DEFORMATION AND SLIP IN CRYSTALS 59 



essentially unchanged whole layers of the crystal 

 slide over one another bodily, and by a sufficient 

 number of such slidings the crystal can adapt 

 itself to the new shape imposed upon it. The way 

 in which the slipping of layers, which themselves 

 remain unchanged in size and shape, can alter 

 the shape of the whole pile is readily seen by 

 piling a number of books upon one another; if 

 now each book is pushed slightly to one side, the 

 pile as a whole becomes very much changed in 

 shape, although each individual book remains 

 unchanged. If this slipping constituted the whole 

 of the process, then we should expect that mechani- 

 cal change of shape would not materially alter 

 the properties of a piece of metal. Actually, 

 however, we find that its properties are very much 

 changed. If a piece of ductile copper, for instance, 

 be drawn out into wire, it gradually increases in 

 strength and hardness but loses in ductility until 

 finally it becomes so brittle that it can be drawn 

 out no further without breaking. The causes of 

 this hardening have been studied, and it has been 

 found that the reason lies in the fact that on 

 the surfaces where crystal layers slide over one 

 another the intimate structure of the crystal 

 itself becomes disturbed. The mere process of 

 polishing the surface of a crystal whether of 

 metal or of other material is found to be sufficient 

 to disturb and more or less to destroy the crystalline 

 structure of a layer of appreciable depth, so that 



