OUTLINES OF GEOLOGY. 



95 



cific heat shows us that, whenever a>body is compressed, its 

 temperature is raised; and as the bodies which compose the 

 interior of the earth are compressed by those which lie be- 

 tween them and the surface, we have a general law which 

 is sufficient to account for the regular increase of tempera- 

 ture which is found in descending into mines. We may next 

 infer by calculation, that at a depth of from 20 to 30 miles, 

 the heat growing out of mere pressure, will be sufficient to 

 fuse all matters of the same nature as those we find at the 

 surface of the earth. Now, as there is no reason to suppose 

 that the interior is composed of elements different from those 

 which exist at the surface, the inference is immediate that 

 the interior of the earth is in a state of igneous fusion. 



In going a step further back, the same law of specific heat 

 will show us : that when the matter which now composes the 

 earth was first brought together, it must, unless the present 

 laws of nature had not begun to act, or had been suspended 

 for the occasion, have been in a state of intense heat, grow- 

 ing out of the mere condensation into a limited space of so 

 great a quantity of matter. The igneous fusion, then, in- 

 stead of being confined to the surface alone, must have pre- 

 vailed throughout the mass, which, existing in a liquid state, 

 would have assumed the very figure which observation 

 shows us the earth at present has. 



A globe thus intensely heated, and situated in a space 

 colder than itself, will part with heat by radiation, and were 

 this the original state of the earth, it would have cooled, with 

 a rapidity having relation to^theheat of the surface, until the 

 quantity of heat radiated, and of that received from the sun, 

 should exactly balance each other. 



This state of calorific equilibrium, the earth, as we learn 

 from an argument founded on astronomic phenomena, has 

 nearly, if not completely, reached. At any rate, the mean 

 temperature of the globe has not, for 2000 years, varied 



