l-Jrl Wyckoff — Crystal Stniciure of Magnesium Oxide. 



of doubt concerning the exact connection between the 

 degree of symmetry as exhibited by the external form and 

 especially by the etch figures and the internal arrange- 

 ment of atoms. In the present instance there is no 

 reason for believing that magnesium oxide is not holo- 

 hedral. If this is true and if n really equals four, then 

 its structure must be assigned to arrangement [c), or of 

 course (g), on the basis of synnnetry considerations 

 alone. On account of the possible uncertainty, however, 

 concerning the class of crystal s^amnetry to which 

 magnesium oxide should be assigned, not only the holo- 



FlGURE 4, 



hedral arrangements but all of those exhibiting cubic 

 symmetry will be compared with experiment. 



The following experimental data are available for dis- 

 tinguishing between the possible structures for magne- 

 sium oxide. The results from the powder photographs-^ 

 are said to be in entire agreement with the ''sodium chlor- 

 ide arrangement" {c). Besides the reflection from the 

 (100) face which was used in finding the amount of mass 

 associated with the unit cube, a series of Laue photo- 

 graphs with the rays normal to the (100) face and 

 inclined as small angles to this normal, was prepared. 

 These photographs {^g. 1), some of which are the 

 results of very long exposures and were prepared in order 

 to give faint reflections a chance to register, were studied 



^ W. p. Davey and E. O. Hoffman, op. cit. 



