It was also exceedingly difficult to burn, requiring a long continued red heat, after which it left an oxide or rather a carbonat of iron, such as remains from the combustion of plumbago. It is in fact to be considered as an artificial plumbago, a substance of whose nature all the charcoals of difficult combustibility partake, deriving their resemblance apparently from the same cause.
The formation of this plumbaginous substance serves to shew a very powerful affinity between iron and carbon, even where the proportions are very different from those which enter into the composition of steel. But to effect this combination, it is necessary that the carbon be in a state of previous union with other substances, and that it be applied to the iron in that state. It will be in vain that we attempt to combine iron with charcoal for this end, unless the charcoal or carbon be in that state of very minute division in which it exists when precipitated by a new affinity from some previous combination.
It is necessary now to account for the iron in this compound.
This distillation of wood for charcoal is carried on in iron vessels, and hence is derived the iron which enters into the composition of the pitch. I will not say that it is solely derived thence, as it is probable that if there were iron contained in the vegetable matter, it would also be found in the same place. When the acetic acid has been separated the iron remains united to the pitch. This fact may shew us, that if in the destructive analysis of vegetable (and probably animal) matter, we trust to find the iron they may contain in the residual matter of the distillation, we may be disappointed, since it may be carried over, together with the substances I have now been describing, in the act of ebullition, as happens in this very case, its tendency being to combine with them, in preference to the charcoal.