224 TERRESTRIAL CURRENTS. 



exerted by it, and naturally the inquiring mind is led 

 at first to believe that electricity is the all-powerful 

 principle of creation ; but a little reflection will serve 

 to convince us that it is a subordinate agent, although 

 a powerful one. 



In proceeding with our examination of the phe- 

 nomena which present themselves in connection with 

 the terrestrial currents, we purposely separate mag- 

 netism from those more distinct electro -chemical agen- 

 cies which play so important a part in the great cosmical 

 operations. 



Electricity, we have already stated, flows through or 

 involves all bodies ; but, like heat, it appears to undergo 

 a very remarkable change in becoming associated with 

 some forms of matter. We have the phenomena of 

 magnetism when an electric current circulates through 

 a metallic wire, and it would appear that all other bodies 

 acquire a peculiar polar condition under the influence of 

 this principle, which will be explained in the next chapter. 



The rocks, taken as masses, will not conduct an 

 electric current when dry : granite, porphyry, slate, and 

 limestone, obstructing its passage even through the 

 smallest spaces. But all the metallic formations admit 

 of its circulating with great freedom. This fact it must, 

 however, be remembered does not in any way interfere 

 with the hypothesis of the existence of electricity in all 

 bodies, in what we must regard as its latent state, from 

 which, under prescribed conditions, it may be readily 

 liberated. Neither does it affect the question of circu- 

 lation, in relation to the great diffusion of electricity 

 which we suppose to exist through all nature, and to 

 move in obedience to some fixed law. We know that 

 through the superficial strata electric currents circulate 

 freely, whether they are composed of clay, sand, or any 

 mixture of these with decomposed organic matter; 

 indeed, that with any substance in a moist state they 

 suffer no interruption. 



