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thru " son it- <>t' the less stable groups have broken up and assumed new configura- 

 tions, but it is quite possible that others more stable may still retain their original 

 configurations, so that the form of the body is determined by the equilibrium between 

 these two sets of groups : but if, on account of rise of temperature, increase of 

 moisture, violent vibration, or any other cause, the breaking up of the less stable 

 groups is facilitated, the more stable groups may again assert their sway, and tend to 

 restore the body to the shape it had before its deformation." 



The semi-plasticity exhibited by recently overstrained iron may thus, on the 

 above theory, be attributed to the less stable groups, which after overstrain are in 

 comparative abundance. And since these less stable groups will tend to break up 

 of themselves, there will be a slow recovery through lapse of time towards elastic 

 behaviour which is associated with the idea of stable groups. 



Increase of temperature has been shown in the present paper to hasten recovery 

 from overstrain to a remarkable extent. This, as indicated by the quotation given 

 above, may be ascribed to a greater facility given by slight warming, to the breaking 

 up of the less stable groups, and possibly to the re-formation of more stable groups. 



Violent mechanical vibration, however, seems to break up the rather more stable 

 groups, rendering the material more semi-plastic and hindering the recovery process. 



That recovery from overstrain, or more generally, that the phenomenon of " elastic 

 after-action," is associated with complexity in the physical structure of the material, 

 is further borne out by the fact that a crystalline body, such as a quartz torsion 

 fibre, exhibits little or no after-action (in the form of zero-creeping) ; while a complex 

 body like glass shows such action in marked degree. An analogy to this difference 

 in the behaviour of material, according as it is simple or complex, is found in the 

 phenomenon of the residual charge in the Leyden jar. Condensers with pure dielec- 

 trics such as sulphur, quartz, air, exhibit little or no residual charges ; while with 

 complex substances like glass, gutta-percha, caoutchouc, the phenomenon is par- 

 ticularly observable. 



