320 wheirry: crystal form and optical properties 



tion, bringing out thereby the lack of a horizontal symmetry 

 plane. The space-lattice shows cube-centered arrangement 

 within the smallest cells, as well as in the whole structure. These 

 features are incorporated into figure 2. 



As this substance possesses a rather complicated space-lattice, 

 certain simplifications are introduced, such as the grouping of the 

 H atoms and the OH radicals. In addition, the symbols are 



HH — 



Fig. 2. Partial space-lattice of penta-erythrite. 



given only for atoms that occur on the surface, except in the 

 upper right forward cell, which is labeled completely; it is under- 

 stood that all the points in any one horizontal plane indicated by 

 dots or rings are occupied by the same atoms or groups that 

 occur where the planes reach the front of the diagram. The 

 space-lattice shown should be. increased one-third in each hori- 

 zontal direction to attain completeness. It will then contain 

 11 chemical molecules and 8^ = 512 of the smallest cells, and the 



