Theory of Induced Magnetism. 215 



resistance to molecular rotations; it occurs whenever a mole- 

 cule turns from one stable position of rest to another through 

 an unstable condition. When it is forced to return, it again 

 passes through a condition of instability. This process, con- 

 sidered 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 ex- 

 pended 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 neigh- 

 bours, 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 magnetization 

 which I have described in a former paper*. 



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 augment- 

 ing magnetic susceptibility and reducing retentiveness are 

 readily explicable 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 changes of magnetic force by 

 reducing their stability during the intervals when they recede 

 from each other, but tends indirectly towards the same result 

 by setting them swinging. 



The effects of temperature which are common to the 

 three magnetic metals may be stated thus : — Let any moderate 

 magnetizing force be applied, not strong enough to produce 

 anything like an approach to magnetic saturation, and let 

 the temperature be raised. Then the permeability increases 

 until the temperature reaches a certain (high) critical value, 

 at which, almost suddenly, there is an almost complete dis- 

 appearance of magnetic equality. As regards the first effect, 

 * Roy. Soc. Proa, June 1889. 



