Lead. 
201 
gins to combine with oxygen. The oxyd of lead, thus 
formed, combines with the earthy matter, which it ren- 
ders so fusible as to become liquid upon the sulphur of 
the melted lead, and defends it from the future action of 
the oxygen. At this stage of the process the fire is raised 
to separate as quickly as possible the melted lead from the 
liquid scoriae. The latter is now let off at the upper tap- 
hole, leaving a small portion still upon the lead to pre- 
serve it from the air. The fire at this period is lowered, 
and a quantity of coal-slack thrown in upon the melted 
mass. This serves as well to facilitate the cooling, and 
to cause the reduction of some oxyd of lead, which also 
tends to stiffen the melted scoriae. This last effect, how- 
ever, is not produced sufficiently, till a quantity of pon- 
dered lime is thrown into the furnace. By this treatment 
the remaining scoriae becomes concrete, and is then bro- 
ken to pieces and pushed to the opposite side by means qf 
a rake, and taken out of the furnace at the different open- 
ings on the same side. The liquid lead is now let out, 
at its proper aperture, into a large iron pan, or cistern, 
from whence it is laded into moulds to cast into pigs. 
The furnace is new ready to be charged again. When 
the ore abounds with much impurity, the oxyd of lead is 
not sufficient to give the proper degree of liquidity to the 
scoriae. In this case a certain quantity of fiuat of lime is 
added, which has the property of forming a very fusible 
compound with sulphat of barytes, an ingredient very 
common in the ore. 
This flux has been used from time immemorial for the 
same purpose, and has no doubt derived its name from its 
properties as a flux. 
The concrete scoriae, which is taken out of the furnace, 
is found to contain some lead, independent of that in the 
state of oxyd, and chemically combined. This is gene- 
rally lodged in the cavities of the spongy mass. These 
Voi*. III. C c 
