366 FRAGMENTS OF SCIENCE. 



the softest iron possesses a certain amount of coercive 

 force. The earth, at this moment, finds in this force 

 an antagonist which opposes the decomposition of the 

 neutral fluid, The component fluids may be figured as 

 meeting an amount of friction, or possessing an amount 

 of adhesion, which prevents them from gliding over the 

 molecules of the poker. Can we assist the earth in this 

 case ? If we wish to remove the residue of a powder 

 from the interior surface of a glass to which the powder 

 clings, we invert the glass, tap it, loosen the hold of 

 the powder, and thus enable the force of gravity to pull 

 it down. So also by tapping the end of the poker we 

 loosen the adhesion of the magnetic fluids to the mole- 

 cules and enable the earth to pull them apart. But> 

 what is the consequence ? The portion of fluid which 

 has been thus forcibly dragged over the molecules 

 refuses to return when the poker has been removed from 

 the line of dip ; the iron, as you see, has become a per- 

 manent magnet. By reversing its position and tapping 

 it again we reverse its magnetism. A thoughtful and 

 competent teacher will know how to place these remark- 

 able facts before his pupils in a manner which will 

 excite their interest. By the use of sensible images, 

 more or less gross, he will first give those whom he 

 teaches definite conceptions, purifying these conceptions 

 afterwards, as the minds of his pupils become more 

 capable of abstraction. By thus giving them a distinct 

 substratum for their reasonings, he will confer upon his 

 pupils a profit and a joy which the mere exhibition of 

 facts without principles, or the appeal to the bodily 

 senses and the power of memory alone, could never 

 inspire. 



