386 FRAGMENTS OF SCIENCTE ' 



periment and satisfy yourselves that the fluids shift their 

 position according to the manner in which the poker is 

 presented to the earth. It has already been stated that 

 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 molecules 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 permanent magnet. By reversing its position 

 and tapping it again we reverse its magnetism. A thought- 

 ful and competent teacher will know how to place these 

 remarkable 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 defi- 

 nite conceptions, purifying these conceptions afterward, as 

 the minds of his pupils become more capable of abstrac- 

 tion. 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, 



