L) PHYSIOGRAPHY. CLASS II. 



OBSERVATIONS. 



1. The most remarkable feature in the series of crystalli- 

 sation of rhombohedral Fluor-haloide is the mode in which 

 the scalene six-sided pyramids enter into combinations 

 with other forms. It is the mode called hemi-rhombohe- 

 dral parallel with faces (Vol. I. . 147-), and is not as yet 

 known with the same peculiarities in any other species. If 

 sufficiently enlarged, the faces marked u^ u &c., and &, & &c., 

 Fig. 148. and 149. will produce forms which by themselves 

 appear as isosceles six-sided pyramids if we give no atten- 

 tion to their connexion with other simple forms of the 

 species. In LEONHARD'S Mineralogy, mention is made of 

 this peculiarity, but it is -said to be unsyrnmetrical, and 

 like the Quartz plagiedrc of HAUY, from which, however, it 

 is distinguished by having the oblique faces disposed on one 

 end to the right, on the other to the left of the faces of 

 2 (R), while in Quartz they are disposed on both ends of 

 the crystals, equally, either to the right or to the left of the 

 same faces. The twelve-sided prisms (P + co) 5 and (P + os) 3 , 

 though derived according to different numbers of deriva- 

 tion, yet possess the same transverse sections, with that 

 difference only that the obtuse edges of the one have the 

 situation of the acute edges of the other. These prisms also 

 produce hemi-dirhombohedral combinations with other 

 forms, so that if we find the alternating faces of (P + cc) 3 

 contiguous to the left side of the faces of 2 (R), the right 

 ones of (P + co) 3 will appear. The product of a combina- 



tion of R + cc, P + co, I 2_f^ and _! 2_^I 3 is 

 r 2 12 



limited parallel to the axis by four times six faces of the 

 same kind, including angles with each other of alternately 

 160 53' 38" and 169 6' 24", so that, proceeding from right 



to left, the edge of combination between - _.fii_ and 



/p , ^ Y j 



11 + co is equal to that between ^ I and P + co, 



2 

 and so all round. 



