LEAD. 
547 
vinegar. The pots are then covered with a thicker 
plate of lead and disposed side by side, sometimes 
to the number of fifteen thousand, in a bed of dung. 
As soon as the heat is sufficient to volatilize the 
vinegar, it attacks the metal, and in about four or 
five weeks the plates are completely covered with a 
coat of white lead. This substance is then detached 
from them by passing the plates between channelled 
rollers under water; a very proper precaution, to 
prevent the noxious exhalations which would other- 
wise arise from the lead to the destruction of the 
workmen. In England this process is conducted 
in a more cleanly manner, tan being used instead of 
dung. 
Red lead is another preparation of this metal, of 
equal importance with that we have just described. 
I11 Derbyshire there are several mills and furnaces 
employed solely for the purpose of making red lead. 
The furnaces resemble a baker’s oven, having a low 
vaulted roof and two party walls, which leave a 
middle space where the pit-coal is burned. The 
flame, being drawn over the party walls, strikes on 
the roof, and thus produces a reflected heat suf- 
ficient to melt the lead on the floor of the furnace. 
The surface of the melted metal being exposed to 
the air becomes instantly covered with a dusky pel- 
licle, which is removed as soon as formed, till most 
of the lead is reduced to a yellowish green powder. 
This powder is afterwards ground in a mill, and 
then being washed and passed through a sieve in 
order to separate the particles of lead which remain, 
2 n 2 
