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



gravity in its upper parts, where the pressure is small, this condition cannot have 

 lasted many minutes. The rigidity of a solid shell of superficial extent so vast 

 in comparison with its thickness, must be as nothing, and the slightest dis- 

 turbance would cause some part to bend down, crack, and allow the liquid to run 

 out over the whole solid. The crust itself would in consequence become shattered 

 into fragments, which must all sink to the bottom, or to meet in the centre and 

 form a nucleus there if there is none to begin with. 



30. It is, however, scarcely possible, that any such continuous crust can ever 

 have formed all over the melted surface at one time, and afterwards have fallen 

 in. The mode of solidification conjectured in § 25, seems on the whole the 

 most consistent with what we know of the physical properties of the matter con- 

 cerned. So far as regards the result, it agrees, I believe, with the view adopted 

 as the most probable by Mr Hopkins.* But whether from the condition being 

 rather that described in § 26, which seems also possible, for the whole or for 

 some parts of the heterogeneous substance of the earth, or from the viscidity as of 

 mortar, which necessarily supen'^enes in a melted fluid, composed of ingredients 

 becoming, as the whole cools, separated by crystallising at different temper- 

 atures before the solidification is perfect, and which we actually see in lava 

 from modern volcanoes; it is probable that when the whole globe, or some 

 very thick superficial layer of it, still liquid or viscid, has cooled down to near 

 its temperature of perfect solidification, incrustation at the surface must com- 

 mence. 



31. It is probable that crust may thus form over wide extents of surface, and 

 may be temporarily buoyed up by the vesicular character it may have retained 

 from the ebullition of the liquid in some places, or, at all events, it may be held 

 up by the viscidity of the liquid; until it has acquired some considerable thickness 

 sufficient to allow gravity to manifest its claim, and sink the heavier solid below 

 the lighter liquid. This process must go on until the sunk portions of crust 

 build up from the bottom a sufficiently close ribbed solid skeleton or frame, to 

 allow fresh incrustations to remain bridging across the now small areas of lava 

 pools or lakes. 



32. In the honey-combed solid and liquid mass thus formed, there must be a 

 continual tendency for the liquid, in consequence of its less specific gravity, to 

 work its way up ; whether by masses of solid falling from the roofs of vesicles or 

 tunnels, and causing earthquake shocks, or by the roof breaking quite through 

 when very thin, so as to cause two such hollows to unite, or the liquid of any of them 

 to flow out freely over the outer surface of the earth ; or by gradual subsidence of 

 the solid, owing to the thermo-dynamic melting, which portions of it, under intense 

 stress, must experience, according to views recently published by my brother, Pro- 



* See his Report on " Earthquakes and Volcanic Action." British Association Report for 

 1847. 



