198- Dr MacCulloc.h on Blade Lead from Cast-Iron. 
hole, I believe,) and it was found to be so exceedingly tough, 
that none of our men were able to break off a trunnion, as is 
the usual practice in these cases, when three or four blows of 
the sledge were commonly found sufficient for this purpose. I 
was particularly induced, therefore, to examine this specimen, 
and was surprised to find that it not only contained a most un- 
usual proportion of plumbago, but that this was in what I may 
call a disengaged state, for want of a better term. The plum- 
bago was not only visible on breaking the metal, giving it the 
appearance of having been rubbed with powdered black lead, 
but the iron was capable of leaving its trace on paper. I have 
neither before nor since ever met with another example of this 
kind. The remaining guns of this lot, which must have been 
made of the same metal, went on service, and some of them are 
probably still existing. I have no doubt that they were the 
best that we received during the war ; and it would have been 
very desirable to have discovered by what means this very un- 
common specimen of gun-metal had been procured, as its tough- 
ness is a matter of such importance. It was from Mr Walker’s 
foundery. 
I do not pretend to account for this singular state of the iron ; 
as, although the presence of plumbago is sufficient to affect the 
colour of the metal, it is never, bating this instance, distinctly 
visible, as far as my experience goes. Yet, in gun-metal, it is 
easy to conjecture its presence and proportion, by the colour of 
the fracture ; which is darkest or most grey, where it is most 
abundant. The trials which I have made, also go to prove that 
the greyest metal is the toughest ; although I know that many 
iron-founders consider that gun-metal may be too grey, and act 
on this principle in the assortment of the pigs for the reverbe- 
ratory. 
I ought to add now, that, on dissolving gun-metal in an acid,, 
the black lead was always obtained in the state of powder, and 
that it is in very small proportion when compared to that ob- 
tained from pig-iron. I am sorry that I cannot now state these 
proportions, as the records of .my experiments are out of reach 
at present. 
It is perhaps unnecessary to say that the inferior proportion 
of plumbago in gun-metal, arises from its approximation nearer 
