504 Geology and Magnetism. 



of their formation is easily solved without having recourse to the 

 igneous theory. 



Mr. Fox has obtained an electro-type copper plate by the agency 

 of these subterranean currents. He found their natural direction to 

 be from south to north in Pennance mine. After a few days crystals 

 were formed in the negative plate, but two months had nearly 

 elapsed before the apparatus was removed from the circuit." 



Mr. Hopkins next enters into a consideration of the heat 

 produced by magnetic and galvanic currents, which he thinks 

 sufficiently powerful to account for all the phenomena, usually 

 but erroneously ascribed to the action of fire in terrestrial 

 physics. Mr. Hopkins objects to the igneous theory, that 

 if the increased heat experienced in deep mines proceeded 

 from an incandescent nucleus, as some profound geologists 

 supposed, it would be found uniform and constant in propor- 

 tion to the depth we descend from the surface. On the 

 contrary Mr. Hopkins states, that in South America, where a 

 great many experiments have been made, the variations in 

 the degree of temperature were irregular, and confined to 

 particular patches ; and in some instances the temperature 

 was even found lower at great depth than at the surface. 

 On the other hand, parts of rocks were sometimes found 

 intensely hot in deep mines at one time, and at another of 

 very low temperature, conditions which though incompati- 

 ble with the igneous theories of the earth, are strictly con- 

 formable with the chemical actions which are going on in 

 rocks. 



Mr. Hopkins here explains the heating phenomena dis- 

 played by metallic solutions in a galvanic battery. The 

 heat developed, and the power of fusion exercised over 

 the most refractory substances, such as platinum, palladium, 

 gold, copper, iron and steel, is in proportion to the amount 

 of the electro-magnetic currents, or in other words, to the 

 power of the battery. 



