SKCT. XXVI. VOLCANIC THEORIES. 249 



The shell of volcanic fire that girds the globe at a 

 small depth below our feet has been attributed to differ- 

 ent causes. By some it is supposed to originate in an 

 ocean of incandescent matter, still existing in the cen- 

 tral abyss of the earth. Some conceive it to be super- 

 ficial, and due to chemical action, in strata at no very 

 great depth when compared with the size of the globe. 

 The more so, as matter on a most extensive scale is 

 passing from old into new combinations, which, if rapidly 

 effected, are capable of producing the most intense heat. 

 According to others, electricity, which is so universally 

 diffused in all its forms throughout the earth, if not the 

 immediate cause of the volcanic phenomena, at least 

 determines the chemical affinities that produce them. 

 It is clear that a subject so involved in mystery must 

 give rise to much speculation, in which every hypothe- 

 sis is attended with difficulties that observation alone 

 can remove. 



But the views of Mr. Babbage and Sir John Herschel 

 on the general cause of volcanic action, and the changes 

 in the equilibrium of the internal heat of the globe, ac- 

 cord more with the laws of mechanics and radiant caloric 

 than any that have been proposed. The theory of these 

 distinguished philosophers, formed independently of each 

 other, is equally consistent with observed phenomena, 

 whether the earth be a solid crust encompassing a nu- 

 cleus of liquid lava, or that there is merely a vast reser- 

 voir or stratum of melted matter at a moderate depth 

 below the superficial crust. The author is indebted to 

 the kindness of Mr. Lyell for the perusal of a most 

 interesting letter from Sir John Herschel, in which he 

 states his views on the subject. 



Supposing that the globe is merely a solid crust, rest- 

 ing upon fluid or semi-fluid matter, whether extending 

 to the center or not, the transfer of pressure from one 

 part of its surface to another by the degradation of ex- 

 isting continents, and the formation of new ones, would 

 be sufficient to subvert the equilibrium of heat in the 

 interior, and occasion volcanic eruptions. For, since 

 the internal heat of the earth is transmitted outwards 

 by radiation, an accession of new matter on any part of 

 the surface, like an addition of clothing, by keeping it in, 



