402 Lord Kelvin on the Formation of 
other atoms far or near. The less dense crowd of atoms in 
those parts constitutes a fluid of the species called gas or 
vapour. Every collision between two atoms in those outlying 
parts, and in the central region every change of speed And of 
direction of atomic motions, due to mutual forces between 
neighbouring atoms, sets ether locally in motion. The motion 
thus produced in ether gives rise to etherial waves which 
travel outwards through the outlying parts of the assemblage ; 
and continue their outward course into void ether all around 
the assemblage. These etherial waves carry away gradually 
into infinite space the kinetic energy of the atoms, originally 
given to them by gravitation between all parts of the con- 
tracting assemblage. A first effect of this loss of energy 
would be to continue the raising of temperature in the central 
region, which in § 10 was said to take place during the whole 
of the gaseous stage of the evolution. As time advances, the 
dense gas or liquid in the central parts comes to a maximum 
of temperature. After this there is a general diminution of 
temperature, by the conduction and radiation of heat out- 
wards : the whole mass goes on cooling, and is automatically 
kept largely stirred by irregular convection-currents, of cooled 
liquid flowing downwards from the surface, and of hotter and 
less dense liquid rising from below. 
§ 12. If, as would be the case were the liquid melted iron, 
or water, the solidified material is less dense than the liquid 
at the same temperature and pressure, a continuous crust 
would form all over the surface of the globe ; which would 
grow thicker and thicker inwards, by freezing of the interior 
liquid in contact with it, in virtue of conduction of heat 
outwards through the crust. This solid crust, completely 
enclosing a liquid, would be burst by the liquid expanding 
as it freezes (just as a closed water-pipe is burst by water 
freezing inside it). If the spherical shell bursts into many 
fragments these would all float. The freezing of the liquid 
exposed in the openings thus produced in the crust would 
quickly fill up the gaps; and thus the process of freezing 
all around would spread inwards to the centre. Such may 
possibly be the history of the earliest solidification of part 
of the Earth's mass, forming a metallic central nucleus by 
coalition of primeval atoms of iron, nickel, gold, platinum, 
or other dense metals. 
§ 13. But the Earth, while not improbably metallic in its 
central parts, is probably in the main of " earthy " or rocky 
materials. It seems highly probable that, unlike the materials 
mentioned in § 12 which expand in freezing, all the " earthy" 
materials of the Earth contract in freezing. Bischoff*, in 
* Bulletin de la Soc. Geol. 2nd series, vol. iv. p. 1312. 
