WRITINGS OF JAMES SMITHSON. 41 



moment to the determination of true or primitive form, nor 

 Lave 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- 

 dral prism, is inferable, not from the strise 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 b, 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 135. 



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 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 in- 

 clinations of the faces </, d, or f; should the face #, as is 

 presumable, result from a decrease of one row 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 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 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 afford angles of 140 

 and 120 by decrements at its corners. 



The sum of the faces of these crystals is 50. 



