Dr MacCulloch on Black Lead from Cast-Iron. 5 



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 off 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 bkck 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. 



