24 STRUCTURE OF MINERALS. 



four-sided pyramids placed base to base. (Figs. 2, 6, 9 ) The 

 plane in which the pyramids meet is called the base of the 

 octahedron ; (bb, fig. 6 ;) the edges of the base are called 

 the basal edges, and the other edges the pyramidal. 

 The dodecahedron* has twelve sides (fig. 3.) 

 The axes of these solids are imaginary lines connecting 

 the centers of opposite faces, of opposite edges, or of oppo. 

 site angles. The inclination of two planes upon one another 

 s called an interfacial angle, f 



The figures here added represent the forms of the bases 

 and faces referred to in the following paragraphs. 



C D E F 



A 



<£0 AA 



A, a square, having the 4 sides equal ; B, a rectangle, dif 

 fering from A, in having only the opposite sides equal ; C, a 

 rhomb, having the angles oblique and the sides equal ; D, a 

 rhomboid, differing from the rhomb in the opposite sides only 

 being equal ; E, an equilateral triangle, having all the sides 

 equal; F, an isosceles triangle, having two sides equal. 

 The lines crossing from one angle to an opposite are called 

 diagonals. 



The fundamental forms of crystals, though thirteen in num. 

 ber, constitute but six systems of crystallization, as follows :— 



What is an octahedron 1 What is its base ? How are the basal and 

 pyramidal edges distinguished 1 What is a dodecahedron 1 What are 

 axes ] What are interfacial angles ? Explain the terms square ; rect- 

 angle ; rhomb ; rhomboid ; equilateral triangle ; isosceles triangle ; 

 diagonal. How many systems of crystallization are there 1 



* From the Greek dodeka, twelve, and hedra, face. 

 t An angle is the amount of divergence of two straight lines from a 

 given point, or of two planes from a given edge. In the annexed figure, 

 ACB is an angle formed by the divergence of two 

 lines from C. If a circle be described with the 

 angular point C as the center, and the circumference 

 DABFE be divided into 360 equal parts, the number 

 of these parts included between A and B will be the 

 number of degrees in the angle ACB ; that is, if 40 

 of these parts are included between A and B, the 

 angle ACB equals 40 degrees (40°). DF being 

 perpendicular to EB, these two lines divide the whole into 4 equal parts, 

 and consequently the angle DCB equals 360°-|-4 equals 90°. This is 

 termed a right angle. An angle more or less than 90° is called an 

 oblique angle ; if leds, as ACB, an acute angle ; if more, as ACE, an 

 abtuee angle. 



