7B Scientific Intelligence. [Jan, 



yielded it in abundance. In 1807 or 1808, Dr. Mac Culloch was 

 requested to go to a London porter brewery to see an appearance 

 which had very much surprised the people employed in it. They had 

 taken some iron out of their porter backs in making repairs, and had 

 found it, as they said, red-hot. This was found to be an exaggeration ; 

 but on removing the iron articles from the porter, they became so hot 

 on scraping off the surface, that it was disagreeable to handle them ; 

 while they smoked from the evaporation of the moisture : they were 

 cast cones, perforated with holes, and about an inch thick, used as 

 strainers to prevent foreign substances from getting into the pipes, and 

 had been immersed in the porter for many years. On examination, 

 some of them were found to be entirely plumbago ; while in others 

 there was a thick coat of that substance on each side, a little iron only 

 remaining in the middle. 



Shortly afterwards, Dr. M. met with the following relation in one of 

 his journeys in the Western Isles. In ] 740 an attempt was made to 

 weigh one of the Spanish Armada, which had foundered off the coast 

 of Mull; this proved unsuccessful, but some brass and iron guns were 

 brought up, the former bearing the date 1584. The iron guns were 

 deeply corroded ; and on scrapingthem, it was said that they were found 

 so hot that they could not be touched, and that they did not become 

 cool till they had been two or or three hours exposed to the air. 



The following are the general results of Dr. Mac Culloch's experi- 

 ments on this subject : The blackest pig-metal appears to yield the 

 greatest quantity of plumbago, 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 has been observed, it is of a much more coarse grain, or scaly 

 granular texture, than natural black-lead. To procure it in perfection, 

 the acid should be very weak, and the operation is then necessarily 

 very tedious. Acetic 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. If the experiment 

 be perfect, the plumbago becomes hot on exposure to the air, smok- 

 ing while there is any moisture to be evaporated, particularly when the 

 surfaces are scraped off in succession ; there is no apparent difference 

 in the plumbago before and after this operation. When the substance 

 does not heat, on being taken out of the fluid, the whole process of 

 oxygenation appears to have taken place in the solution, probably from 

 an excess of strength in the acid. 



From these facts, Dr. Mac Culloch draws the following inferences: 

 that plumbago, by which name he designates the carbon, as it exists in 

 cast-iron, is a metal, and black-lead its oxide: that in white-pig, proba- 

 bly, the combination is this metal and iron ; and that in the black it 

 exists in a state of approximation to black-lead : that the operation of 

 the acid is to dissolve the iron, and to oxygenate the plumbago so as 

 to convert it into black-lead : and that if the acid be strong, the whole 

 operation is completed in the solvent; otherwise some additional oxy- 

 gen is required to produce it in a state of permanence in the air, and 

 that the absorption of this generates the heat in the experiment : 

 lastly, that the metallic nature of the base of charcoal is also proved 

 from these facts. And lest this reasoning should be deemed unsatis- 

 factory, Dr. M. adds the following argument : " The specific gravity 



