SYMMETRY OF SECONDARY PLANES. 



47 



Fig. 112, the same, with its lateral solid angles replaced by single 

 planes, parallel to the perpendicular axis of the Rhomboid. 



Fig. 113, the same, with its lateral solid angles replaced by two 

 planes, meeting at an edge which is parallel to the perpendicular 

 axis of the Rhomboid. 



Fig. 112. 



Fig. 113. 



The relative disposition of the different parts of the regular hexa- 

 gonal Prism is too easy to conceive of, to require a recapitulation of 

 them here; and it will suffice to say, without instancing any exam- 

 ples, that all the terminal edges are modified together, and in a simi- 

 lar manner ; the same is also true of the lateral edges and of all the 

 solid angles. 



The law whose application has just been 

 considered cannot be said to be universal. 

 The Tourmaline, as well as Boracite, pre- 

 sent us with two very remarkable excep- 

 tions. In the first, the primary form is a 

 Rhomboid. Yet it is found, that the three 

 edges formed by the meeting of the three 

 faces P, P, P at each extremity of the six 

 sided prism s, s, s, Fig. 114, are replaced 

 only at one end by the tangent planes n, 

 n, n. It is noticed also, that only the al- 

 ternate three of the lateral solid angles are 

 modified, the remaining three being unal- 



Fig. 114. 



