IN RELATION TO THE EARTH'S INTERNAL STRUCTURE. 41 



(Fall.), for past periods of only 1250, 5000, and 20,000 years, and authorize him 

 to pronounce Poisson's hypothesis impossible, without destruction of life, or relega- 

 tion of the date to so remote an era as to demand an intensely heated stellar region. 



A reheating after solidification which should again fuse the surface to great 

 depths would be, it seems to me, as " inadmissible" for the origin of observed sub- 

 terranean temperatures, as the heat originally " developed by the solidification of 

 the earth." Hence, the hypothesis of a reheating to fusion of the surface by 

 impact of meteoric bodies Avould be likewise excluded by Poisson's theory of in- 

 ternal temperatures. 



In the paper referred to (p. 39), Sir Wm. Thomson discusses the probable circum- 

 stances of solidification of the earth, assuming the known crust-materials (granite, 

 &c.), in a molten state, as the constituent, and reasons that in consequence of the 

 great condensation of granite in freezing, solidification must commence at the centre, 

 and that "there could be no complete permanent incrustation all round the surface 

 " till the globe is solid, with, possibly, the exception of irregular, comparatively 

 " small spaces of liquid ;" such separation of constituents in the process of crystal- 

 lization taking place in all liquids composed of heterogeneous materials, and, indeed, 

 is observa^jle in the lava of modern volcanoes. He infers from the probable 

 phenomena developed, into the discussion of which he goes at some length, " rc- 

 " suits sufficiently great and various to account for all that we see at present, and 

 " all that we learn from geological investigation, of earthquakes, of upheavals, and 

 "subsidences of solid, and of eruptions of melted rock." 



Still we Avould, if possible, find reason to attribute a lower than " the proper 

 melting temperature" 'to the solidified interior. Ice, indeed, preserves its rigidity 

 unimpaired up to the point of fusion, and there may be a few other substances that 

 have the like property; but it seems to be an exceptional one. The known constitu- 

 ents of the earth's crust certainly do not possess it, at least under ordinary pressures. 

 If, as suggested by Prof. Joseph Le Conte (Am. Journal of Science, Nov. Dec. 1872), 

 the "conductivity" be increased by pressure mi^ condensation^ such diminished 

 temperatures may obtain. 



January, 1872 



