166 



or less; the prism is perhaps shorter than here represented. 

 The left-hand outline shows what it should be as a perfect 

 octaedron. The angles of the base of the two pyramids 

 are 90°. Those of the face at the summit are 70° 31' 44", 

 and at the base 54° 44' 8". 



There have been two opinions concerning the primitive 

 form of oxide of tin: the one, thatit is an octaedron ; and the 

 other, that it is a cube. We have obtained very neat frac- 

 tures parallel to four faces of the latter, and signs of faces 

 inclined upon them, so as to form a rhomboidal dodecahe- 

 dron. 



TAB. LXXXI. 



Good specimens of oxide of tin with the proper four- 

 sided column and corresponding pyramid, if the edges are 

 not bevelled, or truncated, are somewhat rare. Some 

 crystals on the present specimen are of this form, and 

 others are truncated on the edge of the column, making a 

 fifth face j which truncation is generally continued up the 

 edge of the pyramid. 



A four-sided column without truncations, or a pyramid, 

 would be a great curiosity. 



