196 
Lead 1 
this application of the pyrites might, probably, be more 
lucrative than the present one— making green vitriol, 
A third circumstance, which requires the utmost care 
of the lead smelter, is the leaving as little lead as possible 
in the slag. Near every smelting- house there are thousands 
of tons of slag, which when properly assayed, are found 
to yield from one-eighth to one-tenth of their weight of 
lead ; though no person has yet discovered a method of 
extracting so much from them when smelted in large 
quantities 5 and indeed the smelters are so little able to 
obtain all the lead contained in them, that in many places 
they never attempt to extract any part of it : in some 
places where they do attempt it, I have known the pro- 
prietor of the slag allow the smelters 20 s. for every pig of 
lead they procured of the value of 38s. besides furnishing 
them with fuel •; and yet the men employed in such an un- 
wholesome business, seldom made above 7s, a week of 
their labour. This fusion of the slag of a cupola furnace 
is made, as has been mentioned, at a hearth furnace ; the 
coal cinder, which they use as fuel, and the slag, are soon 
melted by the strong blast of the bellows into a black 
mass, which, when the fire is very strong, becomes a per- 
fect glass ; this black mass, even in its most liquid state, 
is very tenacious, and hinders many of the particles of lead 
from subsiding, and it being from time to time removed 
from the furnace, a considerable quantity of lead is left in 
it, and thereby lost, A principal part of the lead contain- 
ed in the slag of the cupola furnace, is not, I apprehend, 
in the form of a metal, but in the form of a litharge or 
calcined lead : a portion of the lead, in being smelted 
from its ore, is calcined by the violence of the fire ; this 
calcined lead is not only very vitrifiable of itself, but it 
helps to vitrify the spar which is mixed with the ore, and 
thus constitutes the liquid scoriae ; might it not be useful 
to throw a quantity of charcoal dust upon the liquid scoriae 
