KYSTALUMiKAPHY 



23 



zonal axis. Fig. :U. In the study of crystal faces it will be found 

 that they all belong to a comparatively few zones. The intersec- 

 tion of any two faces mi 

 a crystal will determine 

 the direction of a pos- 

 sible /onal axis. Faces 

 belonging to the same 

 zone must be so related 

 that two of their inter- 

 cepts will bear a con- 

 stant relation, and their 

 intersections with the 

 axial plane in which 

 these two intercepts are 



measured will be paral- 

 lel lines. In Fig. 32 

 four faces belonging to 

 the same zone are rep- 

 resented and extended to the axes a and b ; the ratio of these in- 

 tercepts is easily understood from the similar triangles, and the 



intersections of 

 all the faces with 

 the axial plane 

 aob are parallel 

 lines. A zone may 

 be interrupted at 

 any point by the 

 interposition of 

 other faces not 

 belonging to that 

 zone. Zonal re- 



FIG. 31. Crystal of Topaz in which the Faces 

 c, i. u. o, e, and m are in the Same Zone, the Axis 

 of which is aa'. 



lations help very 

 materially in the 

 measurement of 



crystals, for once a face has been located as a member of a zone, 

 its parameters when determined must fulfill the zonal relations. 



Fundamental forms. Among the faces found on the crystals 

 of any substance, a face which cuts all three axes, and is simply 

 related to all other faces occurring on the crystals, is selected ; its 

 intercept on each axis is taken as the unit of measurement on that 

 axis ; its parameters would be a : b : c ; the form is termed the 



