199 
Dr MacCulloch on Black' J^jad from CasUlrmL 
to the malleable state than pig-metal, or from its greater purity. 
The best malleable iron contains none, and a great part of the 
process of refining iron consists in the combustion of this sub- 
stance ; whence in some degree the' loss of weight. 
During these trials I was led to repeat the same experi- 
ments on the pig-metal used for shells, which is of various qua- 
lities, and is generally distinguished into three kinds, white, 
grey, and black. All these afforded very large proportions of 
black lead after solution, but still various in different specimens. 
The black metal seemed to afford the greatest quantity, as 
might have been expected from the colour ; and yet that is no 
criterion, as the most brilliant silvery metal also afforded it in 
abundance, where, if colour had been a necessary indication of 
its presence, it would not have existed. I think it will appear 
that the plumbago is present in two different states' in iron. It 
will hereafter be shewn, that it is sometimes in the state of a 
metal, and this I conceive to be its form in the white pig. At 
the other extreme it is found in the state of black lead, as in 
the gun-metal just mentioned. But the experiments hereafter 
to be described will shew, that there is some intermediate state 
(or states) between the two ; and this appears to be its condi- 
tion in the grey and black pig. 
I must now observe, that, in these experiments on pig-iron, 
the black lead was always found in the state of powder, or at 
least in such a state as to fall into powder easily. This must 
have arisen from using sulphuric acid, as the cheapest, and from 
using it too strong. Thus the matter rested. In 1807 or 
1808, 1 was requested to go to a London porter brewery, to see 
an appearance which had very much surprised the people em- 
ployed in it, and which had certainly a very marvellous sound 
in the narration. The workmen had taken some iron out of 
their porter backs in making some repairs, and had found it 
red hot, as they said, to their infinite astonishment. On closer 
examination, the red heat was found to be an exaggeration. 
But, on removing the iron articles in question from the porter, 
they became so hot on scraping off the surface that it was dis- 
agreeable to handle them ; while they smoked, from the evapo- 
ration of the moisture. 
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