WycJtoff — Crystal Structure of Magnesium Oxide. 139 



molecules associated with the unit cube of magnesium 

 oxide was found in the usual manner from the density 

 and a spectrum measurement from a principal face. 



7i' _/ 2 sin e V J/ 

 ?n ~\ \ 



V 



where 



m =^ the number of molecules of MgO in the unit cube, 



n =^ the order of the reflection, 



A =z the vrsiYe length of the reflected X-rays, 



^ the angle of the reflection, 



p z= the densit}^ of MgO, 

 M = the weight of one molecule of MgO. 



Figure 2. 



Magnesium oxide crystallizes in the cubic system. 

 From the meager data at hand, its symmetry seems to be 

 holohedral; but for completeness any one of the ^n^ 

 classes will be considered as possible. The number of 



