8 EwiNG, The Structure of Metals. 



We pass on now to another point which is of primary 

 interest. How is it that with this crystalline structure in 

 the grains a metal is capable of being plastic ? We 

 know, for instance, that if we stretch a bar of good iron, 

 or, to take an extreme case, if we draw a piece of iron 

 into wire, it is capable of being changed in form enor- 

 mously without actual rupture. We can bend a plate or a 

 bar, we can stretch it, we can draw it into wire, we can 

 forge it cold into all sorts of shapes in virtue of the 

 property of plasticity which has somehow or other to be 

 reconciled with the fact that it is an aggregate of crystals. 

 How do the crystals behave when the form of the metal 

 is altered ? Examine in the microscope a piece of iron 

 which has been stretched until it has broken in the 

 testing machine. You find that the crystal grains are 

 there just as before, but they now have a predom- 

 inantly long direction one way as compared with the 

 other. Taking a general view over them, although they 

 are very irregular in shape and size, you can easily see 

 that they tend to be longer in the direction in which the 

 piece was stretched. The meaning of that is, each indi- 

 vidual grain has actually been extended in the direction 

 in which the iron has been stretched as a whole. How is 

 it that the crystal grain is capable of being extended, and 

 how is it that the grain remains a crystal after such exten- 

 sion ? Here {Fig. 8, Plate IV) is a very striking example 

 which shows that it does remain a crystal. This is a micro- 

 photograph from the cross section of a bar which was 

 drawn down in the cold state from one inch in diameter 

 to half-an-inch in diameter. That means a very severe 

 straining of the iron, nevertheless the grains remain 

 crystal grains. The particular crystal, a part of which is 

 shown in this field, has been bent round into a shape 

 resembling that of a horse-shoe, but all over it there are 



