Lead 1 
247 
the oxydation, but also for driving the litharge towards 
the gutter formed for its escape. 
We have remarked the inconveniences and even the 
impossibility of making the whole lead penetrate into the 
ashes of large cupells : oxydation, indeed, is not effected 
but in the parts of the mass exposed to the contact 
of the air or to the wind of the bellows ; but as !i- 
tharge, towards the middle of the bason, could not reach 
its edges, it would remain there in a state of stagnation, 
and would necessarily oppose the formation of a new stra- 
tum of oxyd. This has induced metallurgists to expel 
the litharge by the wind of a pair of bellows in proportion 
as it is formed, and to make it flow from the furnace. 
Oxydation then take place only at the surface of the 
mass, and not at its lower part : if the case were other- 
wise, the ashes of cupells would be penetrated by the oxyd 
to a thickness the more unequal, as the operation is long- 
er ; but I have always remarked, that the test , or the part 
of the ashes impregnated with litharge, in refining on a 
large scale, is not thicker in the centre of the bason than 
towards its circumference, though the lead remains thirty 
or forty times as long in the bottom as on the edges ; 
since the mass continually decreases till the whole lead 
is reduced to litharge, and till nothing remains but a cake 
of silver at the bottom of the cupell. 
If the whole lead is imbibed by the assaying cupell, it 
is because this small vessel is exposed to a heat uniform 
in all its parts. As the cupell, in operations on a large 
scale, presents to the action of the caloric only its upper 
surface, the oxyd imbibed ceases to penetrate it at the 
place where the temperature is no longer high enough to 
keep that oxyd in a state of fusion. For this reason, the 
thickness impregnated is equal throughout the whole ex- 
tent of the cupell ; and this prevents the possibility of 
making the whole lead penetrate into the ashes* 
