LEAD 



63 



small railways, by which, when one mass has been removed, its place may readily bo 

 supplied by an empty waggon. When nearly cold, the casings of the waggons are 

 turned over, and the blocts of slag easily made to drop out. In addition to the 

 facility for transport obtained in this way, one of the great advantages obtained by 

 this method of manipulation arises from the circumstance that should the furnaces at 

 any time run lead or matt, without its being detected by tire smelter, the whole of it 

 will be collected at the bottom of the block, from which, when cold, it may be readily 

 detached. 



In working these furnaces care must bo taken to prevent flame from appearing at 

 the tunnel-head, since, provided the slags are sufficiently liquid, the cooler the appa- 

 ratus is kept the less will be the loss of metal through volatilisation. In addition 

 to the greatest attention being paid to the working of the furnace, it is necessary, in 

 order to obtain the best results, that all establishments in which this apparatus is 

 employed should bo provided with long and capacious flues, in which the condensation 

 of the fumes takes place, previous to arriving at the chimney-shaft. These flues 

 should be built at least 8 feet in width, and 6 feet in height, so as readily to admit 

 of being cleaned, and are often made of several thousand yards in length. The value 

 of the fumes so condensed, amounts to many hundreds, and in some instances thousands 

 per annum. 



In order to be advantageously worked in these furnaces, the ores should be first 

 roasted, and subsequently agglomerated into masses, which, after being broken into 

 fragments of about the size of the fist, and mixed with the various fluxes, are charged 

 as before described. 



In an establishment in which the average assay produce of the roasted ore for lead 

 is 42|ths, the furnace yield is 38ff ths, and the weight of coke employed to effect the 

 reduction 22 per cent, of the roasted ore operated on. The mixture charged into the 



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furnace, in this instance, is composed of 100 parts of roasted ore, 42 parts of slags 

 from a previous operation, 8 parts of scrap iron, and 7 parts of limestone. Each 



