12 Prof. W, Thomson on the Secular Cooling of the Earth. 



investigated by my brother, Professor James Thomson*, and 

 verified by myself experimentally for water \, the temperature of 

 solidification will, at great depths, because of the great pressure, 

 be higher there than at the surface if the fluid contracts, or 

 lower than at the surface if it expands, in becoming solid. 



24. How the temperature of solidification, for any pressure, 

 may be related to the corresponding temperature of fluid con- 

 vective equilibrium, it is impossible to say, without knowledge, 

 which we do not yet possess, regarding the expansion 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 freezing, we cannot tell, without definite 

 numerical data regarding those elements, whether the elevation 

 of the temperature of solidification, or of the actual temperature 

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

 duced 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 application 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 actu- 

 ally 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 con- 

 ducted 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 



* " Theoretical Considerations regarding the Effect of Pressure in 

 Lowering the Freezing-Point of Water," Trans. Roy. Soc. Edinb. Jan. 1849. 

 t Proc. Royal Soc. Edino., Session 1849-50. 



