58 MODERN SCIENCE OF METALS 



are merely the sections of polygonal grains or 

 particles are really small crystals of the metal. 

 These crystals are quite as truly crystals as the 

 beautiful and perfectly geometrical pieces of rock- 

 crystal or alum or sugar with which we familiarly 

 associate that term; they have, however, grown 

 up together and the growth of each, instead of 

 finding room to complete itself to full geometrical 

 perfection, has been limited by the presence of 

 its neighbours. The polygonal boundaries which 

 we see in our sections are thus merely the accidental 

 surfaces upon which adjacent growing crystals 

 came into contact with one another. 



This fact alone that metals are aggregates of 

 an immense number of minute crystals constituted 

 a discovery of fundamental importance, particularly 

 when it came to be shown how these crystals behave 

 when the metal is subjected to mechanical deforma- 

 tion such as bending or stretching and ultimately 

 to fracture. While crystals of salt or sugar are 

 fairly hard, they are very brittle any attempt to 

 bend or squeeze or stretch them either fails to 

 produce any permanent change of shape or causes 

 immediate fracture. The crystals of many metals, 

 however, are quite easily deformed they can be 

 squeezed or stretched; the mechanism by which 

 they undergo deformation, however, is of a peculiar 

 kind. Each crystal undergoes a change of shape 

 in the first place merely by a process of slipping 

 which in itself leaves the structure of the crystal 



