324 wherry: crystal form and optical properties 



result when large numbers of both barium and lead atoms are 

 present disturb the refraction effects of the layers in which they 

 lie in one direction, whereas when atoms of one kind are in con- 

 siderable excess the layers, as it were, smooth out, and their effect 

 on the refraction is normal. 



One of several possible interpretations of the structure of these 

 salts is shown in figure 4. It is easy enough to arrange two metal 



Fig. 4. Space-lattice of Ca-Sr propionate. 



atoms of one kind and one of another kind into a group with 

 tetragonal symmetry — the latter may occupy the corners of a 

 square, the former the centers of its edges (only | of the first and 

 J of the second belonging to the square). But the propionic acid 

 radical CH3.CH2.CO is rather clumsy to handle in a space- 

 lattice. The mean positions it probably occupies are indicated 



