204 THE POPULAR SCIENCE MONTHLY. 



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 glu- 

 tinous 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 admit- 

 ted 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 mathematician 

 Hopkins, whose analysis of the molten-nucleus theory was mainly ef- 

 fective in rendering that theory untenable, suggested that there may 

 be isolated subterranean lakes of fiery matter, and that these may be 

 the true seat of volcanic energy. But such lakes could not maintain 

 their heat for ages, if surrounded (as the theory requires) by cooler 

 solid matter, especially as the theory also requiros that water should 

 have access to them. It will be observed also that none of the theo- 

 ries just described affords any direct account of those various features 

 of the earth's surface mountain-ranges, table-lands, volcanic regions, 

 and so on which are undoubtedly due to the action of subterranean 

 forces. The theory advanced by Mr. Mallet is open to none of these 

 objections. It seems, indeed, competent to explain all the facts which 

 have hitherto appeared most perplexing. 



It is recognized by physicists that our earth is gradually parting 

 with its heat. As it cools it contracts. Now, if this process of con- 

 traction took place uniformly, no subterranean action would result. 

 But, if the interior contracts more quickly than the crust, the latter 

 must in some way or other force its way down to the retreating nu- 

 cleus. Mr. Mallet shows that the hotter internal portion must contract 

 faster than the relatively cool crust ; and then he shows that the 

 shrinkage of the crust is competent to occasion all the known phe- 

 nomena of volcanic action. In the distant ages when the earth was 

 still fashioning, the shrinkage produced the irregularities of level 

 which we recognize in the elevation of the land and the depression of 

 the ocean-bed. Then came the period when, as the crust shrank, it 

 formed corrugations ; in other words, when the foldings and elevations 

 of the somewhat thickened crust gave rise to the mountain-ranges of 

 the earth. Lastly, as the globe gradually lost its extremely high tem- 

 perature, the continuance of the same process of shrinkage led no 

 longer to the formation of ridges and table-lands, but to local crush- 

 ing down and dislocation. This process is still going on, and Mr. 

 Mallet not only recognizes here the origin of earthquakes, and of the 

 changes of level now in progress, but the true cause of volcanic heat. 

 The modern theory of heat as a form of motion here comes into play. 

 As the solid crust closes in upon the shrinking nucleus, the work ex- 

 pended in crushing down and dislocating the parts of the crust is 



