2TZ FRA OMENTS OF SCIENCE. 



power to attract its fellow nail ceases. Contact, however, 

 is not necessary. A sheet of glass or paper, or a space of 

 air, may exist between the magnet and the nail; the latter 

 is still magnetized, though not so forcibly as when in actual 

 contact. The nail thus presented to the magnet is itself a 

 temporary magnet. That end which is turned toward the 

 magnetic pole has the opposite magnetism of the pole 

 which excites it; the end most remote from the pole has 

 the same magnetism as the pole itself, and between the 

 two poles, the nail, like the magnet, possesses a magnetic 

 equator. 



Conversant as you now are with the theory of magnetic 

 fluids, you have already, I doubt not, anticipated me in im- 

 agining the exact condition of an iron nail under the influ- 

 ence of the magnet. You picture the iron as possessing 

 the neutral fluid in abundance; yon picture the magnetic 

 pole, when brought near, decomposing the fluid; repelling 

 the fluid of a like kind with itself, and attracting the un- 

 like 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 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 according as the magnet is brought near or withdrawn.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 various positions, and describe the exact magnetic 

 state of the iron in each particular case. The mere facts 

 of magnetism 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 regarded 

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

 petent to cover all the facts,* but which does good practi- 



* This theory breaks down when applied to diatnagnetic bodies 

 which are repelled by magnets. Like soft iron, such bodies are 



