CRYSTALLINE FORMS. 



87 



The planes of a crystal are said to be similar when their 

 corresponding edges are proportional, and their 

 corresponding angles equal. 



Edges are similar when they are produced 

 by the meeting of planes, respectively similar, 

 at equal angles. 



Angles are similar when they are equal, and 

 contained within similar edges respectively. 



Solid angles are similar when they are com- 

 posed of equal numbers of plane angles, of 

 which the corresponding ones are similar. 



An equilateral triangle, fig. 8, is a figure 

 contained within three equal sides, and containing three 

 equal angles. 



FIG. 7. 



FIG. 9. 



An isosceles triangle, fig. 9, has two equal sides, a, I, which 

 may contain either a right angle, or an acute, or obtuse 

 angle. If the contained angle be less than a right angle, it 

 is an acute angle ; if greater, it is obtuse. The line on which 

 c is placed is called the base of the triangle. 



A scalene triangle haa three unequal sides, and contains 

 three unequal angles. 



FIG. 10. 



FIG. 11. 



A square, fig. 11, has four equal sides, containing four 

 right angles. 



A rectangle, fig. 12, has its adjacent sides, a and Z>, unequal, 

 the four contained angles being right angles. 



