ORDER VI. PRISMATIC 1'ETALINE-SPAB. 



137 10', and its short diagonal One of the faces of this 

 prism is smooth and even, and easily obtained, the other is 

 not so easily obtained, and possesses a kind of resinous 

 lustre, similar to that face which passes through the diago- 

 nal, though this seems not properly to be the diagonal of a 

 rhombic prism. Since faces of different physical quality 

 cannot belong to one and the same simple forms, it is ra- 

 ther probable that those two less distinct faces produce a 

 prism of about 95, parallel to the long diagonal of which 

 the more distinct face of cleavage is situated. More ac- 

 curate examinations must decide, whether this more simple 

 hypothesis, which has been introduced in the present work, 

 be correct, or whether the forms of the species, as it is 

 probable, possess a hemi-prismatic or tetarto-prismatic cha- 

 racter. There are, moreover, traces of cleavage nearly 

 perpendicular to the axis of this prism. 



2. According to ARFVEDSON, the varieties of the pre- 

 sent species consist of 



Silica 70-212. 



Alumina 17-225. 



Lithia 5-761. 



If exposed to a high degree of heat, before the blowpipe 

 upon charcoal, it becomes glassy, semi-transparent and 

 white, but melts vfiih difficulty, and only on the edges. If 

 gently heated it emits a blue phosphorescent light. 



3. The prismatic Petaline-spar has been hitherto found 

 only in the Swedish island of Uton, where it is said by 

 some to occur in a narrow vein traversing the beds of octa- 

 hedral Iron-ore, which are worked in that place ; by others, 

 in large boulders. It is accompanied by rhombohedral Lime- 

 haloide, prismatic Feld-spar, rhombohedral Quartz, rhom- 

 bohedral Tourmaline, &c. It has also been said to occur 

 at the Lake Ontario in North America. 



