CHAPTER XVIII. 



THE SOLIDITY OF THE EARTH. 



IN his opening address to the Mathematical and Physical 

 Section of the British Association, Sir William Thomson 

 affirmed, " with almost perfect certainty, that, whatever may 

 be the relative densities of rock, solid and melted, or at about 

 the temperature of liquefaction, it is, I think, quite certain that 

 cold solid rock is denser than hot melted rock ; and no possi- 

 ble degree of rigidity in the crust could prevent it from break- 

 ing in pieces and sinking wholly below the liquid lava," and 

 that " this process must go on until the sunk portions of the 

 crust build up from the bottom a sufficiently close-ribbed 

 skeleton or frame, to allow fresh incrustations to remain bridg- 

 ing across the now small areas of lava-pools or lakes."* 



This would doubtless be the case if the material of the earth 

 were chemically homogeneous or of equal specific gravity 

 throughout, and if it were chemically inert in reference to its 

 superficial or atmospheric surroundings. But such is not the 

 case. Ail we know of the earth shows that it is composed of 

 materials of varying specific gravities, and that the range of 

 this variation exceeds that which is due to the difference 

 between the theoretical internal heat of the earth and its actual 

 surface temperature. 



We know by direct experiment that these materials, when 

 fused together, arrange themselves according to their specific 

 gravities, with the slight modification due to their mutual diffusi- 

 bilities. If we take a mixture of the solid elements of which 

 the earth, so far as we know it, is composed, fuse them, and 

 leave them exposed to atmospheric action, what will occur ? 



The heavy metals will sink, the heaviest to the bottom, the 

 lighter metals (i.e. those that we call the metals of the earth, 

 because they form the basis of the earth's superficial crust) will 

 rise along with the silicon, etc. to the surface ; these and the 

 silicon will oxidize and combine, forming silicates, and a suffi- 

 cient supply of carbonic acid some of them, such as calcium, 

 magnesium, etc., will form carbonates when the temperature 

 sinks below that of the dissociation of such compounds. 



* Aafurr, vol. xir. p, 429. 



