EXAMPLES OF THE EXPLANATION OF LAWS. 573 



substances possessed of the requisite chemical proper- 

 ties, interferes to prevent them from reaching the 

 substances which they are destined to dissolve. 



$ 4. From the foregoing and similar instances, we 

 jmay see the importance, when a law of nature pre- 

 viously unknown has been brought to light, or when 

 new light has been thrown upon a known law by 

 experiment, of examining all cases which present the 

 conditions necessary for bringing that law into action; 

 a process necessarily fertile in demonstrations of spe- 

 cial laws previously unsuspected, and explanations of 

 others already empirically known. 



For instance, Faraday discovered by experiment, 

 that voltaic electricity could be evolved from a natural 

 magnet, provided a conducting body were set in motion 

 at right angles to the direction of the magnet ; and, 

 this he found to hold not only of small magnets, but 

 of that great magnet, the earth. The law being thus 

 established experimentally, that electricity is evolved, 

 by a magnet, and a conductor moving at right angles 

 to the direction of its poles, we may now look out for 

 fresh instances in which these conditions meet. 

 Wherever a conductor moves or revolves at right 

 angles to the direction of the earth's magnetic poles, 

 there we may expect an evolution of electricity. In 

 the northern regions, where the polar direction is 

 nearly perpendicular to the horizon, all horizontal 

 motions of conductors will produce electricity: hori- 

 zontal wheels, for example, made of metal ; likewise 

 all running streams will evolve a current of electricity 

 which will circulate round them ; and the air thus 

 charged with electricity may be one of the causes of 

 the Aurora Borealis. In the equatorial regions, on 

 the contrary, upright wheels placed parallel to the 



