CRYSTALLOGRAPHY 



139 



as to the orientation of the molecule in its surroundings. 

 Let us take as a definite example the oxide of magnesium, 

 periclase (MgO), whose molecules are to be thought of in 

 a determinate arrangement, belonging to the regular 

 system. Then we may ask whether the line joining an 

 atom of magnesium and one of oxygen is parallel to one of 

 the axes of the regular system, or perhaps corresponds to 

 the direction of an octahedral edge, &c. 



The two problems are treated separately in what follows, 

 as : 



A. Relative position of molecule centres in the crystalline 

 figure. 



B. Orientation of the molecules in the crystal. 



A. Relative Position of Molecule Centres in the 

 Crystalline Figure. 



i. The Fundamental Law of Geometrical Crystallography 1 

 (F. C. Neumann). 



Observation shows that normally grown crystals have 

 the form of polyhedra, whose flat bounding faces cut each 

 other at definite an- 

 gles, while the area 

 of the faces depends 

 on accidental circum- 

 stances. The crystal- 

 line form of a body is 

 therefore determined 

 by the angles, i. e. by 

 the relative position 

 of the faces by which 

 it is bounded. With 

 regard to this position, 

 however, observation 



shows further that 

 not every possibility is 



1 Groth, PJnjsikalische Krystallographie, 3rd ed. Leipzig, 1894. 



Fig. 28. 

 found, but that the form of 



