768 On the Evidence that the Earth's Interior is Solid. (August, — 
change would be retarded. Now, when convection no longer — 
caused the heterogeneous materials of the earth to mingle, the — 
cooling rate would change from the comparatively rapid rate of : 
convection-cooling to the very slow rate of cooling by the con- 
duction of liquids. 
It is to be remembered that to have convection in liquids at all 
there must be some external source which shall, at some point, 
continually supply an increment of heat, but for a cooling globe 
no such supply exists. These are facts that ought to be taken 
into account in all discussions relating to the age of the earth 
or sun. 
It would seem, however, that Thomson’s view of the age of 
the earth is based upon the supposition that the earth during its 
liquid state was homogeneous and cooled throughout by conver 
tion, and that later it became solid and likewise cooled by the 
ordinary conduction of a solid body. 
The writer would hold, in contradistinction, that after the 
earliest stages the liquid earth cooled by conduction ima hetero- 
geneous liquid, and after the superficial crust was formed, by con- 
duction not only through a heterogeneous liquid, but also à 
heterogeneous and, at least in its exterior portion, a more or less 
discontinuous or fragmental solid. . In this way it would seem e 
if biologists might gain a portion, if not all, the time desired, | 
which is now denied them by the physicist. 1 
In the same way, if the heavier gases tend to lie nat i 
center in a hot gaseous body, the exceedingly slow rate of z 
ing on account of the poor conductivity of gases ought to 
taken into account in all discussions relating to the ag° of Px 
body formerly gaseous. Another factor would be the heat 
: : form the mineral : 
engaged by the chemical unions necessary to tor ; 
combinations, now existant on the earth, out of the once ee 
FR garea. po Jå 
ut to return ; when the lighter surface materia! ©" ae 
had cooled sufficiently, a crust would be formed which, 
either to its lighter state in its hot condition, or to its sco 
character and the viscidity of the material beneath, wo" | 
sink, It is to be remembered that on account of the aie B 
rocks through the softened or viscous state to the solid, ta" 
viscous material immediately below the solid cru 
nearly the same condition and temperature as the 
st would be 
overlying crust 
