J48 Ktto Method efextraRing Silver from 



hearth of the furnace lies partly within and partly without ; but It is not neceffary, that 

 the projeftlng part of its hearth (hould be of conGderable breadth. The aperture (a), by 

 which the exterior part of the furnace communicates with the interior, fliould be placed 

 immediately above the floor or bottom of the hearth, or which is ftill better in that bottom 

 itfeif. And the outer hearth of the furnace fliould have an Incifion with a gutter, through 

 which the melted copper-mat, rifing above the lead flows off continually, and is collefted 

 in a pot (vortiegel) placed on the floor of the fmelting-houfe. There is, befides, on tlie 

 fide of the outward hearth, another adjoining hearth, into which the lead of the former 

 when fufficiently impregnated with filver may be drawn off, to be afterwards put into iron 

 veflels. The moft convenient and advantageous dimenfions of thofe hearths, efpecially the 

 height of the outward-hearth from the floor up to the twyer of the bellows, can indeed be 

 afcertained by experiments only, but thefe may be eaGly made without much expence. 



In order to deprive the copper-mat of its filver by means of lead in this apparatus of the 

 furnace, and to manage the fufion properly, it is required that the proportion of the fpe- 

 cific gravity of the copper-mat to that of the lead fliould be known. For, as this proccfs 

 is grounded on the well known hydroftatical principle, that the heights of two fluids of 

 different kinds, placed in connected tubes, if required to be in aqullibrio, muft be to each 

 other in the inverfe ratio of their fpecific gravities, it is evident, that the melted copper-mat 

 in the furnace, to keep the equilibrium with the fufsd lead in the outward hearth, muft 

 form a column proportionably fo much higher, as it is exceeded by the lead in fpecific 

 gravity. It is therefore from the fpecific gravities of both thefe fubftances, that we may 

 find how far diftant the twyer muft be from the loweft point of the hearth, and to what 

 height the lead may be futTered to rife in the outer hearth, with caufing the fluid copper- 

 mat to enter into that aperture. The fpecific gravity of copper-mat is variable, according 

 to the difl^erent degrees of its roafting. That of fufed copper-mat varies from 4.66 after 

 the fecond roafting, to 5.20 after the third roafting. 



But for the fake of greater fecurity, the leaft weight only fliould in this cafe be afliimed 

 as a ftandard. The fpecific gravity of lead, at a mean rate, is 11.35. 



From thefe data it is obvious, that if the height of the lead in the outward hearth he fix 

 inches, the copper-mat m the interior furnace, to be equiponderant, muft ^znA fourteen 

 inches and one half \\\^. — If then the copper-mat, rifing through the lead in the outward 

 hearth, covers the furface of this laft to the height of \ an inch, its column muft likewife 

 be proportionally higher in the furnace, and confequently reach to 16 inches. It is there- 

 fore evident, that, when the If ad rifes to 6 Inches height in the outward hearth, the dif- 

 tance of the twyer, or nozzle of the bellows, muft amount to at leaft 18 or 20 inches from 

 the loweft point of the hearth. 



'According to the above ftated fpecific gravity of lead, one Rhinland cubic * foot of that 

 metal weighs 7501b. Cologne weight. When therefore 300 pounds of lead are employed 



• The Rhinland foot is reckoned at 12,396 Englifti inches, and the mark of Cologne is 3611 grains, 

 wheoce the pound will be ■jz%^ grain's, or about half an ounce more th.-ui our avoirdupois pound. ---N, 



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