356 Prof. J. A. Ewing. Contributions to the [June 19, 



and the contraction of transverse lines will not only alter the stability 

 of those molecules which continue to lie in their original rows, but 

 will tend to make the members of those rows which are much 

 lengthened swing round and form transverse 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 may be 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 magnetising force be applied, and then begin to 

 apply stress. The first effects are in general large, for the strain pre- 

 cipitates into instability those molecular magnets which were already 

 on the verge of instability. This is beautifully apparent in iron (see 

 ' Phil. Trans.,' 1885, 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 applica- 

 tions 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 temperature change 

 either on induced or residual magnetism. Again, the theory shows 

 that when a cyclic change of stress is repeated, there will be hyste- 

 resis in the corresponding changes of magnetism, whether induced or 

 residual, unless either the cyclic range is very small or the magnetism 

 approaches saturation. During each application of the stress some 

 molecular magnets will swing through unstable positions ; 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 

 magnetisation 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 with 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 loaded and partially unloaded down to a given load before 



