262 Dr. A. C. Crehore on the Construction of the 



equilateral triangle, each axis being parallel to that of 

 and in the same direction, thus assisting the stability and 

 making the moments zero. 



Plane — 2, containing atoms 4, 5, and 6, ma}' be similarly- 

 studied, the whole plane making for stability, giving zero 

 moments. Similarly, each plane not yet considered in detail, 

 particularly planes — 1, zero, and +1, may be shown to give 

 zero moments and produce stability. Each atom lies in the 

 centre of a tetrahedron and the axes of the four corner atoms 

 are always in different directions from each other, parallel to 

 one of the lines joining a centre of some tetrahedron and a 

 corner of it. There are but four directions of axes of rotation 

 in the whole crystal determined by these four fundamental 

 medial lines of the tetrahedron. 



Fk. 4. 



The tetrahedron of which is the centre is omitted from 

 fig. 3 to avoid complication, but is shown in fig. 4. The 

 atom J, however, is in the centre of the tetrahedron 0, 1, 2, 3. 

 Since it is always true that the sum of the moments of an} r 



