FIGURES OF CRYSTALS. 



435 



In drawing the secondary forms of crystals, it very 

 frequently happens that the law of decrement will 

 suggest some relation between the position of the 

 secondary edges or angles, and some known points or 

 lines of the primary form, which will supercede the 

 necessity of any directing diagram. One instance of 

 this will be seen if we turn to p. 420, where the 

 rhombic dodecahedron is derived from the cube, 

 through a previous knowledge of the relation of the 

 two forms to each other. 



And many expedients will probably occur to those 

 who are accustomed to draw crystals, which will 

 greatly abridge the laborious processes just described. 

 These will, however, form particular cases, and will 

 depend on the degree of attention and ingenuity 

 employed in framing the diagrams. 



The following figure will supply another example 

 of the delineation of a secondary form, from ascer- 

 taining its relation to the primary. 



Let it be proposed to circumscribe a cube with a 

 figure contained within 24 trapezoidal planes, be- 

 longing to mod. class b of the cube, the law of decre- 



