OF FORMS IX GENERAL. 13 



The planes, or faces of a crystal receive certain names according 

 to these faces, as for instance, triangular faces, rhombic faces, &c. ; 

 and also according to the forms which they limit, as/aces of the 

 Cube, of the Octahedron, &c. 



. 22. EDGES, 



The lines produced by the meeting of the planes or fa- 

 ces are termed edges. 



The edges are denominated, not only according to the forms to 

 which they belong, but also as respects their peculiar situation 

 in these forms, as will be illustrated hereafter. 



. 23. PLANE ANGLE. 

 The meeting of any two edges forms a plane angle.* 



* Elementary definitions. The measure, or, as it is sometimes termed, 

 the value of an angle, is the number of degrees, minutes, &c. of which 

 it consists ; these being determined by the portion of a circle which 

 would be intercepted by the two lines forming the angle, supposing the 

 point of their meeting to be in the centre of the circle. For the pur- 

 pose of measuring angles, the circle is dividecUinto 360 equal parts, which 

 are called degrees ; each degree into 60 equal parts which are called 

 minutes; and each minute into 60 seconds ; and these divisions are thus 

 designated : 360, 60', 60", the signifying degrees, the ' minutes, the 

 " seconds. 



If of the circle, or 90, be intercepted by the 

 two lines a o, o b, Fig. 1, which meet at an an- 

 gle a o b in the the centre, those lines are 

 perpendicular to each other, and the angle at 

 which they meet is said to measure 90, and 

 i termed a right angle. If less than 4 of the a 

 circle be so intercepted, as by the lines b 0, 

 o c. the angle b o c, will measure less than 90. 

 and is said to be acute. If it measure more 

 than 90, a^ it would if the angle were formed by the lines a o, o c, ii is 

 called obtuse. 



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