A LECTURE ON MAGNETISM. 367 



stand the test of a comparison with facts, may be of emi- 

 nent use in enabling us to 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, I will make use of a strong and vivid image. 

 In optics, red and 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 green repels green ; but that red 

 attracts green and green attracts red, the attraction of the 

 dissimilar colors being equal to the repulsion of the similar 

 ones. Imagine the two colors mixed so as to produce 

 white, and suppose two strips of wood painted with this 

 white ; what will be their action upon each other ? Sus- 

 pend 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 re- 

 pel the red component of your suspended strip, but then it 

 will attract the green ; and the forces being equal they neu- 

 tralize each other.. In fact, the least reflection shows you 

 that the strips will be as indifferent to each other as two 

 unmagnetized darning-needles would be under the same 

 circumstances. 



But suppose, instead of mixing the colors, we painted 

 one half of each strip from centre to end red, and the other 

 half green, it is perfectly manifest that the two strips would 

 now behave toward each other exactly as our two magnet- 

 ized darning-needles the red end would repel the red and 

 attract the green, the green would repel the green and at- 

 tract the red ; so that, assuming two colors thus related to 

 each other, we could by their mixture produce the neutral- 

 ity of an unmagnetized body, while by their separation we 

 could produce the duality of action of magnetized bodies. 



But you have already anticipated a defect in my con- 



