152 PLEASANT WAYS IN SCIENCE. 



substances, and leaving them to undergo the chemical changes 

 due to their association. As a familiar instance of explosive 

 action thus occasioned, we need only mention the results 

 experienced when any one unfamiliar with the methods of 

 treating lime endeavours over hastily to " slake " or " slack " 

 it with water. Indeed, one of the strong points of the 

 chemical theory consisted in the circumstance that vol- 

 canoes only occur where water can reach the subterranean 

 regions or, as Mallet expresses it, that "without water 

 there is no volcano." But the theory is disposed of by the 

 fact, now generally admitted, that the chemical energies of 

 our earth's materials were almost wholly exhausted before 

 the surface was consolidated. 



Another inviting theory is that according to which the 

 earth is regarded as a mere shell of solid matter surround- 

 ing a molten nucleus. There is every reason to believe 

 that the whole interior of the earth is in a state of intense 

 heat ; and if the increase of heat with depth (as shown in 

 our mines) is supposed to continue uniformly, we find that 

 at very moderate depths a degree of heat must prevail 

 sufficient to liquefy any known solids under ordinary con- 

 ditions. But the conditions under which matter exists a 

 few miles only below the surface of the earth are not 

 ordinary. The pressure enormously exceeds any which our 

 physicists can obtain experimentally. The ordinary dis- 

 tinction between solids and liquids cannot exist at that 

 enormous pressure. A mass of cold steel could be as plastic 

 as any of the glutinous liquids, while the structural change 

 which a solid undergoes in the process of liquefying could 

 not take place under such pressure even at an enormously 

 high temperature. It is now generally admitted that if the 

 earth really has a molten nucleus, the solid crust must, 

 nevertheless, be far too thick to be in any way disturbed 

 by changes affecting the liquid matter beneath. 



Yet another theory has found advocates. The mathe 

 matician Hopkins, whose analysis of the molten-nuclem 

 theory was mainly effective in showing that theory to be un 



