52 Rev. J. Conybeare on Plumbago in Coal Gas Retorts. [Jan. 



on nitre deflagrate, but not rapidly, leaving after combustion 

 traces of iron. Some rude experiments gave me reason to think, 

 that the quantity of iron varied in different specimens, and that 

 it scarcely amounted, at the most, to the nine per cent, stated 

 by Berthollet to exist in native graphite. That the substance 

 in question is a true artificial plumbago admits, I apprehend, of 

 no doubt. It must be formed plentifully in many other gas 

 works besides those of Bath, and must, therefore, in all probabi- 

 lity have attracted the notice of persons far better able to inquire 

 into its production and properties than myself. In our works it 

 has not yet been converted to any useful purpose. In writing or 

 drawing, its hardness and general want of compactness render 

 it in its crude state altogether unserviceable. When finely pow- 

 dered and washed, it might be used in composition pencils. For 

 the domestic uses to which plumbago is applied in coating 

 grates, &c. I have found it to answer sufficiently well ; but it is 

 objected by the sava>ites in housewifery, that it has not a suffi- 

 cient lustre, and would, therefore, I suppose, be seldom used by 

 them ; but where preservation only, and not ornament, was the 

 object,* I have been informed that it has been applied with suc- 

 cess to the purpose of covering razor strops. For that of dimi- 

 nishing friction, and for the manufactory of crucibles, furnaces, 

 &c. it would, I should apprehend, answer sufficiently well, for the 

 latter especially. It appears from a paragraph in the article 

 Coinage (Supplement to Encyclopaedia Britannica), that the blue 

 pots used in the mint are all of foreign manufactory, those 

 made in this country containing too sma 1 7 an admixture of black 

 lead. Is this to be attributed to the scarceness of that material, 

 or to the want of sufficient tenacity in the English clay with 

 which it is worked up, rendering its addition beyond a certain 

 point impracticable? 



But whether this substance prove useful or worthless in an eco- 

 nomical view, its occurrence cannot fail to strike the more spe- 

 culative inquirer as adding one more to the many instances in 

 which the unintentional products of art have been found to 

 resemble those of nature, and as contributing, remotely at least, 

 to throw fresh light upon one of the most controverted points of 

 geological theory. Plumbago is well known to be among the 

 most infusible of mineral substances. Now in the present cases, 

 if not the whole of its mass, at least all those portions in which 

 the mammillated structure is discernible, and vet more its stalac- 

 titic form, must have been brought to a state of fusion by a heat 

 inferior to that at which cast-iron begins to run. Will this be 



* I know not at what price it might be rendered, but as its production is matter of 

 necessity, and it is at present considered as useless, the charge could scarcely be high. 

 The retail price of the black lead of the shops is 2s. per lb. and it is said to be much 

 adulterated by the admixture of a micaceous haematite obtained near Bovey Traccy 

 (C. Devon), which must be injurious to its preservative qualities. 



