ELEMENTARY MAGNETISM 377 



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 to- 

 gether again, and neutrality is restored. Imagination must 

 be quite nimble in picturing 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 is brought near or withdrawn.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 vari- 

 ous 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 de- 

 pend. 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 regarded 

 as a symbol merely a symbol, moreover, which is incom- 

 petent to cover all the facts, 1 but which does good prac. 

 tical service while we are waiting for the actual truth. 



The state of excitement into which iron is thrown by 

 the influence of a magnet is sometimes called "magneti- 

 zation by influence." More commonly, however, the mag- 



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 

 ite own futility. 



