PRIMARY FORMS. 



. 42. THE RIGHT RHOMBIC PRISM. 



The right rhombic Prism is a quadrangular prism, whose 

 bases are equal rhombs, and whose lateral planes are either 

 equal squares, or equal rectangles. 



Fig. 30 is drawn with the greater 

 diagonal of the bases horizontal. 

 The solid angles at A are the ob- 

 tuse, and those at E the acute, so- 

 lid angles. The edge G and its 

 opposite are the acute, and the edge 

 H and its opposite 'the obtuse, lat- 

 eral edges. 



The right rhombic Prism, beside 

 the prismatic axis, has two greater 

 and two lesser, axes. The greater axes pass through the solid an- 

 gles, which terminate the acute edges of the prism, as E E, and the 

 lesser through those, which terminate the obtuse edges of the prism, 

 as A A. 



The individuals belonging to this class will differ from each other 

 in the inclination of M on M', or in the ratio of the edge H to the 

 edge B. 



The right rhombic Prism is one of the most frequent forms of 

 crystals ; examples are found in Andalusite, Sulphate of Barytes, 

 Yenite, &c. 



. 43. THE RIGHT OBLIQUE-ANGLED PRISM. 



The right oblique-angled Prism is a quadrangular prism, 

 whose bases are equal oblique-angled parallelograms. 



The adjacent lateral faces, 

 Fig. 31, are unequal. T is a 

 rectangle ; but M may b'e ei- 

 ther a square or rectangle. 



The angles and edges of this 

 class of prisms are designated 

 as those of the right rhombic 

 Prism, (. 42.) The solid an- 

 gles at A are the obtuse, those 



3 



Fig. 31. 



