ELEMENTARY MAGNETISM. 353 



if from an assumed cause the observed acts neces- 

 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 im- 

 agination 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 inter- 

 poses to carry out the principle to its logical conse- 

 quences. A perfect theory gives dominion over natural 

 facts; and even an assumption which can only par- 

 tially 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 mag- 

 netic 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 comple- 

 mentary colours; their mixture produces white. 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 darn- 

 ing-needle, and bring the other near it; what will 

 occur? The red component of the strip you hold in 



