TERMINOLOGY. 



The lateral edges GG, Fig. 28. are always lon- 

 ger or shorter than the terminal ones BB. If these 

 edges are equal, the form becomes a cube. 



The right square Prism has an axis in four di- 

 rections, similar to the Cube. It also has a line 

 connecting the centres of the bases, called the 

 prismatic axis. 



The individuals of this class may differ from each 

 other in the comparative length of the edges G and B. 



Scapolite, Apophyllite and Idocrase, may be mentioned 

 erals which assume this form. 



28. 



. 41. THE RIGHT RECTANGULAR PllISM. 



The right rectangular Prism is a quadrangular prism, 

 whose bases are equal rectangles. 



The lateral edges GG, Fig. 29. 

 of this form are similar ; but dif- 

 fer in length from the terminal 

 dges CB, which are not equal. 

 If G was equal to C or B, the form 

 would be a right square Prism. 



The same number of axes exist 

 in this form, as in that of the right 

 square Prism. 



The individuals of this class will 



differ from each other in the comparative length of the three, adja* 

 cent sides C, B, G. 



The present form may be seen among the crystals of Anhydrite, 

 Harmofone and Bournonite. 



line connecting the centres of two opposite faces, and parallel to the 

 edges of four other faces between them : this line onght therefore to be 

 taken for a prismatic axis, the four face? for the lateral planes, and the 

 other two for the bases. 



When the prismatic axis is perpendicular to the bases of these *oli<is, 

 they are called right quadrangular prisn.s ; when oblique, they are call" 

 ed oblique quadrangular prisms. 



