194 



Professor W. Lawrence Bragg 



[May 28, 



of X-rays of known wave-length falls on the crystal, and it is found 

 that when the beam makes a certain glancing angle with the planes 

 of the structure it is strongly diffracted. By measuring this angle it 

 is possible to calculate the distance between the planes of the crystal 

 structure, just as the distance between the lines of the grating is 

 obtained by employing monochromatic light. The distance between 

 the planes in all directions is thus measured up, and leads to the 

 fixing of the atoms in the crystal structure at their intersections. In 

 this way the crystalline arrangements of some twenty or thirty 

 elements, of compounds such as NaCl,MgO, ZnS, the carbonates, the 



S2 



5A- 



< 



3-80A 



Fig. 2. 



spinel group of minerals, the alums, the oxides isomorphous with 

 ruby, pyrites, fluor, galena and many others have been analysed. F " 

 3. In many simple crystalline substances the atoms are so arranged 

 that their exact positions are determined by the symmetry of the 

 crystal. In the diamond, for example, each carbon atom is at the 

 centre of four other carbon atoms. In the cubic crystal of potassium 

 chloride, the atoms are arranged so that potassium and chlorine atoms 

 alternate at the corners of the cubes in the structure. Every potassium 

 atom is surrounded symmetrically by six chlorine atoms, every 

 chlorine atom by six potassium atoms. The atoms cannot be displaced 



