174 Account of the Blach-lead Mine, 
published: but the most extraordinary fact 
is, that the method should remain so long 
undiscovered. Great improvement has also 
been. made in the composition pencils ; these 
are made of the saw dust, and of pieces too 
small, for use in the common way; which, 
being ground to an impalpable powder, is 
mixed with some adhesive substance, and the 
mixture is sometimes in a soft state forced into 
the grove of the pencil: but a better kind, 
being made up with a cement of amore resinous 
nature, is formed in a mould, into square 
blocks, consolidated by a strong pressure, 
and then treated in the same manner as solid 
black lead: and this method is now brought 
to such a degree of perfection, that the pen- 
cils answer for some purposes as well as those 
made from the best black-lead, and can be 
afforded much cheaper; they can scarcely be 
distinguished by cutting, but by holding the 
point to the flame of a candle they are easily 
detected. 
The specific gravity of the best wad, is to 
that of water as two to one nearly ; the coar- 
ser is heavier as it contains more stony matter ; 
black-lead is incapable of fusion, and was for- 
merly thought to be incombustible: but it is 
found tobe a carburet of iron, in the proportion 
of about nine parts carbon to one of iron; and 
