PRIMARY FORMS. 



27 



The oblique rhombic Prism has, beside a prismatic axis, a greater, 

 a lesser, and two transverse, axes. The greater axis is that which 

 passes through the two acute solid angles ; the lesser, that which 

 passes through the two obtuse solid angles, and the transverse, those 

 which pass through the lateral solid angles. 



The planes M M f may meet at an acute or an obtuse angle: in 

 the former case, the prism is said to be oblique from an acute edge, 

 and in the latter, oblique from an obtuse edge. 



The individuals of this class will differ from each other in the in- 

 clination of M on M r , and in the ratio of the edge H to the edge D. 



This is a frequent form among crystals. Laumonite, Pyroxene, 

 Mica, &c., are examples. 



. 45. THE DOUBLY OBLIQUE PRISM. 

 The doubly oblique Prism is a quadrangular prism, whose 

 bases are equal oblique-angled parallelograms, and whose 

 prismatic axis inclines from a perpendicular. 



in Figs. 33 and 34, a plane or section perpendicular to their edges, or 



prismatic axes. The relations observable between the portions of the 



lateral edges of the prism situated above and below the perpendicular 



Fig. 33. Fig. 34. 



section xy v z, render apparent the different positions of the bases which 

 we wish to indicate. An obtuse lateral edge is supposed to be in front, 

 in both Figs. 33 and 34. In Fig. 33 the bases are so disposed that the ob- 

 tuse edge iy and e z above the plane xy v z, or the corresponding ones 

 below, y n and zr, are equal to each other ; while the acute edges ax 

 and vo, and the corresponding ones below the horizontal section xyvz, 

 x m and v s, are unequal. On the contrary, in Fig. 34, precisely the op- 

 posite takes place. It may be added, what indeed is sufficiently appa- 

 rent, perhaps, that the base avoi, Fig. 34, forma an equal angle with 

 the planes amre and erso. 



