Dr MacCulloch on Black Lead from Cast-Iron, 'S03 
must be far more different from that of pure iron than it is. If 
pure iron, indeed, is freed from the effects of condensation by 
heating, it scarcely differs in specific gravity from pig-metal. 
Thus, while we conclude that the plumbago combined with the 
iron is a metal, we may also infer, that the specific gravity of 
that metal is not very different from that of iron. 
A problem has long been held out, namely, that of making 
black-lead by an artificial process. This may be considered as 
now solved in a certain way. I attempted to improve its quality 
by heating, in the usual manner, but with indifferent success. 
It may be procured, however, so compact, as to cut into pencils, 
as formerly observed ; but I never could free it from the brown 
colour which it gives to paper, arising from some rust which it re- 
tains. Whether this could be effected or not, seems a matter of 
indifference, as far as economical purposes are concerned ; since it 
could not be produced so cheap in this manner, as to compete in 
the market with mineral black-lead. 
I may conclude this communication with remarking, that the 
effects of heat on black-lead, as well as on charcoal, are, in some 
measure, explained by the views here held out. If soft black- 
lead, which yields a black as well as a soft streak, be heated 
without burning, it diminishes in bulk and increases in sjfecific 
gravity, while it becomes hard and gives a pale streak. This is 
the process us^d in making hard and pale pencils ; and it ap- 
pears to consist in the loss of a portion of oxygen, or in the re- 
duction of the black-lead, to a state somewhat nearer to that of 
metallic plumbago. Thus, in drawing, artists harden the points 
of their soft pencils in the flame of a candle. 
Something very similar to this happens in charcoal. After 
giving out hydrogen ^t becomes harder and more brilliant ; so 
as, from only scratching the softer metals, to become capable of 
corroding glass. In this state it loses one at least of its valuable 
qualities, so as to be no longer fit for making gunpowder. Na- 
turally, some woods produce hard charcoal, and others a soft 
kind ; and the latter alone are fit for this purpose. But the softest 
may be injured by over heating, or they can be reduced to this 
hard state ; so that the wood of the willow or alder may become 
as bad as that of the oak. This is an accident far more likely 
to happen in the method of charring in retorts, than in the com- 
