depending on their Internal Changes. 287 



The mineral usually designated by the name of Blue Lead, 

 is in some respects the converse of the changes considered 

 above. Its forms are those of the rhombohedral lead-baryte, 

 namely, regular six-sided prisms. The compound of phos- 

 phate of lead and chloride of lead, of which their substance 

 originally consisted, has given way to the sulphuret, which 

 usually appears in granular compositions, filling the crystals. 

 The first varieties that were noticed by mineralogists were 

 those from Tschopau in Saxony. I remember having seen 

 specimens of it, entirely consisting of compact galena, but I 

 have not had an opportunity of comparing any again, after hav- 

 ing examined some of the other varieties of the same substance. 

 At Huelgoet in Brittany, six-sided and twelve-sided prisms 

 are found, often upwards of an inch in length, and nearly half 

 an inch in thickness, which consist of a coarse-grained com- 

 pound variety of lead-glance, the component individuals being 

 so large that it is very easy to ascertain their hexahedral cleavage. 

 Sometimes these individuals have oneof their hexahedral facesof 

 crystallization coincident with the original surface of the hexa- 

 gonal prism. The stratum of lead-glance contiguous to the sur- 

 face of the original crystal is usually separated from the body of 

 it by an empty space, so that it may be very easily broken off. 

 Sometimes only this stratum is in the state of lead-glance, 

 while remains of the original species are still visible in the in- 

 terior, or part of the crystal only has begun to have a portion 

 contiguous to the surface converted into lead-glance, while the 

 rest presents the adamantine lustre and brown colour of the 

 rhombohedral lead-baryte. In the six-sided prisms of the 

 same kind of formation met with at Wheal Hope in Cornwall, 

 generally a film of lead-glance is also observed near the sur- 

 face ; but the crystals of the sulphuret in their interior are 

 often much more curiously arranged. Partly they are simply 

 composed of a mass of very compact galena, partly also they pre- 

 sent, when broken, the appearance of being cleavable with great 

 facility perpendicular to their axis, and at the same time also pa- 

 rallel to the sides of the six-sided prisms, and parallel also to the 

 planes replacing their edges. The smooth planes obtained in 

 this manner are actually the faces of cleavage of the hexahedron 

 peculiar to lead-glance. The individuals of the sulphuret 



