•220 Analyjis of a new Variety of Lead Ore. 



devoted his attention; and to call in the aid of chemiftry to (hew with \vhat certitude the 

 penetrating eye of the able cryftallographer can, from external ftrudure, arrive at an 

 intimate knowledge of pure mineralogical fubftances. 



In the valuable coUedion of the Right Honourable Charles Greville, there are many 

 different varieties of carbonated lead, acknowledged and clafled as fuch from their external 

 charaders : one, however, which, as far as I can learn, does not exift elfewhere, excited 

 fome doubt in the mind of Monf. le Comte de Bournon, from its affuming a peculiar 

 modification of form, not hitherto remarked. He offered it to me for chemical examina- 

 tion ; obferving at the fame time, that though thoroughly convinced it was carbonated 

 lead, he was equally fure it muft be in fome particular flate of combination, either in 

 the proportion of its known principles, or by containing fome fubftance not found in the 

 other varieties. I undertook the analyfis, and to render the approximation more ftriking, 

 he has favoured me with the following accurate mineralogical defcription. 



Mineralogical CharaSlers, in ivUch it chiefly differs from funple Carbonated Lead, hy 



Monf. le Comte de Bournon. 



This variety of lead ore differs from fimple carbonated lead. 



1. It is much fofter than carbonated lead, making no impreffion whatever on it, and 

 being eafily fcratched and worn by rubbing againft it. 



2. Its fpecific gravity is much lefs, being 60.6^ r. Many experiments on pure cryftals 

 of carbonated lead give a fpecific gravity of 72.357. Many authors fix it at 44,63, but 

 they have undoubtedly taken the heavy fpar for it. 



3. Its frafture is glaffy, and much like that of the precious ftones. 



4. Its form is different in many refpe£ts. Like mod other natural oxides of lead, its 

 lamellse appear to be redangular ; but their union, which very feldom forms a perfeft cube 

 in the carbonated lead, produces that form more frequently in this ore. The various laws 

 of decreafe, which thefe lamellx undergo, in order to produce that cube (which is very 

 frequently lengthened, or flattened) as well as the different cryftalUne modifications, are 

 »lfo very different. 



Sometimes the decreafe is at the angles, fo as to replace thofe of the cube or paral- 

 lelipipedon by planes of greater or lefs extent, and perpendicular to the axis of the cube. 

 Sometimes, if we confider the cube or parallel ipipedon as a prifm, we fliall find that 

 the longitudinal edges of the prifm are replaced by planes perpendicular to thofe edges. 



Very frequently, the cryftal being ftill confidered as a prifm, the edges of the terminat- 

 ing faces are replaced by planes inclined to the faces of the prifm, fo as to form an angle 

 of 148°, and another angle of 122°, with the terminating faces. The cryftallization 

 under that form has fometimes been of fufficient duration, to allow at the two extremities 

 of the cryftal a tetraedal pyramid, feldom complete, but which fometimes preferves no 

 marks of ternrinating faces, but a fmall plane replacing the fummit of the pyramid j when 

 complete, the oppofite faces join at the fummit in an angle of 64°. 



At 



