34 EEPORT — 1891. 



to mention here : If an iron wire be heated to a bright-red 

 heat, and allowed to cool in the air, you will see, if you watch 

 it carefully, that just when it has become of a very dull red 

 colour it suddenly glows out quite bright, and afterwards cools 

 regularly. Some molecular change must occur here in which 

 latent heat is liberated, for the iron, though giving out heat by 

 radiation, suddenly increases in temperature. 



Every circumstance that affects the molecular condition 

 of iron has a considerable effect on its magnetic properties. 

 Mechanical vibration, stress, even light, falling on iron affects, 

 and admixture with other metals in a wonderful degree changes, 

 its magnetic properties. Iron with 10 per cent, of manganese 

 is almost non-magnetic. 



Within the last few months Ewing, who had already made 

 his name famous by his researches on the subject about which 

 I have just been speaking, has crowned these labours by giving 

 to the world a new theory of magnetism — a theory that 

 works, or, in other words, explains the known facts of mag- 

 netism. 



All the older theories have failed : Poisson's, in which every 

 molecule of a piece of iron contained two magnetic fluids that 

 were separated on magnetization ; Weber's, in which the 

 molecules of a magnetic body were pernianent magnets whose 

 axes lay in all directions irregularly in an unmagnetized body, 

 while the act of magnetization imparted a certain amount of 

 direction to them, they at the same time offering a certain 

 resistance to change of direction ; Maxwell's, which is merely 

 Weber's with the additional proviso that if the molecules by 

 any chance got turned through more than a certain angle they 

 were unable to return to their original positions ; Hughes's 

 chain-theory, in which the molecules of unmagnetized iron are 

 magnets arranged in groups forming closed chains, and thus 

 exert no external action, while the act of magnetization breaks 

 several of these chains, and thus a certain amount of magnetic 

 moment is imparted to the mass. 



From what I have already said on magnetic induction it is 

 evident that any magnetic theory has a great number of facts 

 to account for. It must be in agreement with what is known 

 about permeability, residual magnetism, hysteresis, the effects 

 of stress, vibration, and temperature. Poisson's theory ex- 

 plained nothing. He was righc only in his idea that each mole- 

 cule was a magnet. Weber's accounted for the saturation 

 limit of magnetization ; while Maxwell's only added the ex- 

 planation of residual magnetism. 



Ampere did not propose a theory of magnetism. He only 

 stated that each molecule was a magnet, its magnetism being 

 due to electric currents circulating incessantly, flowing round 

 it, in fact, in a circuit of no resistance. With this conception 



