188 CHEMISTRY OF THE EARTH. 



outer crust.* The same conclusion was subsequently adopted by Han- 

 steen. 



§ 1.'). Apart from tliese considerations, however, many of the best 

 modern physicists and geologists have found numerous leasons for 

 rejecting the popular notion which regards our globe as a liqui<l molten 

 mass covered by a layer of twenty or thirty miles of solidified rock. 

 The deductions of Hopkins from the phenomena of precession and nuta- 

 tion, those of Pratt from the crushing force of iiumeuse mountain masses 

 like those of the Himalaya, and those of Sir William Thompson from the 

 tides, showing the great rigidity of the earth, all unite to i)rove that 

 the earth, if not solid to the center, must have a firm and solid crust 

 several hundred miles in thickness. Under these conditions, if there 

 still exist a liquid center, it must, so far as superficial phenomena are 

 concerned, be as inert as if it were not. We are thus prepared to accept 

 the coiK^lusions to which the line of argument in § 11 leads us, and admit 

 that our globe solidified from the center. 



§ 14. It is, then, with the superficial portions of the earth, alone, that 

 we have to do from the moment of its solidification ; and as in the sub- 

 sequent pages of our history air and water must play an important ]>art, 

 it becomes necessary, bofore going further, to consicler brietly the nature 

 of the process of aqueous solution and its relations to pressure. Solu- 

 tion may, for our i^resent purpose, be defined as a chemical union between 

 two or more bodies, of which one is liquid, resulting in a liquid product, 

 and acconqianied by a change of volume.! In ordinary cases, as in that 

 of most bodies dissolving in water, this change is in the direction of con- 

 densation, and hence, as might be expected, pressure exerts an inllnence 

 similar to that in the liquefaction of certain bodies by fusion, explained 

 in §11. Pressure facilitates the liquefaction of ice, which is attended 

 by condensation, and acts in like manner in the case of solution, so that 

 the solubility of salts in water, as shown by the experiments of Sorby, is 

 increased by pressure. We can scarcely doubt that these phenomena 

 of fusion and solution come under one general i)hysical law, and that 

 for all those bodies which contract in dissolving, (to which there are 

 but few exceptions,) their solubility in water must be augmented by 

 and in proportion to the pressure. As expressed by Mr. Sorby, mechani- 

 cal is thus converted into chemical force. | 



§ 15. Eeverting now to the solidified globe, in whose superficial por- 

 tions and in the surrounding gases and vapors were present all the 

 chemical elements with which we have to do, it is necessary to consider 

 briefly its physical and its chemical condition at this early period. 



The formation of a crust at the surface of the viscid layer which still 

 enveloped the solidified mass of the globe, as conceived by Hopkins, 

 is readily admissible; but that this process commenced when the remain- 

 ing envelope of liquid matfer was yet so deep that the refrigeration up 

 to the present time has not been sufficient for its entire solidification 

 is not probable. Such a crust on the cooling superficial layer would, 

 from the contraction consequent on the further refrigeration of the 

 liquid stratum beneath, become more or less depressed, corrugated, and 



* The elevated temperature of the interior of tlie globe would probably otFer no ob- 

 stacle to the development of magnetism. In a recent experiment of M. Treve, com- 

 mnnicated by M. Faye to the French Academy of Sciences, it was found that molten 

 cast iron when poured into a mold, surrounded by a helix which was traversed by an 

 electric current, became a strong magnet when liquid at a temperature of 1,300-^ C, 

 .and retained its magnetism while cooling. {Comptes-Bencliis de I' Academie des Sciences, 

 February, 1869.) 



t T. S. Hunt, Thoughts on Solution, American Journal of Science, [2,] xix, 100. 



t Bakeriau Lectui-e for 1863, L. E. and D, Philosophical Magazine, February, 1884 



