218 



ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1920. 



and calcium, cadmium and nickel oxides, 31 all have this same struc- 

 ture. Galena (lead 

 #Cu °° sulphide) exhibits 



the same grouping. 

 Zinc blende (zinc 

 sulphide), on the 

 other hand, has a 

 different structure. 

 (Fig. 24.) In this 

 case each zinc atom 

 is surrounded by 

 four equally dis- 

 tant sulphur atoms, 

 while each sulphur 

 atom has about it 

 four zinc atoms. 

 Zinc oxide exhibits 

 a still different 

 grouping of atoms, 

 but again each zinc 

 atom is surrounded 

 by four oxygen 

 atoms, while each oxygen atom has about it four zinc atoms. Cad- 

 mium sulphide and 

 wurzite (hexagonal 

 zinc sulphide) have 

 the same atomic ar- 

 rangement as zinc 

 oxide. 32 



The immediate 

 conclusion to be 

 drawn from these 

 structures is that 

 there is no unique 

 connection between 

 the arrangement of 

 the atoms compos- 

 ing them and their 

 chemical valencies. 

 A qualitative ex- 

 planation of the 

 structures of these 

 compounds is nevertheless possible. From other evidence it has 



Fig. 23. — The cuprous oxide arrangement. Each monovalent 

 copper atom is surrounded by two equally distant oxygen 

 atoms and each divalent oxygen atom by four copper atoms. 

 The white atoms alone have the body-centered grouping. 



Of— 



Hm 



'C 



Fig. 24. — The zinc sulphide arrangement. In this arrange- 

 ment each white atom is distant from four black atoms 

 and each black atom from four white ones. 



31 W. P. Davey and E. O. Hoffman, Phys. Rev., 15, 333, 1920. 

 82 W. L. Bragg, Phil. Mag. (6), 39, 647, 1920. 



