Dr MacCulloch on Black Lead from Cast-iron. 7 



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 specific 

 gravity, while it becomes hard and gives a pale streak. This is 

 the process used 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 it 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- 



