'^02 Dr MacCuiioch on Black Lead from Cast-Iron. 
The theory of this experiment appears very plain, and it 
proves, with tolerable certainty, what has been supposed, but 
what has not yet been proved in any other way, namely, that 
plumbago is a metal, and black lead its oxide, if I may be 
allowed to use that term for the present, instead of carbon. 
In white-pig, as already suggested, it is probable that the 
combination is pure plumbago and iron. In the black, the 
colour would lead us to imagine, that there was already an ap- 
proximation to black lead. The operation of the acid, in either 
case, is to dissolve the iron, and to oxygenate the plumbago, so 
as to convert it into black lead. Thus, when in small quantity, 
it is obtained in the form of powder, when very abundant^ in a 
solid state. If the acid is strong, the whole operation is com- 
pleted in the solvent ; otherwise some additional oxygen is re- 
quired to produce in it a state of rest or permanence in the air ; 
and this takes place by a species of combustion, generating the 
heat in the experiment, analogous to that which occurs with the 
alkaline bases. 
Thus, black lead is an oxide of plumbago, or of carbon, if 
we choose to use this term for the presumed element. It is 
scarcely necessary to say that the metallic nature of the base of 
charcoal is proved by the same experiment. Nor need I say 
that iron is not a necessary ingredient in black lead. The best 
kinds, indeed, are those which contain least. 
This experiment, and these conclusions, would be much more 
satisfactory, if we could produce the metal of black lead in its 
separate state. No method of doing that has yet occurred to 
any one; and it will probably be found a very difficult pro- 
blem, as this is evidently a highly combustible substance. But 
chemistry does so much every day that once appeared hopeless, 
that we have no reason to despair. 
If the foregoing reasoning respecting the metallic nature of 
this substance, should be deemed unsatisfactory, the following 
argument may be added. 
The specific gravity of pig-iron is about 7.6, and that of 
black-lead is 2, or less. Now, the bulk of black-lead procured 
in this experiment, is equal to that of the original iron exposed 
to solution. Two such bodies could not co-exist in the sarpe space ; 
tor, if that could be imagined, the specific gravity of such iron 
