8 
12. It is clear, from this very statement, how much remains 
merely hypothetical in this solution, on which the calculation of 
the age of the earth’s crust is to depend. Professor Tait has 
since replaced the estimate of the limits of 20 and 400 millions 
of years by a suggested period of 10 millions only. In the 
statement quoted it is owned that three further data must be 
supplied, before the solution can be altered so as to suit the 
real conditions. The view, which Sir William rejects as more 
hypothetical, that the heat is generated by chemical change, 
seems to me less hypothetical and more natural than his 
own ; and needs only to be carried a step further and applied 
to the formation of the chemical elements themselves, by 
pressure, to supply a far more complete solution of the great 
problem. 
The rejection of uniformity of action through many millions of 
years is justified, I conceive, on many grounds. But instead 
of grounding it on the certain steady decrease of solar heat by 
exhaustion and dissipation, I think it may be based more 
reasonably on the opposite ground of its increase. For if the 
present amount has ensued after solar condensation, and the 
sun was once a diffused mass of low temperature, variation 
by increase for long ages must be one constituent element of 
the theory; but a reversal of the process, and a greater loss 
than gain of heat for many millions of years must be wholly 
improbable in the absence of any direct expei’imental 
evidence. 
13. Those theorems of Fourier, on which the reckoning is 
based, all rest on the hypothesis that the heat transferred 
from a hot to a cool body is strictly as the difference of their 
temperatures, and that the temperature is the quotient of the 
amount of heat in any body, divided by the mass. This 
implies the hypothesis that heat is a specific fluid. For it 
reasons as if the total heat of the system, between the parts 
of which conduction takes place, were a fixed quantity, not 
capable of increase or diminution, by forces generating motion, 
or motion being extinguished by expansion. But tho opposite 
view, the doctrine of Bacon and Bumford, that heat is simply 
atomic motion, is now fully established, and Sir William is one 
of those who have had no mean share in its confirmation. Hence 
the conditions of the problem of conduction, for long periods, 
must be wholly altered. There is no longer a fixed amount 
of heat, of which a small part is transferred by a definite law 
from a hot to a cooler body. It may be generated in the one 
by condensation, and conversely by expansion be destroyed in 
the other to an unknown extent. Potential may be turned 
into kinetic energy on one side, and on the other kinetic into 
