6o MODERN SCIENCE OF METALS 



it is not surprising to find that where two adjacent 

 crystal layers move over one another under the 

 heavy pressures which must exist within a metal 

 undergoing plastic deformation, similar but more 

 intense disturbance occurs. The essence of crystal- 

 line structure is the regular arrangement of the 

 atoms on certain geometrical patterns which allow 

 among other things of the existence of "gliding 

 planes" or surfaces upon which this sliding of 

 layers can take place. Where the sliding has 

 actually taken place, however, this regular arrange- 

 ment is locally destroyed and with it the " gliding 

 planes." Further gliding is thus rendered more 

 difficult and the metal appears to be harder but 

 also more brittle than before. Gradually, as the 

 crystalline structure becomes more and more 

 deeply disturbed, no power of gliding remains, 

 and if sufficient force is applied the crystal or 

 such remnants of it as remain breaks without 

 further change of shape; in this state the metal 

 is hard and brittle and behaves in much the same 

 way as a material like glass, which is devoid of 

 any regular structure. 



Up to this point we have followed in outline 

 the course of a purely scientific discovery, none 

 the less purely scientific in character because it 

 deals with the inner meaning of phenomena which 

 occur in daily technological practice, when metals 

 are "worked" in the cold into many complicated 

 shapes. We shall now see how a result of the 



