132 PHYSIOGRAPHY. CLASS II. 



revolution perpendicular, face of composition par- 

 allel to one of the faces of Pr. The composition is 

 often repeated, not only in parallel laminae, as in 

 prismatic Lime-haloide, but likewise parallel to both 

 the faces of ?r. The individuals are generally con- 

 tinued beyond the face of composition. Thus are 

 formed the well known star-like compound crystals 

 of the present species, as represented in Fig. 38. 

 Massive : composition often granular, or even impal- 

 pable, and strongly connected ; more rarely co- 

 lumnar. Faces of composition rough, or longitu- 

 dinally or irregularly streaked. 



OBSERVATIONS. 



1. The species distinguished by mineralogists within the 

 varieties of di-prismatic Lead-baryte, are White Lead-Spar, 

 Black Lead-Spar, and Earthy Lead-Spar. The last of these 

 comprehends impalpable compositions, often mixed with 

 clay, silica, oxide of iron, &c., and thence variously co- 

 loured. It is farther named indurated or friable, agreeably 

 to the state of coherence of its particles. The other two 

 are distinguished only in colour, so that those whose colour 

 is not black are called white lead-spar, while the rest forms 

 the Hack lead-spar. The varieties of the latter are gene- 

 rally less perfectly formed than those of the former, which, 

 like the colour, seems to be a consequence of mere acciden- 

 tal circumstances in their formation. 



2. A variety of the present species has yielded to KLAP- 

 ROTH, 



Oxide of Lead 82-00. 



Carbonic Acid 16-00. 



Water 2-00. 



It is expressed by the formula Pb C 2 , which corresponds 

 to 83-52 oxide of lead, and 16-48 carbonic acid. The black 

 varieties are said to contain a small proportion of carbon. 



