PROFESSOR W. THOMSON ON THE SECULAR COOLING OF THE EARTH. 167 



with heat, and the specific heat of the fluid, and the change of volume, and the 

 latent heat developed in the transition from fluid to solid. 



25. For instance, supposing, as is most probably true, both that the liquid 

 contracts in cooling towards its freezing-point, and that it contracts in freez- 

 ing, we cannot tell, without definite numerical data regarding those elements, 

 whether the elevation of the temperature of solidification, or of the actual tem- 

 perature of a portion of the fluid given just above its freezing-point, produced 

 by a given application of pressure, is the greater. If the former is greater than 

 the latter, solidification would commence at the bottom, or at the centre, if there 

 is no solid nucleus to begin with, and would proceed outwards ; and there could 

 be no complete permanent incrustation all round the surface till the whole globe 

 is solid, with, possibly, the exception of irregular, comparatively small spaces of 

 liquid. 



26. If, on the contrary, the elevation of temperature, produced by an appli- 

 cation of pressure to a given portion of the fluid, is greater than the elevation 

 of the freezing temperature produced by the same amount of pressure, the 

 superficial layer of the fluid would be the first to reach its freezing-point, and the 

 first actually to freeze. 



27. But if, according to the second supposition of § 22, the liquid expanded 

 in cooling near its freezing-point, the solid would probably likewise be of less 

 specific gravity than the liquid at its freezing-point. Hence the surface would 

 crust over permanently with a crust of solid, constantly increasing inwards by 

 the freezing of the interior fluid in consequence of heat conducted out through 

 the crust. The condition most commonly assumed by geologists would thus be 

 produced. 



28. But Bischof's experiments, upon the validity of which, so far as I am 

 aware, no doubt has ever been thrown, show that melted granite, slate, and 

 trachyte, all contract by something about 20 per cent, in freezing. We ought, 

 indeed, to have more experiments on this most important point, both to verify 

 Bischof's results on rocks, and to learn how the case is with iron and other 

 unoxydised metals. In the meantime we must consider it as probable that the 

 melted substance of the earth did really contract by a very considerable 

 amount in becoming solid. 



29. Hence, if according to any relations whatever among the complicated 

 physical circumstances concerned, freezing did really commence at the surface, 



i either all round or in any part, before the whole globe had become solid, the 

 solidified superficial layer must have broken up and sunk to the bottom, or to 

 the centre, before it could have attained a sufiicient thickness to rest stably on 

 the lighter liquid below. It is quite clear, indeed, that if at any time the earth 

 were in the condition of a thin solid shell of, let us suppose 50 feet or 100 feet 

 thick of granite, enclosing a continuous melted mass ot 20 per cent, less specific 



