611 
only exhaustion of potential energy, which in this case could scarcely 
be other than chemical affinity between substances forming part of 
the earth’s mass. But it is certain that either the earth is becom- 
ing, on the whole, cooler from age to age, or that the heat conducted 
out is generated in the interior by temporary dynamical action (such 
as chemical combination). To suppose, as Lyell has done,* that 
the substances combining together, according to the chemical hypo- 
thesis of terrestrial heat, may be again separated electrolytically by 
thermo-electric currents due to the heat generated by their combin- 
ation, and thus the chemical action and its heat continued in an end- 
less cycle, violates the first principles of natural philosophy in ex- 
actly the same manner and to the same degree, as to believe that 
a clock constructed with a self-winding movement may fulfil the ex- 
pectations of its ingenious inventor by going for ever. 
Adopting as the more probable, the simpler hypothesis that the 
earth is merely a heated body cooling, and not, on the whole, influ- 
enced to any sensible degree by interior chemical action, the author 
applies Fourier’s theory of the conduction of heat to trace the 
earth’s thermal history backwards. From data regarding the spe- 
cific heat and thermal conductivity of the earth’s substance, he in- 
vestigates the time that must elapse from an epoch of any given 
uniform high temperature throughout the interior, until the present 
condition of underground temperature could be reached. Taking 
into account the very uncertain character of the data when high 
temperatures are concerned, he infers that most probably either the 
whole earth must have been incandescent at some time from 
50,000,000 to 500,000,000 years ago, or that at some less ancient 
date, but still anterior to the earliest human history, there must have 
been up to the surface a temperature above the boiling-point of 
water. Either alternative — or indeed any theory whatever con- 
sistent with the principles of natural philosophy regarding previous 
conditions of the earth — is as decisive against the views of those 
naturalists who acknowledge no creation of life on the earth within 
fathomable periods of time, as the plainest elements of dynamics 
are against those who maintain that we have no evidence in nature 
O 
of an end. 
* Principles of Geology. 
4 N 
VOL. IV. 
