CRYSTALLOGRAPHY 



17 



. arc interchangeable ami equal. The axes are designated, 

 a : a : c. 



III. Hexagonal. Includes all those crystals which may be 

 referred to four axes, three of which are equal and interchangeable, 

 being in the same plane at an angle of 60 with each other ; all are 

 at 90 to the fourth, or c axis. The axes are designated, ai : a 2 : a 3 : c. 



IV. Orthorhombic. Includes all those crystals which may be 

 referred to three unequal axes, all at right angles. The axes are 

 designated, 4 : b : 6. 



V. Monoclinic. Includes all those crystals which may be 

 referred to three axes, all unequal; two of these, the lateral axes, 

 are at right angles to each other. One of these is at right angles 

 to the third, or c, axis ; the other is inclined. The axes are desig- 

 nated, a : 6 : c. 



VI. Triclinic. Includes all those crystals which may be re- 

 ferred to three axes, all unequal and all inclined. They are desig- 

 nated, & : b : c. 



Some are accustomed to add to these six systems a seventh system, 

 the rhombohedral or trigonal system, referred to axes parallel to 

 the edges of the rhombohedron. The forms included in this sys- 

 tem are very closely re- , 

 lated to the hexagonal 

 system, and can be in- 

 cluded in that system 

 equally as well. 



Parameters. The 

 distance from the origin 

 at wljich any plane, or 

 face, cuts a crystallo- 

 graphical axis, Fig. 22, 

 as ob', is the intercept of 

 that plane a'bV on the 

 axis b. This intercept 

 expressed in terms of 

 the unit on that axis, 

 and written as a co- 

 efficient of the symbol FIG. 22. Axial Intercepts, 

 standing for or repre- 

 senting the axis, is the parameter of the plane on the axis. If ob is 



the unit on the axis b, then ^-, in this case 8, 8b, is the parameter 



ob 



