internal heat to maintain the exterior at a warm
and indeed at a very hot temperature. Nor is there any bound to our
retrospect arising from the operation or intervention of any other
agent, so far as we know; consequently the hotter and the hotter grows
the surface the further and the further we look back. Nor can we stop
until, at an antiquity so great that I do not venture on any estimate
of the date, we discover that this earth must have consisted of
glowing hot material. Further and further we can look back, and we see
the rocks--or whatever other term we choose to apply to the then
ingredients of the earth's crust--in a white-hot and even in a molten
condition. Thus our argument has led us to the belief that time was
when this now solid globe of ours was a ball of white-hot fluid.

On the argument which I have here used there are just two remarks
which I particularly wish to make. Note in the first place, that our
reasoning is founded on the fact that the earth is at present, to some
extent, heated. It matters not whether this heat be much or little;
our argument would have been equally valid had the earth only
contained a single particle of its mass at a somewhat higher
temperature than the temperature of space. I am, of course, not
alluding in this to heat which can be generated by combustion. The
other point to which I refer is to remove an objection which may
possibly be urged against this line of reasoning. I have argued that
because the temperature is continually increasing as we look
backwards, that therefore a very great temperature must once have
prevailed. Without some explanation this argument is not logically
complete. There is, it is well known, the old paradox of the geometric
series; you may add a farthing to a halfpenny, and then a
half-farthing, and then a quarter-farthing, and then the eighth of a
farthing, followed by the sixteenth, and thirty-second, and so on,
halving the contribution each time. Now no matter how long you
continue this process, even if you went on with it for ever, and thus
made an infinite number of contributions, you would never accomplish
the task of raising the original halfpenny to the dignity of a penny.
An infinite number of quantities may therefore, as this illustration
shows, never succeed in attaining any considerable dimensions. Our
argument, however, with regard to the increase of heat as we look back
is the very opposite of this. It is the essence of a cooling bo