MODIFICATIONS OF PRIMARY FORMS. 



31 



<. 47. THE REGULAR HEXAGONAL PRISM. 



The regular hexagonal Prism is a right prism whose 

 bases are regular hexagons. 



Fig. 41. 



G M 



M' 



Any two of the adjacent lateral faces, 

 as M, M' Fig. 41, incline to each other 

 under angles of 120. 



The hexagonal prism has as many 

 axes as it has opposite solid angles, 

 (viewed through the centre of the sol- 

 id,) but ^the prismatic axis is the only 

 one often used in the description of its 

 modifications. 



The individuals of this class can dif- 

 fer from one another only in the ratio 

 of the edge G to the edge B. 



The Emerald, Phosphate of Lime and Arseniate of Lead, may be 

 instanced, as among minerals which possess this form. 



CONSIDERATION OF THE CHANGES TO WHICH THE PRIMA- 



RY FORMS OF CRYSTALS ARE SUBJECT. 



I 



<. 48. MODIFICATIONS OF PRIMARY FORMS. 



Primary forms are said to undergo modifications 

 ever they become more or less altered in their figure. 



en this axis, the obtuse Rhomboid is immediately generated ; and when 

 the axis entirely disappears, the six faces fall into one plane of a hexa- 

 gonal figure. On the other hand, by elongating the axis of the Cube, a 

 series of acute Rhomboids is produced, which go on forming with indefi- 

 nite variations, till that axis becomes a line of indefinite extent, when 

 the solid vanishes. Thus the Rhomboid is limited in its variations, on one 

 side by a /me, and on the other by a glane; while in the intermediary 

 state, it is a solid. 



