ELEMENTARY MAGNETISM. 353 



sarily follow, we call the assumption a theory, and, 

 once possessing it, we can not only revive at pleasure 

 facts already known, but we can predict others which 

 we have never seen. Thus, then, in the prosecution 

 of physical science, our powers of observation, memory, 

 imagination, and inference, are all drawn upon. We 

 observe facts and store them up ; the constructive imagi- 

 nation broods upon these memories, tries to discern their 

 interdependence and weave them to an organic whole. 

 The theoretic principle flashes or slowly dawns upon 

 the mind; and 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 partiallv stand 

 the test of a comparison with facts, may be of eminent 

 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 our- 

 selves 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 comple- 

 mentary colours ; their mixture produces ivhite. Now 

 I ask you to imagine each of these colours to possess 

 a self-repulsive power ; that red repels red, that green 

 repels green ; but that red attracts green and green 

 attracts red, the attraction of the dissimilar colours 

 being equal to the repulsion of the similar ones. 

 Imagine the two colours mixed so as to produce white, 

 and suppose two strips of wood painted with this white ; 

 what will be their action upon each other ? 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 

 repel the red component of your suspended strip ; but 



