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 

 he 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 basea 

 and faces referred to in the following paragraphs. 



A B C D E F 



OAA 



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

 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 2 What is its base ? /How are the basal and 

 pyramidal edges distinguished ? What is a dodecahedron 1 What are 

 axes ? What are interfacial angles 1 Explain the terms square ; rect- 

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

 diagonal. How many systems of crystallization are there? 



* 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, 

 D ACB is an rngle formed by the divergence of two 



K A. lines from CY^/lT a circle be described with the 

 angular point p.as the center, and the circumference 

 }b 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 36Q°-f-4 equals 90°. This is 

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

 oblique angle ; if less,- as ACB, an acute angle ; if more, as ACE, an 

 obtuse angle. 



