72 SCIENCE IN SHORT CHAPTERS. 



United Kingdom during the first five months of 1882 

 amounted to 26,297,346 gallons.) 



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 possible degree of rigidity in the crust could 

 prevent it from breaking in pieces and sinking wholly be- 

 low the liquid lava," and that "this process must goon 

 until the sunk portions of the crust build up from the bot- 

 tom a sufficiently close-ribbd skeleton or frame, to allow 

 fresh incrustations to remain bridged 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 refer- 

 ence to its superficial or atmospheric surroundings. But 

 such is not the case. All we know of the earth sbows 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 spe- 

 cific gravities, with the slight modification due to their 

 mutual diffusibilities. If we take a mixture of the solid 

 elements of which the earth, so far as we know it, is com- 

 posed, fused them, and leave them exposed to atmospheric 

 action, what will occur? 



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



* Nature, vol. xiv. p. 429. 



