﻿Molecular Structure of Metals. 263 



to him three N poles and one S pole. He has the choice of 

 conforming to the majority, or of throwing in his lot with the 

 dissenters ; and he has a third possible position of equilibrium 

 (very feeble equilibrium) which is reached when his two 

 S poles are turned until the one neighbouring south pole 

 faces just between them. 1 have laboured these points a 

 little because they seem important when we come to speak of 

 the effects of strain. 



Consider now a straining action, which we may imitate 

 in the model by sliding one part of the group past the other 

 part. For this purpose the centres on which the '' molecules " 

 are pivotted are cemented to two glass plates which can slide 

 parallel to one of the axes. 



At first, when the displacement by sliding is exceedingly 

 small, the strain is a purely elastic one. The molecules 

 adjacent to the plane of sliding pull one another round a little, 

 but without breaking bond, and if in this stage the strain is 

 removed, by letting the plate slide back to its original position, 

 there is no dissipation of energy. The work done in dis- 

 placing the molecules is recovered in the return movement. 

 We have here a representation of what happens between each 

 pair of adjoining rows in the elastic straining of a metal. So 

 far the action is within the limit of elasticity; it leaves no 

 permanent effect : it is completed}- reversible. 



But now let the process of straining be carried further. The 

 opposing molecules try to preserve their rows intact, but a 

 stage is reached when their resistance is overcome ; the bonds 

 are broken, and they swing back, unable to exert further 

 opposition to the slip. The limit of elasticity has now been 

 passed. Energy is dissipated ; set has been produced ; the 

 action is now no longer reversible. The model shows well 

 the general disturbance that is set up in the molecules adjoining 

 the plane of slip, which we may take to account for the 

 work that is expended in a metal in producing plastic strain. 



Moreover, when the slip on any plane stops and the mole- 

 cules settle down again, the chances are much against their 

 all taking up the normal oiientation which they had before 

 the disturbance. What I have called dissenting groups or 

 unstable coteries are formed as a result of the disturbance. 

 Here and there like poles are found in juxtaposition. Viewed 

 as a whole, the molecular constitution of the metal in the 

 region adjacent to the plane of slip is now uncertain and 

 patchy- It includes parts whose stability is much less than 

 normal. Individual molecules or small groups in it are very 

 feebly stable ; a touch would make them tumble into positions 

 of greater stability. 



Observe how all this agrees with what we know about the 



