CRYSTALLOGRAPHY 



ml perfection of the natural polish of cry.-tal faces and eleav- 

 ur faces. 

 It a crystalline molecule is placed at each corner of a cube, the 



distance between each molecule will be measured by the length of 

 the edge, Fig. t a. The cube may be considered aii elementary form 

 or unit of a homogeneous 

 point-system which may 

 be built up, as in Fig. 5. 

 If the cubical unit is 

 lengthened in one direc- 

 tion, it will now possess 

 edges of two different 

 values, Fig. 4b. This 

 unit when packed to- 

 gether so as to fill space 

 will produce a regular 

 point -system of another 

 typo, Fig. 6. The sec- 

 ond unit may now be 

 broadened, when it will 

 possess edges of three 

 different values, Fig. 4 c, 

 and when packed to- 

 gether will fill space, pro- 

 ducing a point system of 

 still another type. There 

 are fourteen such ele- 

 mentary units, which 

 when packed together 

 will fulfill the crystalline 

 requirements of com- 

 pletely filling space, and 

 place each molecule of 

 the system in such a position that its surroundings shall be 

 exactly the same as the conditions surrounding every other 

 molecule. 



The shape of the fourteen elementary units must in no way be 

 considered to represent the shape of the molecule, as the space 

 between molecules is far greater than the diameter of the molecule, 

 and there is no method by means of which the shape of a molecule 

 can be determined. These dimensions of space may be considered 



Fio. 5. 



_. 6 T 



-6 



Fia. 6. 



