212 THE IltlOMIiOll), AND 



. The modifications of (he rhomboid, and the second- 

 ary figures to which they lead, are generally dis- 

 tinguishable from those of the oblique rhombic prism. 

 But those which mark the distinction with the great- 

 est certainty, are modifications a, e, g*, A", and m, of 

 the rhomboid. In the oblique rhombic prism, modifi- 

 cation c, corresponding with a on the rhomboid, is 

 not a tangent plane ; and modification e of the prism, 

 corresponding in position to some of the planes of 

 modification e, g*, k, and m, of the rhomboid, affects 

 only four solid angles of the prism instead of the six 

 which are simultaneously modified on the rhomboid. 

 The three edges of the rhombic prism which meet 

 at the solid angle A, are not generally all modified 

 at the same time, as those of the rhomboid are ; nor 

 are the edges G and D modified together as the cor- 

 responding edges are in class m of the rhomboid. 



When the primary planes of the rhomboid are 

 effaced, it is frequently only by observing the direc- 

 tion of the natural joints, or cleavage planes, that 

 we are enabled to determine the classes of modifi- 

 cation to which its secondary forms belong. 



Several of the preceding classes of primary forms 

 stand in certain relations to each other, which it has 

 not fallen within the scope of the tables to point out. 



If we imagine the lateral edges of the cube to be 

 lengthened or shortened, a square prism would be 

 produced. If while the lateral edges are lengthened, 

 or shortened, we conceive four parallel terminal 

 edges to be lengthened or shortened also, but in a 

 different ratio to the remaining edges of the cube 

 from that in which the lateral edges have been varied, 

 we shall then have the right rectangular prism. 



It will facilitate our description of the relation of 

 some of the primary forms to certain others, if we 



