TRANSFORMATION ADD EQUIVALENCE OF FORCES. 213 



conclude that the heat of the Earth's still molten nucleus is 

 but a remnant of the heat which once made molten the 

 entire Earth. The mountainous surfaces of the Moon and 

 of Venus (which alone are near enough to be scrutinized), 

 indicating, as they do, crusts that have, like our own, been 

 corrugated by contraction, imply that these bodies too have 

 undergone refrigeration. Lastly, we have in the Sun a still- 

 continued production of this heat and light, which must 

 result from the arrest of diffused matter moving towards a 

 common centre of gravity. Here also, as before, a 



quantitative relation is traceable. Among the bodies which 

 make up the Solar System, those containing comparatively 

 small amounts of matter whose centripetal motion has been 

 destroyed, have already lost nearly all the produced heat : a 

 result which their relatively larger surfaces have facilitated. 

 But the Sun, a thousand times as great in mass as the 

 largest planet, and having therefore to give off an enormous- 

 ly greater quantity of heat and light due to arrest of moving 

 matter, is still radiating with great intensity. 



§ 69. If we inquire the origin of those forces which have 

 wrought the surface of our planet into its present shape, we 

 find them traceable to the primordial source just assigned. 

 Assuming the solar system to have arisen as above supposed, 

 then geologic changes are either direct or indirect results 

 of the unexpended heat caused by nebular condensation. 

 These changes are commonly divided into igneous and aque- 

 ous — heads under which we may most conveniently con- 

 sider them. 



All those periodic disturbances which we call earth- 

 quakes, all those elevations and subsidences which they sev- 

 erally produce, all those accumulated effects of many such 

 elevations and subsidences exhibited in ocean-basins, islands, 

 continents, table-lands, mountain-chains, and all those for- 

 mations which are distinguished as volcanic, geologists now 

 regard as modifications of the Earth's crust produced by the 



