378 FRAGMENTS OF SCIENCE. 



needle. I now reverse the poker, bringing its knob under- 

 most ; the knob is now a north pole and attracts the south 

 end of a magnetic needle. Get such a poker and carefully 

 repeat this experiment ; 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 an- 

 tagonist which opposes the full decomposition of the neu- 

 tral fluid. The component fluids may be figured as meet- 

 ing an amount of friction, or possessing an amount of ad- 

 hesion, which prevents them from gliding over the atoms 

 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 fluid to the atoms 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 atoms 

 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 thoughtful 

 and competent teacher will well know how to place these 

 remarkable facts before his pupils in a manner which will 

 excite their interest; he will know, and if not, will try to 

 learn, how, by the use of sensible images, more or less gross, 

 to give those he teaches definite conceptions, purifying 

 these conceptions more and more as the minds of his pupils 

 become more capable of abstraction. He will cause his 

 logic to run like a line of light through these images, and 

 by thus acting he will cause his boys to march at his 

 side with a profit and a joy, which the mere exhibition of 



