ELEMENTARY MAGNETISM 873 



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, that green repels green; but that red at- 

 tracts green and green attracts red, the attraction of the 

 dissimilar colors being equal to the repulsion of the sim- 

 ilar 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 repel the red component of your suspended strip; 

 but then it will attract the green, and, the forces being 

 equal, they neutralize each other. In fact, the least re- 

 flection 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 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 two col- 

 ors thus related to each other, we could by their mixture 

 produce the neutrality of an unmagnetized body, while 



