PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 635 



of the same temperature throughout, the equability being main- 

 tained b}^ the process of convection; and Lycll not only makes the 

 same assertion, but declares that if the earth ever was an entirely 

 molten mass, this process of convection must have prevented the 

 solidification of any portion of the globe, until the whole was of 

 the temperature at which it solidifies; and that the solidification 

 would begin at the centre, not at the surface, so that the latter 

 could not solidify till all the rest had. 



If, as recent experiments render probable, all bodies attain their 

 maximum liquid density at a temperature, which is a few degrees 

 above that at which they solidify, the formation of a solid crust 

 while the interior remained molten would have been possible, after 

 the manner in which ice is formed on the surface of water; but 

 as in all such cases, the temperature of the whole mass must be 

 that of its maximum liquid density before the first film of a solid 

 surface can be foi-med. 



At temperatures between that of the maximum liquid density 

 «ind of solidification all bodies expand; but as soon as any portion 

 solidifies, it, if still cooling, contracts. If this cooling is continued 

 long enough, the solid will soon attain a density equal to, and 

 then greater than that of the liquid mass, and will sink. 



Hence the existence of a solid crust or surface produced from 

 and npon a liquid mass by cooling, is only possible when the 

 cooling is not continued, till that solid surface attains a greater 

 specific gravity than the underlying liquid mass. As this con- 

 dition is not fulfilled in the earth's case, the conclusion follows, 

 that its solid surface never was formed l:>y the cooling of a liquid 

 body. 



The fact that some portions of the earth in the Arctic and Tem- 

 perate zones have once possessed a much warmer climate than 

 now, is proved beyond doubt by their fossil remains, and seems 

 to afford a strong point in favor of La Place's theory; but geolog- 

 ical investigations have shown that some portions of the earth, 

 especially during the glacial epoch, possessed a much colder 

 climate than now. 



If the foct that some parts of the earth are colder now than they 

 have been, is explained upon the theor}'- that the earth is cooling; 

 how can the fact that some portions of the earth are warmer now 

 than formerly, be gotten over ? 



