33S 



LESSONS IN ELECTRICITY. 



sumption a theory, and, once possessing it,we 

 can not only revive at pleasure facts already 

 known, but we can predict ot tiers which we 

 luwe never seen. Thus, tnen, in the prose- 

 cut ir/n of physical science, our powers of ob 

 serration, memory, imagination, and in- 

 teren-e, are nil drawn upon. We observe 

 Jncts and i-tore them tip ; imagination breed 6 ' 

 upon these memoiies, end by the aid of 

 leasou tries to discern their interdependence. 

 The theoretic principle flashes, or slovly 

 dawns upon the mind, aud then the deductive 

 faculty interposes to carry out the principle 

 to its logical consequences, A perfect theory 

 gives dominion over natural facts ; and even 

 an assumption which can only partially stand 

 the test of a comparison with facts, may be 

 of eminent 11*0 in enabling us tq connect and 

 classify groups of phenomena. The theory 

 of magnetic fluids is of this latter character, 

 and with it we must now make ourselves 

 familiar. 



With the view of stamping the thing more 

 firmly on your minds, 1 will make use of a 

 strong and vivid image. In optics, red aud 

 green are called complementary colors ; their 

 mixture produces white. Now I ask you to 

 | imagine each of these colors to possess a self- 

 \ repulsive power ; that red repels red, and 

 * that greeti repels green ; but that red attracts 

 green and green attracts red, the attraction of 

 5 the dissimilar colors being equal to the repul- 

 sion of the similar ones. Imagine the two 

 colors mixed so as to produce white, and sup- 

 pose two strips of wood painted with this 

 white ; what will be their action upon each 

 othei ? Suspend one of them freely as we 

 suspended our darning-needle, and bring the 

 other near it; what will occur? The red 

 component of the strip you hold in your hand 

 will *epel the red component of your sus- 

 pended strip, but then it will attract the 

 green ; and the forces being equal they neu- 

 tralize each other. In fact, the least reflec- 

 tion shows you that the strips will be as in- 

 different to each other as two unmagnetized 

 darning-needles would be under the same cir- 

 cumstances. 



But suppose, instead of mixing the colors, 

 w*i painted one half of each strip from centre 

 to end red, and the other half green, it is per- 

 fectly manifest that the two strips would now 

 behave toward each other exactly as our two 

 magnetized darning - needles the red end 

 would repel the red and attract the green, 

 the green would repel the green and attract 

 the red ; so that, assuming tw r o colors thus 

 related to each other, we could by their mix- 

 ture produce the neutrality of an unmag- 

 netized body, while by their separation we 

 could produce the duality of action of mag- 

 netized bodies. 



But you have already anticipated a defect 

 in my conception ; for if we break one of our 

 strips of wood in the middle we have one half 

 entirely red and the other entirely green, and 

 with these it would be impossible to imitate 

 the action of our broken magnet. How, then, 

 must we modify our conception ? We must 

 evidently suppose each atom of wood painted 



green on one face and red on the opposite? 

 one. If this were done the resultant action 

 of all the atoms would exactly resemble tho 

 action of a magnet. Here, also, if the two 

 opposite colois of each atom could be caused, 

 to mix so as to produce white, we should 

 have, as before, pei feet neutrality. 



Substitute in your minds far these two self 

 repeilant anl mutually attractive colors two 

 invisible self-repellant and mutually attrac- 

 tive fluids, which in ordinary steel are mixed 

 to form a neutral compound, but which tho 

 act of magnetization separnl.es from each 

 other, placing the opposite fluids on the op- 

 posite faces of each atom, and you have a 

 perfectly distinct conception of the celebrated 

 theory of magnetic fluids. The strength of 

 the magnetism excited is supposed to be pro- 

 portional to the quantity of neutral fluid de- 

 composed. According to this theory nothing 

 is actually transferred from the exciting mag 

 net to the excited steel. The act of mag- 

 netization consists in the forcible separation 

 of two powers which existed in the steel be- 

 fore it was magnetizefl, but which then neu- 

 tralized each other by their coalescence. And 

 i f you lest your magnet after it has excited a 

 hundred pieces of steel, you will find that it 

 has lost no force no more, indeed, th>m I 

 should lose had my words such a magnetic in- 

 fluence on your minds as to excite m them p. 

 strong resolve to study natural philosophy. 

 I should, in fact, be the gainer by my own 

 utterance and by the reaction of your 

 strength ; and so also the magnet is the gainer 

 by the reaction of the body which it mag- 

 netizes. 



Look now to your excited piece of steel ; 

 figure each atom to your minds with its op- 

 posed fluids spread over its opposite faces. 

 How can this state of things be permanent ? 

 The fluids, by hypothesis, attract each other : 

 what, then, keeps them apart ? Why do they 

 not instantly rush together across the equator 

 of the atom, aud thus neutralize each other? 

 To meet this question, philosophers have 

 been obliged to infertile existence of a special 

 force which holds the fluids asunder. They 

 call it coercive force ; and it is found that those 

 kinds of steel which offer most resistance to 

 being magnetized, which require the greatest 

 amount of coercion to tear their fluids asunder, 

 are the very ones which offer the greatest re- 

 sistance to the reunion of the fluids after they 

 have been once separated. Such kinds of 

 steel are most suited to the formation of 

 permanent magnets. It is manifest, indeed, 

 that without coercive force a permanent mag- 

 net would not be at all possible. 



You have not forgotten that, previous to 

 magnetizing your darning-needle, botfi its 

 ends were attracted by your magnet ; and 

 that both ends of your bit of iron wire were 

 acted upon in the same way. Probably also 

 long before this you will have dipped the end 

 of your magnet among iron filings, and ob- 

 served how they cling to it, or into a nail- 

 box, and found how it drags the nails after 

 it. I know very well that Vf you are not the 

 slaves of routine, you will have by this time 



