6 Dr MacCulloch on Black Leadjrom 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 same space ; 

 or, if that could be imagined, the specific gravity of such iror 



