30 



TERMINOLOGY, 



Its angular extremities are of two kinds ; one of these consists of 

 but two similar solid angles, and the line which connects them, as 

 ab Fig. 39, is the perpendicular axis of the 

 solid : when this is vertical, the Rhomboid is 

 said to be in position. The lines cd, gh, ef, 

 which connect the other similar solid angles, 

 are called the transverse axes of the Rhom- 

 boid.* 



The two opposite solid angles, a and b, situ- 

 ated at the extremities of the perpendicular 

 axis, are called the solid angles of the summit, 

 or the terminal solid angles : the six remaining solid angles, and 

 which are all equal, are termed the lateral solid angles. The 

 edges ah, ac and ae, for the summit a, and the edges bd, 6/*and bg, 

 for the opposite summit 6, are called the terminal edges, or the 

 edges of the summit ; while the six other edges de, eg, gc, cf, fh 

 and hd, are denominated the lower edges, or the lateral edges of 

 the Rhomboid. 



The individuals belonging to this class are distinguished from 

 each other by the inclination of P on P', Fig. 40. When P on P' 1 

 measures more than 90, the Rhomboid 

 is called obtuse; when less^ acute. 

 In the first of these, the summits are 

 formed by the meeting of three obtuse 

 plane angles, and the lateral solid an- 

 gles are formed by the meeting of one 

 obtuse and two acute plane angles. In 

 the acute Rhomboid, the summits are 

 formed of three acute plane angles, and 

 the lateral solid angles are each formed 

 by the meeting of two obtuse and one 

 acute plane angle. t 



The Rhomboid is one of the most fre- 

 quent forms among crystals. Carbon- 

 ate of Lime, Bitter Spar and Chabasie, are obvious examples. 



Fig. 40. 



* The lines ab and cd, Fig. 39, have sometimes been called the 

 greater and the lesser axes of the Rhomboid. 



t It is apparent that the Cube, taking any one of its diagonals for the per- 

 pendicular axis, may be regarded as a Rhomboid, whose &ces are square, 

 and whose inclination to each other is 90 throughout. Now if we short- 



