CRYSTALS. 281 



commonly occurring combination, is not restricted to a 

 perfectly symmetrical shape, like that above presented. 

 It may develop one surface and diminish the others to 

 any extent. Forms such as 

 are represented in the margin 

 result. Different as they seem, 

 it will be observed that they 

 agree precisely with the per- 

 fect shape in the angles be- 

 tween the surfaces of the prism and pyramid, and the 

 different surfaces of each. In this their identity 

 as crystalline forms consists. It would thus seem that 

 nature pays exclusive attention to the corners and an- 

 gles in her various systems of crystalline architecture. 

 703. The least variation of the relative 



What consti- ..... , 



tutes a new length of the vertical axis that is not by 

 some simple ratio, constitutes a new and 

 distinct form. This has its related forms as before, 

 the whole making a new and distinct set, to which the 

 choice of any substance that enters it, is limited. 



704. SYSTEMS OF CRYSTAL FORMS. It 



Define ano- 

 ther system of will be obvious to the student that the sub- 



C forms liUe stitution of an octahedron, such as is re- 

 presented in the accompanying figure, for 

 the double six-sided pyramid, would be the 

 starting point, of an entirely distinct system of 

 forms. Within its limits there might be in- 

 numerable sets as before. It would be, as it 

 were, the type of a new order of crystalline architec- 

 ture, susceptible of variations consistent with the ge- 

 neral style. 



