A LECTURE ON MAGNETISM. 371 



brought near or withdrawn. Fixing a definite pole in your 

 imagination, you must picture the precise arrangement of 

 the two fluids with reference to this pole. And you must 

 not only be well drilled in the use of this mental imagery 

 yourself, but you must be able to arouse the same pictures 

 in the minds of your pupils, and satisfy yourself that they 

 possess this power of placing actually before themselves 

 magnets and iron in various positions, and describing the 

 exact magnetic state of the iron in each particular case. 

 The mere facts of magnetism will have their interest im- 

 mensely augmented by an acquaintance with those hidden 

 principles whereon the facts depend. Still, w r hile you use 

 this theory of magnetic fluids to track out the phenomena 

 and link them together, be sure to tell your pupils that it 

 is to be regarded as a symbol merely a symbol, more- 

 over, which is incompetent to cover all the facts, 1 but which 

 does good practical service while we are waiting for the 

 actual truth. 



This state of excitement into which the soft iron is 

 thrown by the influence of the magnet, is sometimes called 

 " magnetization by influence." More commonly, however, 

 the magnetism is said to be " induced " in the soft iron, 

 and hence this way of magnetizing is called "magnetic 

 induction." Now, there is nothing theoretically perfect in 

 Nature : there is no iron so soft as not to possess a certain 

 amount of coercive force, and no steel so hard as not to be 

 capable, in some degree, of magnetic induction. The 

 quality of steel is in some measure possessed by iron, and 

 the quality of iron is shared in some degree by steel. It is 

 in virtue of this latter fact that the unmagnetized darning- 



1 This theory breaks down when applied to diamagnetic bodies, which 

 are repelled by magnets. Like soft iron, such bodies are thrown into a 

 state of temporary excitement in virtue of which they are repelled, but 

 any attempt to explain such a repulsion by the decomposition of a fluid 

 will demonstrate its own futility. 



