ESSAY XIII. 



EXPERIMENTS ON PLUMBAGO. 1779. 



I HAD the satisfaction of seeing my experiments on molyb- 

 dena inserted in the third quarter of the Transactions of 

 last year ; and as, in the beginning of that paper, I advanced 

 that the black-lead or plumbago which is generally known 

 in commerce is very different from molybdena, I shall now 

 show by experiments that this is really the case. The 

 mineral of which I am to speak at present is called by 

 Cronstedt, Molybdoena texturd micaced et granulatd. 



SECTION I. 



Finely levigated and sifted plumbago, after being digested 

 and boiled in all the known acids, both concentrated and 

 diluted, showed no sign of decomposition, excepting that 

 the menstrua were impregnated with a small portion of 

 iron. The acid of arsenic is the only acid which has any 

 effect upon this mineral, but it produces it only in the dry 

 way. I mixed two parts of dry acid of arsenic with one 

 part of pulverised plumbago ; and, after having distilled this 

 compound in a retort, found the neck of the vessel filled 

 with arsenic. That this reduction of the acid of arsenic 

 was not owing to the heat, I shall show in the sequel of 

 this paper. 

 12 



