1 8 EwiNG, The Structure of Metals. 



see-saw sets up a large amount of movement in the 

 adjacent molecules, and sets up dissenting communities 

 which spread themselves on both sides of the plane of 

 slip ; so that not only molecules immediately adjacent to 

 the plane of slip are affected, but molecules extending for 

 some distance inwards on both sides. An interesting 

 confirmation of this is given by the fact (noticed by 

 Rosenhain) that after you have fatigued a piece by 

 repeated straining backwards and forwards, and then 

 polish and etch it, you find a marking which recalls the 

 presence of slip lines, although you have a perfectly 

 fresh etched surface to examine. This means that the 

 metal has really had a difference in structure set up in 

 it by the repeated alternations, a difference in structure 

 which is sufficient to manifest itself in the etching. That 

 difference, I believe, is due to a corresponding per- 

 manent disturbance of those molecules which are adjacent 

 to the planes in which slip has occurred. 



The time is too short for me to attempt a complete 

 account of all the points which are explainable by this 

 theory. I will only mention one more. Overstraining 

 causes a species of fatigue in a metal which shows itself 

 conspicuously in a loss of elasticity. After this fatigue 

 has been set up it is possible, by means of rest cure, to 

 restore the elasticity which has been lost. It is a well- 

 known fact, that if you take a piece which is beginning 

 to be fatigued, and give it a rest, it recovers, and you can 

 accelerate that recovery very much by raising the tem- 

 perature, as, for example, by such treatment as dipping 

 it into hot water. There is a remarkable instance of 

 this in a practice that is probably familiar to many now 

 present, when you dip a razor into hot water after strop- 

 ping it. The stropping of a razor means the alternate 

 bending of the edge to one side and the other. That 



