ELEMENTARY MAGNETISM. 409 



two poles the nail, like the magnet, possesses a mag- 

 netic equator. 



Conversant as you now are with the theory of mag- 

 netic fluids, you have already, I doubt not, anticipated 

 me in imagining the exact condition of an iron nail under 

 the influence of the magnet. You picture the iron as 

 possessing the neutral fluid in abundance ; you picture 

 the magnetic pole, when brought near, decomposing 

 the fluid ; repelling the fluid of a like kind with itself, 

 and attracting the unlike fluid ; thus exciting in the 

 parts of the iron nearest to itself the opposite polarity. 

 But the iron is incapable of becoming a permanent 

 magnet. It only shows its virtue as long as the magnet 

 acts upon it. What, then, does the iron lack which the 

 steel possesses ? It lacks coercive force. Its fluids are 

 separated with ease ; but, once the separating cause is 

 removed, they flow together again, and neutrality is 

 restored. Imagination must be quite nimble in pictur- 

 ing these changes able to see the fluids dividing and 

 reuniting, according as the magnet is brought near or 

 withdrawn. Fixing a definite pole in your mind, you 

 must picture the precise arrangement of the two fluids 

 with reference to this pole, and be able to arouse 

 similar pictures in the minds of your pupils. You 

 will cause them to place magnets and iron in various 

 positions, and describe the exact magnetic state of the 

 iron in each particular case. The mere facts of mag- 

 netism will have their interest immensely augmented 

 by an acquaintance with the principles whereon the facts 

 depend. Still, while you use this theory of magnetic fluids 

 to track out the phenomena and link them together, 

 you will not forget to tell your pupils that it is to be re- 

 garded as a symbol merely, a symbol, moreover, which 

 is incompetent to cover all the facts, 1 but which does good 



1 This theory breaks down when applied to diamagnetic bodies 



