SECT, xxxv.] ELECTRICITY OF METALLIC VEINS. 379 



same manner as it influences the parallelism of the iso- 

 thermal lines. That such currents do exist in metalliferous 

 veins appears from the experiments of Mr. Fox, in the 

 Cornish mines. Even since the last edition of this book 

 was published, Mr. Fox has obtained additional proof of the 

 activity of electro-magnetism under the earth's surface. He 

 has shown that not only the nature of the metalliferous 

 deposits must have been determined by their relative elec-- 

 trical conditions, but that the direction of the metallic 

 veins must have been influenced by the direction of the 

 magnetic meridians; and, in fact, almost all the metallic 

 deposits in the world tend from east to west, or from north- 

 east to south-west. Though it is impossible to say, in the 

 present state of our knowledge, how far the sun may be 

 concerned in the phenomena of terrestrial magnetism, it is 

 probable that the secular and periodic disturbances in the 

 magnetic force are occasioned by a variety of other com-* 

 biuing circumstances. Among these, M. Biot mentions the 

 vicinity of mountain chains to the place of observation, and, 

 still more, the action of extensive volcanic fires, which 

 change the chemical state of the terrestrial surface, they 

 .themselves varying from age to age, some becoming ex- 

 tinct, while others burst into activity. Should the ethereal 

 medium which fills space be the same with the electric 

 fluid, as M. Mossotti supposes, may not the heat of the sun 

 rarefy it at the earth's equator, and thus, by the inequality 

 of its distribution, and its superior density at the poles, 

 occasion some of the magnetic phenomena of the globe ? 

 and may not the sun's motion in declination cause tem- 

 porary variations of density in the fluid, and produce 

 periodic changes in the magnetic equator and intensity 1 ? 

 Were this the case, all the planets would be magnets, like 

 the earth, being precisely in similar circumstances. 



It is moreover probable that terrestrial magnetism may 

 be owing, in a certain extent, to the earth's rotation. Dr. 

 Faraday has proved that all the phenomena of revolving 



