HfiFlNlNG OF tEAD* 207 



pofed (o fire with the contaa of atmofpheric air, while the fil- 

 ver it may contain, preferves its metallic form ; it only re- 

 mained for them to contrive a method of feparating theie two 

 metals. They were led to this method byobferviBg that the 

 oxide of lead, in its ftate of liquefadion eafily penetrates the 

 iubftances with which it may be in contact, particular bone 

 aflies, without deftroying the figure of the veflel made of that 

 material. In fa6i there is no fubftance whatever which is 

 better adapted to form the fmall cupels for aflaying. 



The difficulty, and often the impoffibility of procuring f^^** ^"»^ »<* 

 I r t r, ^ r, r, ■ A r > ^- bone aflies the 



three or tour bumels of bone alncs in Germany, tor each time afl,eg ^f ^^^ 

 of refining, has led to the adoption of the athes of wood. But are ufedfor c«- 

 not to mention that thefe are of conliderable price, and not ^^^^^ ii^^. 

 always to be had, they prefent another inconvenience, by encca. 

 often riftng and floating on the furface of the lead. When this 

 happens, the jyrocefs rauft fail; and it does happen as often 

 as the aflies are ill prepared, or the cupel not fufhciently or 

 irregularly beaten, or that the openings lefi for the evaporation 

 of the humidity are not properly difpofed, or enough in num- 

 ber, or clofed by a portion of the fcoriae upon which the floor 

 is made to receive the athes. This floor ought to be conflni^ted 

 of the mofl porous bricks, in order that the water with whiclj 

 the aflies mud be wetted may penetrate them and evaporate 

 into the bed of fcoria, and efcape through the opening at the 

 bottom of the furnace. 



The elailicjty of this aqiieous vapour frequently caufes ex- 

 plofions, which not only diforder the cupel, but even the ma- 

 fonry of the furnace, if it be not properly conflrudied« 



In order to afcertain the proportion of filver in any quantity Cupelling rf 

 of lead, it is only neceflary to pafs a few pennyweights into a ^ " 

 fmall cupel of bone aflies placed under the muffle of an aflay- 

 er's furnace; as the lead becomes oxided it is imbibed in tiie 

 cupel, and at length the phenomenon of brightening takes 

 place upon the metallic button. This appearance fiiews thait 

 all the lead is diflipated, and the remaining Hlver in a pure 

 flate. 



In the large way of refining, the fame objed of feparating Refining la rfw 

 the filver from the lead is aimed at, but the lead is not in- ^'^''^^ ^«y ^f ' 

 tended to penetrate into the cupel, which in fact is impratli- the adyiog ;pa»- 

 cable. For the total abforption of this melal would require a ^^^^ whereta ickie 

 much greater quantity of aflies, with the confumplion of a ten 

 5 foJd 



