400 



Professor J. A. Ewing 



[May 22, 



to be. It falls over, and then its neighbours, weakened by the loss of 

 its support, follow suit, and gradually the disturbance propagates 

 itself from molecule to molecule throughout the group. In a very 

 thin piece of iron — a fine wire, for instance — there are so many 

 surface molecules, in comparison with the whole number, and con- 

 sequently so many points which may become origins of disturbance, 

 that the breaking up of the molecular communities is too soon over 

 to allow much of this kind of lagging to be noticed. 



Effects of temperature, again, may be interpreted by help of the 

 molecular theory. When iron or steel or nickel is heated in a weak 

 magnetic field, its susceptibility to magnetic induction is observed to 

 increase, until a stage is reached, at a rather high temperature, when 

 the magnetic quality vanishes almost suddenly and almost completely. 

 Fig. 15, from one of Hopkinson's papers, shows what is observed as 

 the temperature of a piece of steel is grad ually raised. The sudden loss 

 of magnetic quality occurs when the metal has become red-hot ; the 



Fig. 15. 



C 1400 - 

 " ^ /200 - 

 _ t '000 - 



■ ^ aoQ - 



O CL 



f:< 600- 



^<^ 400- 



100 200 300 400 



Temperature 



500 600 700° C 



Effects of rising temperature on the magnetic inductive capacity of steel (Hopkinson). 



magnetic quality is recovered when it cools again sufficiently to cease 

 to glow.^ Now, as regards the first effect— the increase of suscepti- 

 bility with increase of temperature —I think that is a consequence of 

 two independent effects of heating. The structure is expanded, so 

 that the molecular centres lie further aj^art. But the freedom with 

 which the molecules obey the direction of any applied magnetic force 

 is increased not by that only, but perhaps even more by their being 

 thrown into vibration. When the magnetic field is weak heating 

 consequently assists magnetisation, sometimes very greatly, by hasten- 

 ing the passage from stage a to stage h of the magnetising process. And 

 it is at least a conjecture worth consideration whether the sudden loss 

 of magnetic quality at a higher temperature is not due to the vibra- 

 tions becoming so violent as to set the molecules spinning, when, of 

 course, their pcdarity would be of no avail to produce magnetisation. 

 We know, at all events, that when the change from tlie magnetic to 

 the non-magnetic state occurs, there is a profound molecular change, 



