702 Some Results of Gravitation . '^December, 
would be the following : — If the compression of gravity first 
condensed the region of matter adjoining the surface, and 
yielded sensible heat, which heat was distributed to regions 
containing less sensible heat, this would take place as well 
towards the region of lower temperature at the centre as 
towards that at the surface. As a necessary consequence 
the central region must have grown hotter instead of cooler, 
and rarer instead of denser. And long ere this effect could 
have been reversed by the gradual approach of condensation 
to the centre, the solidification of the surface was probably 
produced, and the ready outflow of heat prevented. 
The argument often advanced of late, that as solid matter 
was formed it would sink to the centre, and thus gradually 
produce a solidified core, does not seem well founded. The 
descent of this solid matter would be opposed by friction 
with the interior gas, while its weight would decrease as it 
descended. The decreased pressure and increased tempera- 
ture to which it was exposed interiorly would tend to re- 
vapourise it. Long ere this process could have carried 
condensation to any extreme depth the local formation of 
solid matter at the surface would most probably have become 
general, and a continuous shell of solid matter have embraced 
the earth. The rapid radiation from the surface undoubtedly 
tended to produce this effect long ere the full results of com- 
pression could have been felt interiorly. In the present 
condition of the sun we have an illustration of this earlier 
stage of terrestrial evolution. In the solar orb the sensible 
heat yielded by the growing compression of the interior sub- 
stance comes to the surface by a rapid convention, and is 
rapidly radiated outwards. Reversely, the dark spots seem 
regions of downrush of the condensed surface substance. 
Such a process must continue long after the surface con- 
densation becomes sufficient to produce local solidity. Yet 
as interior compression becomes greater the convective aCtion 
must become less rapid, and the outflow of sensible heat 
less aCtive. Also increased compression must decrease the 
facility of descent of solid matter. Consequently the surface 
condensation must grow more declared as it is less hindered 
by upflowing heat, and solidification in time become general. 
This effedt must eventually be produced in the sun. It 
has, since a remote period, been produced in the earth. 
When once the solid shell had attained a certain thickness 
and rigidity it must have become permanent, and the out- 
flow of internal heat have been, in great measure, confined 
to the slow process of conduction through this solid material. 
As the solid surface became thicker and more rigid its 
