238 STRUCTURE AND MODE OF ACTION 



canic phenomena soon himself relinquished it. Observations 



made in mines and caverns in all climates, and which in 



concert with M. Arago I have collected in a separate 



memoir, shew that, even at what may be considered a very 



small depth, the temperature of the Earth is much above 



the mean temperature of the atmosphere at the same place. 



A fact so remarkable, and so generally confirmed, connects 



itself with that which we learn from .volcanic phenomena. 



The depth at which the globe may be regarded as a molten 



mass has been calculated. The primitive cause of this 



subterranean heat is, as in all planets, the process of formation 



itself, the separation of the spherically condensing mass from 



a cosmical gaseous fluid, and the cooling of the terrestrial 



strata at different depths by the loss of heat parted with by 



radiation. All volcanic phenomena are probably the result 



of a communication either permanent or transient between 



the interior and exterior of the globe. Elastic vapours 



press the molten oxydising substances upwards through 



deep fissures. Yolcanos might thus be termed intermitting 



springs or fountains of earthy substances ; i. e. of the fluid 



mixture of metals, alkalis, and earths which solidify into 



lava currents and flow softly and tranquilly, when being 



upheaved they find a passage by which to escape. In a 



similar manner the Ancients represented (according to Plato's 



Phaedon) all volcanic fiery currents as streams flowing from 



the Pyriphlegethon. 



To these considerations and views let me be permitted to 

 add another more bold. May we not find in this internal 

 heat of our globe, (a heat indicated by thennometric 



