136 PHYSIOGRAPHY. CLASS n. 



Formerly two species used to be distinguished, the 

 Green and Brown Lead-Spar, merely according to the shades 

 of colours, which, however, as in rhombohedral Emerald, 

 or other well determined species, form but one uninter- 

 rupted species, in which only arbitrary limits can be fixed. 

 Green Lead-spar referred to varieties of green or greenish 

 colours, while Brown Lead-spar comprehended those in 

 which the colours approached more to brown tints. What 

 has been called Blue Lead possesses the shape of the crys- 

 tals of the present species, the substance of which has been 

 replaced by hexahedral Lead-glance. There are varieties 

 of Blue Lead, which consist of rhombohedral Lead-baryte 

 of a dark blueish-grey colour. 



2. In two varieties, one of a brown colour from Huel- 

 goet, the other a green one from Zschopau in Saxony, 

 KLAPROTH found the following ingredients : 



Oxide of Lead 78-58 78-40. 



Phosphoric Acid 1973 18-37. 



Muriatic Acid 1-65 1-70. 



Oxide of Iron 0-00 0-10. 



The proportion of oxide of lead and phosporic acid, cor- 

 responding to the formula Pb 5 P 4 , is that of 79-27 : 20-73. 

 Two specimens containing arsenic acid, yielded to ROSE, 



Oxide of Lead 77'50 77'50. 



Phosphoric Acid 0-00 7*50. 



Arsenic Acid 19-00 12-50. 



Muriatic Acid 1-53 1-50. 



Oxide of Iron 0-25 0-00. 



The rhombohedral Lead-baryte is soluble without efferves- 

 cence in heated nitric acid. Before the blowpipe it melts 

 by itself upon charcoal, and the bead assumes a polyhedral 

 form of a dark colour. In the interior flame the globule 

 becomes blueish, in the moment of crystallisation it is lu- 

 minous, and the faces become larger. The form itself has 

 not been accurately examined ; it seems to consist of several 

 crystalline individuals. 



3. The varieties of the present species occur chiefly 



