EL EM UN TAR Y MA GNKTISM. 69 



capable of seizing firmly on a physical image as a principle, 

 of discerning its consequences, and of devising means 

 whereby these forecasts of thought may be brought to an 

 experimental test. If such a principle be adequate to 

 account for all the phenomena if from an assumed cause 

 the observed acts necessarily 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 constuctive 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 partially 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 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 

 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 

 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 reflection shows 

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

 two unmagnetized darning-needles would be under the 

 same circumstances. 



