Dr MacCuiloch on Black Lead from Cast-Iron, SOI 
The astonishment of the Highlanders on finding guns still 
hot, after having been more than a century under water, may 
easily be imagined ; and it is not surprising that the story was 
not believed, and that not being believed, it was forgotten. 
This may afford us an useful hint on the subject of physical 
incredulity : since, assuredly a fact thus nakedly stated, with- 
out a knowledge of the explanation here given, must have been 
pronounced impossible by every one, chemist or not. I escaped 
this, however, as the circumstances above mentioned had put 
me in possession of the solution before the tale, which proved a 
very agreeable and unexpected confirmation of my own experi- 
ments. 
I may now state the general result of the experiments. The 
blackest pig-metal appears to yield the greatest quantity of 
black lead, and in the most solid state. When the experiment 
is complete, the produce equals the iron in bulk, and is a solid 
mass, capable of being cut by a knife, even into pencils ; but, 
as far as I have ever observed, it is of a much more coarse 
grain, or scaly granular texture, than any natural black lead 
that has occurred to me. 
To procure it in perfection, the acid should be very weak, 
and the operation is then necessarily very tedious. Acetous 
acid appears to be the best, and it is by this that it is produced 
in porter-backs, in the waste-pipes of breweries, and in calico- 
printing-houses, where sour paste is employed. The process 
by water is insufferably tedious. Very dilute mineral acids also 
succeed ; but, with these, one of the results is sometimes not 
obtained. 
If the experiment is perfect, the black lead becomes hot on 
exposure to air, smoking while there is any moisture to be eva- 
porated, particularly when the surfaces are scraped oflF in suc- 
cession, so as to give access to the air. Oxygen is absorbed in 
this case ; yet, as far as I have perceived, the eye cannot de- 
tect any difference in the appearance of the black lead before 
and after this operation. In those instances where the substance 
does not heat, on being taken out of the fluid, it appears to 
arise from the whole process of oxygenation having been per- 
formed in the solution, and probably from an excess of strength 
in the acid. 
