BEHAVIOUR OF OVER-STRAINED MATERIALS 481 



in solid bodies subjected to stresses greater than the 

 elastic limit, thus presenting similarities to the well-known 

 phenomenon of magnetic hysteresis. Prof. Ewing, whose 

 theory of magnetism has been so successful, imagines the 

 resemblance between the two phenomena to be so close that 

 they may be accounted for by very similar molecular theories. 

 In his presidential address to Section G of the British 

 Association x this year, he conceives the molecules in an elastic 

 body as being endowed with certain polarities whose mutual 

 actions cause the molecules to arrange themselves, in an 

 unstrained body, in a very stable order. Small stresses effect 

 relative movement between them, but they are not sufficiently 

 disturbed to destroy the stability of their configuration, and 

 they return to their previous position when the stress is 

 removed. But if the body is subjected to a stress great enough 

 to produce too great relative motion, the arrangement of the 

 molecules is no longer stable, and slipping occurs. (This 

 slipping has actually been observed with the aid of a micro- 

 scope.) This represents the stage at which the elastic limit is 

 exceeded, and the deformation is gradually increasing. When 

 the stress is removed, the molecules do not regain their original 

 configuration, but arrange themselves in the stable order which 

 happens to be nearest their present position. This movement 

 corresponds to the immediate recovery on release, which is, 

 however, not complete, and the gradual recovery which follows 

 may be regarded as due to slow movements of the molecules 

 into more stable positions. 



A further physical phenomenon which is analogous is that of 

 phosphorescence. A body which possesses this property, after 

 being exposed to the action of light, continues for some time to 

 emit light on its own account when placed in darkness. The 

 intensity of this light falls off with the lapse of time. Mr. 

 Whiteside, at the British Association this year, reported some 

 experiments, the object of which was to examine this decay of 

 intensity. He finds that if I denote the intensity at any time t 

 after the phosphorescent body has been placed in darkness, its 

 value can be denoted by the equation 



a + bt 



It is worthy of note, in this connection, that phosphorescence 



1 Prof. J. A. Ewing, Phil. Mag., September 1906. 



