DEFINITIONS. 1^ 



t 



The diagonals and axes of crystals are imaginary 

 lines, by means of which the secondary planes of crys- 

 tals may frequently be described with greater pre- 

 cision than could be attained without their assistance; 

 they also facilitate the mathematical investigations 

 into the relations which subsist between the primary 

 and secondary forms. 



The diagonal planes are imaginary planes of a 

 similar character. 



A crystal is said to be in position, when it is so 

 placed, or held, as to permit its being the most easily 

 and precisely observed and described. 



For this purpose tetrahedrons are made to rest on 

 one of their planes, as in the figure already given. 



Octahedrons are supposed to be held with the axis 

 vertical, and in this position the plane angles at a 

 and 6, fig. 28, are called the terminal angles, and the 

 edges a c, a d, a e, af y the terminal edges, or edges of 

 the pyramid. 



The edges e d, df, &c. may be termed edges of the 

 base ; and the angles a e d, a df, lateral angles. 



The angles of the base are the angles c e d, or e df> 



The cube stands on one of its planes, and all prisms 

 on their respective bases. 



Rhombic dodecahedrons are supposed to be held 

 with a greater axis vertical, as in the former figure. 



The rhomboid is also supposed to be held with its 

 perpendicular axis vertical, 







Crystals are supposed to be first formed by the 

 aggregation of a few homogeneous molecules, which 

 arrange themselves around a single central molecule 

 in some determinate manner ; and they are conceived 

 to increase in magnitude, by the continual additions 

 of similar molecules to their surfaces. 



