€2 Mr. Smithson on the Composition , &c. 
atoms along the edge of the cube, and the angle it forms with 
the face m is really of 135 0 . 
The face c is produced by a decrease of two rows of atoms 
at the corners of the cube, and the angle it forms with the 
face b is = 125 0 15' 52". 
The face b being produced like the face a , forms the same 
angle with the face m. 
No crystal I possess, has enabled me to measure the incli- 
nations of the faces g , d , or/; should the face g, as is presum- 
able, result from a decrease of one row of atoms at the corners 
of the cube, it will form with the face b, an angle of 14,4,° 44/ 
8", and if the faces d and/ are, as is also probable, produced 
by a decrease of two rows of atoms along the edges of the 
cube, the first will form an angle of ii 6°33 / 54", and the 
latter one of 153° 2 6' 6", with the face m. 
The angles assigned here differ considerably from those 
given in the former account of these crystals ; but the angles 
there given have not only appeared to me to be contradicted 
by observation, but, crystallographically considered, are in- 
consistent with each other, as the tetraedral prism of dimen- 
sions to produce an angle of 135 0 by a decrement along its 
edge, would not afford angles of 140° and i2o°by decrements 
at its corners. 
The sum of the faces of these crystals is 50. 
