2 Dr MacCulloch 011 Black Lead from Cast-Iran. 



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 WalkerV 

 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 

 it present. 



It is perhaps unnecessary to say that the inferior proportion 

 ol plumbago in gun-metal, arises from its approximation nearer 



