Prof. W. Thomson on the Secular Cooling of the Earth. 13 



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 unoxidized metals. In the mean time we must con- 

 sider 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, 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 sufficient 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 of 20 per cent, less specific 

 gravity in its upper parts, where the pressure is small, this con- 

 dition cannot have lasted many minutes. The rigidity of a solid 

 shell of superficial extent so vast in comparison with its thick- 

 ness, must be as nothing, and the slightest disturbance would 

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

 run out over the whole solid. The crust itself would in conse- 

 quence 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, s"eems on the whole the most consistent 

 with what we know of the physical properties of the matter 

 concerned. 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 vis- 

 cidity as of mortar, which necessarily supervenes in a melted 

 fluid, composed of ingredients becoming, as the whole cools, 

 separated by crystallizing at different temperatures 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 commence. 



31. It is probable that crust may thus form over wide extents 



* See his Report on " Earthquakes and Volcanic Action/' British Asso- 

 ciation Report for 1847. 



