32 DEVELOPMENT AND HEREDITY. 



change in the phenomena to which these names are 

 given is accompanied by a change, either temporary 

 or permanent, in the matter affected by them, so 

 many recent experiments on magnetisation have 

 connected magnetic phenomena with a molecular 

 change in the subject-matter. Thus M. Wertheim 

 has shown that the elasticity of iron and steel is 

 altered by magnetisation, the coefficient of elasticity 

 in iron being temporarily, in steel permanently, 

 diminished. 



" He has also examined the effect of torsion upon 

 magnetised iron, and concludes from his experiments, 

 that in a bar of iron, arrived at a state of magnetic 

 equilibrium, temporary torsion diminishes the mag- 

 netism, and that the untwisting, or return to its 

 primitive state, restores the original degree of mag- 

 netisation. 



" M. Guillemin observed that a bar of iron, slightly 

 curved by its own weight, is straightened by being 

 magnetised. Mr. Page and Mr. Marrion discovered 

 that a sound is emitted when iron or steel is rapidly 

 magnetised or demagnetised, and Mr. Joule found 

 that a bar of iron is slightly elongated by magnet- 

 isation. 



"Again, with regard to diamagnetic bodies, Mr. 

 Matteucci found that the mechanical compression of 

 glass altered the rotary power upon a ray of polarised 

 light which it transmitted." 



