MODIFICATIONS OF CRYSTALS. 85 



raid — and it will be admitted that the powers of crystalliz- 

 ation scarcely yield to vitality in the forms of beauty they 

 produce. 



These results are not more wonderful than the simplicity 

 of the laws that lead to them. 



The various secondary forms proceed from the occurrence 

 of planes on the angles or edges of the fundamental forms, 

 which planes are called secondary planes. Figures 20, 

 21, are secondaries to the cube, and the planes a and e are 

 secondary planes ; figures 28, 29, 30, are secondaries to 

 the rhombohedron, and the planes e and a are secondary 

 planes.* These secondary planes however numerous, con- 

 form in their positions to a certain law called the law of 

 symmetry. Previous to stating this law a few explanations 

 are added. 



The cube, it has been remarked, has six equal square faces. 

 The twelve edges are therefore all equal, and so also the 

 eight angles. In the square prism the vertical edges differ 

 in length from the basal, and are therefore not similar. In 

 the rectangular prism, not only the vertical differ from the 

 basal, but two of the basal at each extremity differ from the 

 other two basal. This will be seen at once in the models. 

 In the right rhombic and rhomboidal, two of the lateral edges 

 are acute and two obtuse ; these then are not similar to one 

 another. In the oblique prisms some of the basal edges are 

 acute and some obtuse. After tracing out the similar and 

 dissimilar angles and edges in the primaries, with the models, 

 the following laws may be easily applied : Either — 



1. All the similar parts of a crystal are similarly and 

 simultaneously modified ;* or, — 



Explain the relation of secondary planes to the fundamental form. 

 What is said of the cube 1 of the square prism 1 the rectangular prism 1 

 the right rhombic and rhomboidal ? the oblique prisms'? What is the first 

 law repecting secondary planes 1 



Note. — What is meant by replacement, bevelment, and truncation? 



* To avoid circumlocutions, the following technical terms are employed 

 in describing the modifications of crystals. 



Replacement. An edge or angle is replaced, when cat off by one or 

 more secondary planes, (figs. 20, 21, 32.) 



Truncation. An edge or angle is truncated, when the replacing 

 plane is equally inclined to the adjacent faces, (figs. 20, 21.) 



Bevelment. An edge is beveled, when replaced by two planes, which 

 are respectively inclined at equal angles to the adjacent faces, (fig. 32.) 

 Truncation and bevelment can occur only on edges formed by the meet' 

 ing of equal planes. 



