210 RKFININO OF LEAD. 



balh conftantly diminifties till all the lead is reduced into li- 

 tharge, and nothing at laft remains but the button of filver at 

 the bottom of the cupel* 

 Other reafons That the whole of the lead is abforbed in the cupel of aflay, 

 ^rocefscannS ^""^^^^ ^^^^ ^^^^ unequal aftion of heat upon every part of the 

 be, made by ab- fmall veflel. As the cupel in the large way prefents only its 

 forption. faperior furface to the action of the heat, the oxide thus im- 



bibed ceafes to penetrate at the place where the temperature is 

 np longer in a ftate to hold the oxiding fulion. For this reafon 

 it is that the whole of the cupel is throughout impregnated to 

 an equal depth, and it is impoflible to caufe all the lead to 

 penetrate the allies* 

 Litharge is From the preceding obfervations it will be eafy to conclude 



fought as a pro- that though the aflay of lead muft be niade in fmall cupels of 



u in t e ^^S^^jQj^g aflies, in order that the whole of the oxided metal may 



way. ■' 



either be abforbed or partly evaporated ; yet the cafe is very 



ditierent in the large operation, where the objedl is to proceed 



with celerity, and to obtain as much litharge as poffible. 



Addition of (and I baVe before ftated that the wood aflies uted in forming 



lo the wood large cupels are expentive, and frequently not to be procured 



cs, c. ^^ fufficient quantity ; to which I have added their being fub- 



je(5t lo blow up or rife entirely, which occaiions a conlider- 



able lofs. It muft further be mentioned, that in order to give 



more weight and confiftence to thefe cupels it is often necelfary. 



to mix a confiderable quantity of (and with them, particularly 



li the lead (hould contain foreign fubftances, fuch as arfenic, 



cobalt, antimony, zinc, tin and other matters. If the lead be 



Uierely-arfenical, after having taken off the firft fcum, it is 



^fual to throw from time to tinje, on the whole furface of the 



bath, about 2()lbs. of iron fcales or granulated crude iron. 



This iron being lighter than the lead, floats on the top and ab- 



forbs the arfenic, after which it is cleared away, and then the 



litharge is formed without any obflacle. This method is ufed 



in Saxony. 



The neceffity of adding fan^l to the aflies of the cupels ought 



to have led to a difcovery which I propofe : it is as follows. 



Neiv ConjlruBion of the Cupels or Bajoiis for refining Lead. 



New conftruc- Without making any change in the mafonry of the furnaces 

 t«on of tii« •**- for refining by what we call the German method, it is only 



|:)n>i or vcUels ^ , ,- , i r rr- • i c 



for refining lead, neceflary to be carciui to make a luttjcient nuuiber oi vents in 



their 



