﻿258 Dr. J. A.. Ewino- on the 



ft 



and etched it appears, under the microscope, to be to all intents 

 and purposes as regular in the tactical grouping o£ its elements 

 as any other crystal. 



Further j in the process of straining we have, first, an elastic 

 stage, extending through very small movements, in which 

 there is no dissipation of energy and no permanent set. When 

 this is exceeded, slip occurs suddenly; the work done in 

 straining is dissipated; if the straining force is removed a strain 

 persists, forming a permanent " set"; if the force continues to 

 act it goes on (within certain limits) producing augmented 

 strain. In general a large amount of strain may take place 

 without the cohesion between the gliding surfaces being 

 destroyed. Immediately after the strain has occurred there 

 is marked fatigue, showing itself in a loss of perfect elasticity; 

 but this will disappear with the lapse of time, and the piece 

 will then be harder than at first. If, on the other hand, a 

 process of alternate straining back and forth be many times 

 repeated, the piece breaks. 



These are now familiar facts. Can we attempt to explain 

 them on the basis of a molecular theory which will at the 

 same time offer a clue to the process of crystal-building as 

 we find it in metals ? I venture to make this Address the 

 occasion of inviting attention to some more or less speculative 

 considerations which may be held to go some little way towards 

 furnishing the material for such an explanation 



At the Leeds Meeting of this Association, in 1890, it was 

 my privilege to bring forward certain contributions to the 

 molecular theory of magnetism, and to show a model which 

 demonstrated that the rather complex phenomena of magneti- 

 zation were explainable on the very simple assumption that 

 the magnetic molecules are constrained by no other forces 

 than those which they mutually exert on one another in con- 

 sequence of their polarities*. From this were found to 

 result all the chief phenomena of permeability and magnetic 

 hysteresis. Let us attempt to-day to apply considerations of 

 a similar character to another group of physical facts, namely, 

 those that are associated with the crystalline structure of 

 metals and with the manner of their yielding under strain. 

 Just as in dealing with magnetic phenomena, I take as 

 starting-point the idea that the stability of the structure is 

 due to mutual forces exerted on one another by its elementary 

 parts or molecules, and that the clue to the phenomena is to 

 be sought in the play of these mutual forces when displacement 

 of the molecules occurs. 



* " Contributions to the Molecular Theory of Induced Magnetism," 

 Proc. Roy. Soc. vol. xlviii. June 19, 1890, or Phil. Mag. September 1890. 



