137 



parting with its sulphur, it cannot endure both heat and the 

 current without desulphuration. The lead, therefore, sepa- 

 rates in the metallic state ; any foreign matter which the ore 

 contained melts along with the lead, and swims upon its sur- 

 face. This matter, called scoria, or slag, would run off with 

 the lead when the furnace is tapped, but for a process of 

 coagulation or thickening, which it is made to undergo by 

 sprinkling lime upon its surface. The slag is finally removed, 

 and melted with more lime in another furnace, called the slag- 

 hearth, urged by bellows, and then it affords an additional 

 quantity of lead. The first run is called soft, or ore lead ; the 

 second is hard, or slag lead, and bears a somewhat lower 

 price. 



*' The blast-hearth is a small furnace, constructed of a 

 few blocks of cast iron placed upon a bed of masonry, in such 

 a manner as to include a square shallow well, in which is con- 

 tained the burning fuel, consisting, according to circum- 

 stances, of wood, charcoal, common coal, coke, or turf, or all 

 of these. A double bellows, of considerable size, worked by 

 a water-wheel, or by manual labour, assisted by a heavy 

 swinging pendulum, is made to blow a stream of air towards 

 the centre of the fire, and being there obstructed by a burning 

 sod of turf, placed for that purpose, the air is driven in all di- 

 rections through the fuel ; and thus is established an equal 

 heat, as well as an equal blast, to carry off the sulphureous 

 vapours through the chimney which surmounts the hearth. 

 Lime, which ought to be very small in quantity, is occasion- 

 ally thrown on to coagulate the slag ; the melted lead trickles 

 down to the well, which soon fills, and which ought to be 

 allowed to remain full. New portions of lead will cause the 

 well to overflow, and the melted metal will run down a gutter 

 made in an attached inclined plane, called the apron, and 

 thence into an iron pot placed beneath. The fire, after the 

 charge has been smelted, is drawn out on the apron ; the slags 

 are picked out as soon as visible, and the fire is returned to its 



VOL. IV. M 



