cnci the Inconvenience of JJh Cupdh. 213 



in a ftatfi of ftagnation, and would neceffiirily oppofe the 

 formation of a new ftratiim of oxide. This has induced me- 

 tallurgifts to expel the litharge by tlie wind of a pair of bel- 

 lows ni proportion as it is formed, and to make it flow from 

 the furnace. 



Oxidation then takes place only at the furface of the mafs, 

 and not at its lower part: if the cafe were otherwile, the 

 aihes of cupells would be penetrated by the oxide to a thick- 

 nefs the more unequal, as the operation is longer ; but I have 

 ahvavs remarked, that the tejl, or the part of the afhes im- 

 -preonated with litharge, in refining on a large fcale, is not 

 thicker in the centre of the bafon than towards its circum- 

 ference, though the lead remains thirty or forty times as long 

 in the bottom as on the edges; fince the mafs contmuallv 

 <iccreafes till the whole lead is reduced to litharge, and till 

 nothing remains but a cake of filver at the bottom of the 



<-"upelI. . n ■ • 



If the whole lead is imbibed by the afTaying cupell, it 13 

 ^H'caufc this fmall veilel is expofed to a heat uniform in all 

 its parts. As the cupcll, in operations on a large Icale, pre- 

 fcnis to the a6tion of the caloric only its upper furface, the 

 oxide iml)ibed ccafes to penetrate it at the place where the 

 temperature is no longer high enough to keep that oxide in 

 a Ibte oftufion. For this reafon, the thicknufs impregnated 

 is equal throughout the whole extent of the cupell ; and this 

 prevents the poflibility of making the whole lead penetrate 

 into the afiies^ 



From the above obfer\'ations it may eafily be conceived, 

 that if the alTaying of lead ought to be performed in fmall 

 cupells of bone-'afhes, in order that the oxidated metal may 

 I)enetrate them, and be in part evaporated, the cafe is dif- 

 ferent with refiriinnr on a large fcale, where it is iiecefTary to 

 accelerate the operation, and to obtain as much litharge as 



pofTible, r 1 • I 



1 have alreadv faid, that the wood-a(hcs of which cupells 

 are made for refinins: on a large fcale are expenflve; that 

 wry often a fufficient'quantitv cannot be procured ; and that 

 bcfides tliis,part of the allies', and lometimes the whole, fe- 

 mrate thenifclvrs entirely from ihefole; which occafion a 

 confidcrable lofs. I Ihall' add, that to give more weight and 

 confiflcnce to the cuinlls, it is often neceilary to mix with 

 the alhes a lartre quantity of fand ; cfpecially il the lead con- 

 tains forei.Mi i'ubliaiices, hich as arfenic, cobalt, antimony, 

 tin, ?cc. ff the lead be only arftnical, after having ieparaled 

 the firllfcum, a quarter of a hundred wciglitof iion lilirigs or 

 caft iron turnin«is arc now and then thrown over the whole 

 Q T furlacc 



