1 6 EwiNG, The Structure of Metals. 



Text-fig. 4. Now notice how I establish a community of 

 two dissenters. They are just keeping one another in 

 countenance and no more. They have very little stability 

 indeed. But now I increase the number of dissenters 

 considerably, and the stability of the dissenting group is 

 clearly strengthened. 



If you have a dissenting group set up in any way, 

 they are likely to show that they are different from 

 the rest when you come to etch the metal. They are 

 less stable than the rest, and consequently one may 

 anticipate that they will be more readily attacked by 

 any etching re-agent. Probably that is the explanation 

 of those geometrical pits which we see when we etch the 

 polished surface of forged iron. It is an interesting 

 point that when you come to etch an iron surface, even 

 although the rest of the iron is scarcely attacked at all, 

 here and there the iron is so strongly acted on that a hole 

 is formed, which has a well-defined rectangular shape. I 

 imagine that hole was originally a dissenting community 

 which has yielded to the acid more readily than the rest. 

 To that extent the crystal was imperfect : the condition 

 of absolutely uniform tactical grouping was not rigorously 

 fulfilled ; for a dissenting group introduces a want of 

 homogeneity within the grain in which it occurs, the 

 general nature of which will be apparent when one 

 compares Text-fig. 5 with the perfectly regular tactics of 

 the preceding figure. 



Passing from that, I want to show you how this 

 theory allows us to explain the characteristics of straining. 

 First of all, it is quite obvious that a crystal of this sort is 

 capable of transmitting any elastic disturbance. Let us 

 displace one particular molecule by turning it some way 

 round, and letting it go suddenly. You see that in 

 swinging back it starts a wave which is transmitted 



