24 CONSTITUENT MATERIALS OF THE EARTH. 



in the phenomena of volcanoes and thermal springs, as well as 

 in what is ascertained with regard to the density of the 

 entire mass of the earth. This approximates five and a half 

 times the weight of water; hut the actual weight of the prin- 

 cipal solid substances composing the outer crust is as two 

 and a half times the weight of water ; and this, we know, if 

 the globe were solid and cold, should increase greatly towards 

 the centre, water acquiring the density of quicksilver at 362 

 miles below the surface, and other things in proportion, and 

 these densities becoming much greater at greater depths ; 

 so that the entire mass of a cool globe should be of a gravity 

 infinitely exceeding five and a half times the weight of water. 

 The only alternative supposition is, that the central materials 

 are greatly expanded or diffused by some means ; and by 

 what means could they be so expanded but by heat ? Indeed, 

 the existence of this central heat, a residuum of that which kept 

 all matter in a vaporiform chaos at first, is amongst the most 

 solid discoveries of modern science,( 9 ) and the support which 

 it gives to the nebular hypothesis is highly important. We 

 shall heratefter see what have been supposed by some to be 

 traces of an operation of this heat upon the surface of the 

 earth in very remote times ; an effect, however, which has 

 long passed entirely away. 



