9 
potential. There may thus be both an indefinite demand, and 
an equally unlimited supply. The real problem will depend 
mainly on these two elements, which are entirely absent in 
the solution Sir William has proposed. The calculation is 
really a partial survival from that fluid- caloric theory which is 
now universally abandoned. 
14. The doctrine of uniformity, as held by Sir C. Lyell, 
rests on a confusion of two things wholly distinct, — the con- 
stancy of natural laws, such as gravitation and cohesive 
affinity, and the sameness of the conditions under which they 
operate at widely separated periods of time. But these con- 
ditions are changing hourly through the action of the laws 
themselves, and the difference in the course of ages becomes 
so great as wholly to falsify any conclusions which are based 
on the assumption of their near approach to identity. I fully 
agree, then, with Sir W. Thomson, in his protest against that 
theory ; but I cannot accept, as reasonable or true, the special 
ground on which he bases his opposition. Mr. Croll sets the 
two doctrines in contrast in the following passage, which 
shows the immense scale of time adopted by uniformitarian 
theoi’ists. 
“ It was the modern doctrine that the great changes under- 
gone by the earth’s crust were produced not by convulsions 
of nature, but by the slow and almost imperceptible action of 
sun, rivers, snow, frost, ice, which impressed so strongly on 
the minds of geologists the vast duration of geological periods. 
When it was considered that the rocky face of our globe had 
been carved into hill and dale, and worn down to the sea-level 
by these apparently trifling agents, not once or twice but many 
times, in past ages, it was not surprising that the views enter- 
tained by geologists on the immense antiquity of our globe 
should not have harmonized with the deductions of physical 
science. It had been shown by Sir W. Thomson and others, 
from physical considerations of the sun’s heat and the secular 
cooling of our globe, that the history of the earth’s crust must 
be limited to a period of something like a hundred millions 
of years. But these speculations had little weight when 
pitted against the stern and undeniable fact of subaerial 
denudation. How were the two to be reconciled ? Was it the 
physicist who had under-estimated geological time, or the 
gelogist who had over-estimated it? Few familiar with 
modern physics, who have given attention to the subject, would 
admit that the sun could have been dissipating his heat at the 
present enormous rate for a pei’iod much beyond a hundred 
millions of years.” 
15. In this conflict of the two theories, I believe that there 
