1890.] Molecular Theory of Indwed Magnetism. 351 



Rayleigh has shown,* is what happens in a magnetic metal, and the 

 susceptibility with respect to small cyclic changes is small in the 

 model, just as it is in the actual solid. 



The chief facts of permeability and retentiveness, and hysteresis 

 generally, are therefore at once explicable by supposing that Weber's 

 molecular magnets are constrained by no other forces than those due 

 to their own mutual magnetic attractions and repulsions. No arbi 

 trary constraining forces are required. In the model the centres of 

 rotation are fixed ; in regard to the actual solid we may make an 

 equivalent supposition, namely that the distances between the mole- 

 cular centres do not change (except in so far as they may be changed 

 by strain). 



Hysteresis, then, is not the result of any quasi-frictional resistance 

 to molecular rotations; it occurs whenever a molecule turns from 

 one stable position of rest to another through an unstable con- 

 dition. When it is forced to return it again passes through a 

 condition of instability. This process, considered mechanically, is not 

 reversible ; the forces are different for the same displacement going and 

 coming, and there is dissipation of energy. In the model the energy 

 thus expended sets the little bars swinging, and their swings take 

 some time to subside. In the actual solid the energy which the 

 molecular magnet loses as it swings through unstable positions 

 generates eddy currents in surrounding matter. Let the magnets of 

 the model be furnished with air- vanes to damp their swings, and the 

 correspondence is complete. 



A regular group of elementary magnets, especially when furnished 

 with air-vanes, gives a good illustration of what has been called 

 magnetic viscosity. When the imposed force reaches a critical 

 value one of the outer members of the group becomes unstable, and 

 swings slowly round; its next neighbours, finding their stability 

 weakened, follow suit, and the disturbance spreads through the 

 group in a way eminently suggestive of those phenomena of time-lag 

 in magnetisation which I have described in a former paper. f 



The model shows equally well other magnetic phenomena which 

 presumably depend on the inertia of the molecules, such as the fact 

 that a given force causes more magnetic induction when suddenly 

 applied than when gradually applied, and leaves less residual 

 magnetism when suddenly removed than when gradually removed. 



The well known effects of mechanical vibration in augmenting 

 magnetic susceptibility and reducing retentiveness are readily explic- 

 able when we consider that vibration will cause periodic changes in 

 the distances between molecular centres. This has not only a direct 

 influence in making the molecular magnets respond more easily to 



* 'Phil. Mag.,' March, 1887. 

 t ' Boy. Soc. Proc.,' June, 1889. 



