674 PHILOSOPHICAL TRANSACTIONS. [aNNO l/QO. 



cularly noticed. He reduced the molybdaena into fine powder, and poured on it 

 concentrated nitrous acid : " the mixture," he says, " was hardly lukewarm in 

 the retort, when it passed all together into the recipient with great heat ;" and it 

 was for this reason that he afterwards used diluted acid. Presuming that this vio- 

 lent action of the concentrated nitrous acid might afford a decisive criterion of 

 molybdaena, Mr. W. had the black residuum, after 5 or 6 abstractions of the 

 diluted acid, ground fine on a levigating glass, and returned into the retort, with 

 6 times its weight of smoking spirit of nitre. The heat was increased cautiously 

 far beyond lukewarm, but no commotion could be perceived, except the explo- 

 sions already mentioned, which always took place when the mixture was near 

 boiling. The distillation was continued to dryness, and repeated 5 times with 

 smoking acid ; but the mineral remained just as black as it was at first. 



Now, as Scheele's molybdaena is slowly decomposed by the diluted nitrous acid, 

 and rapidly acted on by the concentrated acid, while the black part of this mineral 

 obstinately resists both, we cannot hesitate to conclude, that this black substance 

 is not Scheele's molybdaena. There are some other circumstances which con- 

 firm this conclusion, though taken singly they would not perhaps be of much 

 weight, considering the great proportion of other matter here mixed with the 

 black. The principal of these circumstances are, that it yields no flowers before 

 a blow-pipe, and that its particles seem to have no flexibility or elasticity, the 

 only difficulty of reducing it into fine powder arising from a property of another 

 kind, unctuosity. The difference, above noticed, between this black matter and 

 common black-lead, consists only in the former leaving on calcination a white 

 substance, seemingly siliceous, and the latter a brown ferrugineous one. In their 

 aspect, unctuosity, resistance to acids, and the volatility (in open fire) of that 

 part in which the blackness consists, they perfectly agree ; and they appear to 

 agree also in the nature or constitution of this volatile part ; for the Sydney-Cove 

 mineral, as well as black-lead, deflagrates and effervesces very strongly with nitre, 

 produces an hepatic impregnation on fusion with vitriolated alkali, but none with 

 pure alkali, and is manifestly rich in inflammable matter, without sulphur. 



^ It seems tlierefore, that this substance is a pure species of plumbago, or black 

 lead, not taken notice of by any writer. Fourcroy, in the last edition of his' 

 Chemistry, considers iron as an essential component part of black-lead, to which 

 accordingly he gives a new name, expressive of that metal, carbure de fer. 

 Lavoisier, in his Elements of Chemistry, lately published, mentions a carbure of 

 zinc also, and says that both these carbures are called plumbago, or black-lead. 

 The quantity of mineral Mr. W. had been furnished with was too far exhausted, 

 before he met with this observation, to admit of any further experiments, for 

 determining the presence of zinc in it ; but those already stated, with the re- 

 collection of some circumstances attending them, persuade him, that that metallic 



