220 Prof. J. A. Ewing on the Molecular 



to 



rows which are much lengthened swing round and form trans- 

 verse lines in which they will be more stable than before. 

 We may therefore reasonably expect that the permeability 

 with regard to strong fields will be reduced by pull, as it 

 actually is both in iron and in nickel, though with regard to 

 weak fields the permeability maybe increased, as it is in iron. 



Again, the theory explains well why the effects of stress 

 are by no means the same (1) when the stress is applied first 

 and the magnetic force after, and (2) when the magnetic force 

 is applied first and the stress after. 



Let a moderate magnetizing force be applied and then begin 

 to apply stress. The first effects are in general large, for the 

 strain precipitates into instability those molecular magnets 

 which were already on the verge of instability. This is 

 beautifully apparent in iron (see Phil. Trans. 1885, part ii., 

 plates 63 and 64) ; and the theory shows why the first effects 

 are not reversible, why they do not disappear when the stress 

 is removed, and why it is only in subsequent applications and 

 removals of the stress that the magnetic changes become 

 cyclic. 



The same remark evidently applies to the first effects of 

 stress on residual magnetism ; also to the first effects of tem- 

 perature-change either on induced or residual magnetism. 

 Again, the theory shows that when a cyclic change of stress 

 is repeated, there will be hysteresis in the corresponding 

 changes of magnetism, whether induced or residual, unless 

 either the cyclic range is very small or the magnetism ap- 

 proaches saturation. During each application of the stress 

 some molecular magnets will swing through unstable posi- 

 tions; during the removal of stress they will swing back, but 

 not at the same values of stress. And it will be characteristic 

 of this hysteresis that the variation in magnetism will begin 

 slowly when the change from applying to removing stress, or 

 from removing to applying stress, begins. All this agrees 

 with the facts. 



Moreover, the theory shows that even in the absence of any 

 resultant magnetization a cycle of stress makes the molecular 

 configuration pass through a series of changes which will at 

 first not be cyclic, but will become cyclic when the stress-cycle 

 is repeated, and in which any intermediate value of the stress 

 will be associated w r ith different configurations during the 

 going and coming parts of the process. In other words, we 

 see that there will be hysteresis in the relation of molecular 

 configuration to stress when a cycle of stress is repeated. 

 Hence certain rather obscure effects which have been observed 

 in magnetic experiments ; for instance, where an iron wire is 



