ITS MODIFICATIONS. 189 



Class k. Lateral edges of the prism replaced by 

 tangent planes. 





Class /. Lateral edges of the prism replaced by 

 two planes. 



Classes /, and /, would produce other oblique rhom 

 bic prisms, varying from the primary, and from each 

 other, in the angles at which their lateral planes 

 would meet. 



In this class of primary forms the cleavage planes, 

 parallel to one of the lateral planes, are sometimes 

 brighter than those parallel to the other planes, which 

 is not the general character of symmetric cleavage. 



This class of prisms may generally be distinguished 

 from rhomboids, by the unequal angles at which the 

 adjacent planes incline to each other at a terminal 

 edge, and at an adjacent lateral oblique edge; but if 

 these planes should respectively meet at equal angles, 

 as it is possible they may do, the distinction then would 

 arise from the lateral edge being greater or less than 

 the terminal one. For it is possible that the in- 

 clination of P on M, or M' 3 should be equal to that of 

 M on M', but in this case the edges D would be 

 greater or less than the edges H, and this prism 

 would then bear the same analogy to the rhomboid, 

 that the square prism does to the cube. 



