WRITINGS OF JAMES SMITH80N. 41 



moment to the determination of true or primitive form, nor 

 have any quantities been assigned to the decrements sup- 

 posed. I will, therefore, supply these very important omis- 

 sions. 



That the atom of this substance is a rectangular tetrae- 

 <lral prism, is inferable, not from the striae on the crystals, 

 for striae are by no means invariably indicative of a decre- 

 ment in the direction of them ; but from the angles which 

 the faces a and c make with the faces m and 6, and these 

 angles also prove, that the height of this prism is equal to 

 the side of its base, that is, that it is a cube. 



Hence the face a is produced by a decrease of one row of 

 atoms along the edge of the cube, and the angle it forms 

 with the face m is really of 186°. 



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 6 is = 125° 15' 52". 



The face h being produced like the face a, forms the same 

 angle with the face m. 



No crystal I possess, has enabled me to measure the in- 

 clinations of the faces g, d, or /; should the face g, as is 

 presumable, result from a decrease of one rov7 of atoms at 

 the corners of the cube, it will form with the face 6, an 

 angle of 144° 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 

 116° 33' 54", and the latter one of 153° 26' 6", with the 

 face m. 



The angles assigned here difier considerably from those 

 given in the former account of these crystals ; but the 

 angles there given have not only appeared to me to be con- 

 tradicted by observation, but, crystallographically consid- 

 ered, are inconsistent with each other, as the tetraedral 

 prism of dimensions to produce an angle of 135° by a 

 decrement along its edge, would not afibrd angles of 140° 

 and 120° by decrements at its corners. 



The sum of the faces of these crystals is 50. 



