322 wherry: crystal form and optical properties 



the number of atoms represented in a square occupied at 'both 

 corners and centers of edges is figured out, it is found to be three; 

 or if the atoms are moved from the corners part way toward the 

 center — in the figure they are half way^ — ■, six prove to be present. 

 The latter arrangement seems to fill the requirements in the pres- 

 ent case the best. 



The R groups are regarded, then, as lying in planes, with the 

 Al atoms arranged, as required by the symmetry, along the verti- 



Fig. 3. Space-lattice of mellite. 



cal axis between them. A sort of spool-shaped structure is there- 

 by produced, and it is only natural that water of crystallization 

 should, as it were, take the place of the thread. The 18 mole- 

 cules of this can best be arranged in three layers of six each. 



If the atoms of the (C.COO) group are actually spread out in 

 the same vertical layers, occupying nodes where these intersect 



