ything that can be proved to the contrary, one or two
hundred million years might serve the purpose, even of a thoroughgoing
Huttonian uniformitarian, very well.

But if, on the other hand, the 100,000,000 or 200,000,000 years appear to
be insufficient for geological purposes, we must closely criticise the
method by which the limit is reached. The argument is simple enough.
_Assuming_ the earth to be nothing but a cooling mass, the quantity of
heat lost per year, _supposing_ the rate of cooling to have been uniform,
multiplied by any given number of years, will be given the minimum
temperature that number of years ago.

But is the earth nothing but a cooling mass, "like a hot-water jar such
as is used in carriages," or "a globe of sandstone," and has its cooling
been uniform? An affirmative answer to both these questions seems to be
necessary to the validity of the calculations on which Sir W. Thomson
lays so much stress.

Nevertheless it surely may be urged that such affirmative answers are
purely hypothetical, and that other suppositions have an equal right to
consideration.

For example, is it not possible that, at the prodigious temperature which
would seem to exist at 100 miles below the surface, all the metallic
bases may behave as mercury does at a red heat, when it refuses to
combine with oxygen; while, nearer the surface, and therefore at a lower
temperature, they may enter into combination (as mercury does with oxygen
a few degrees below its boiling-point), and so give rise to a heat
totally distinct from that which they possess as cooling bodies? And has
it not also been proved by recent researches that the quality of the
atmosphere may immensely affect its permeability to heat; and,
consequently, profoundly modify the rate of cooling the globe as a whole?

I do not think it can be denied that such conditions may exist, and may
so greatly affect the supply, and the loss, of terrestrial heat as to
destroy the value of any calculations which leave them out of sight.

My functions as your advocate are at an end. I speak with more than the
sincerity of a mere advocate when I express the belief that the case
against us has entirely broken down. The cry for reform which has been
raised without, is superfluous, inasmuch as we have long been reforming
from within, with all needful speed. And the critical examination of the
grounds upon which the very grave charge of opposition to the principles
of Natural Philo