galena ore
bodies which lie below the permanent ground water level, retain their original form and the pure unaltered galena mineral is found in deposits of this kind. Portions of the ore bodies above the permanent ground water level are affected by the oxidizing action of the air and percolating surface water containing air and in these upper portions of the ore deposits the galena becomes oxidized and forms carbonate of lead and sulphate of lead. Generally the localities in which the large deposits of lead ore are found have permanent ground water level not far below the surface and for this reason the amount of altered galena found in the oxidized surface portions of the ore bodies is very small as compared to the very large amount of pure unaltered galena in the undecomposed ore bodies below the permanent ground water level.
The commercially important deposits of lead ore from which the world's production of lead is obtained occur in many dif ferent geologic formations, and most of the deposits extend into the earth to great depths so that their recovery requires deep min ing operations. In some lead mines the ore recovered is rich enough to be smelted directly into metallic lead but the ore from most of the mines contains so much closely associated waste min eral that a preliminary milling treatment is required to remove mechanically the waste rock and concentrate the galena. Miner als containing some other valuable metals such as copper, zinc, gold and silver are frequently associated with galena and are separated and recovered in the treatment of the ore.