“Now, are geologists prepared to admit that, at some time within the last 20,000 years, there has been all over the earth so high a temperature as that?  I presume not; no geologist—­no modern geologist—­would for a moment admit the hypothesis that the present state of underground heat is due to a heating of the surface at so late a period as 20,000 years ago.  If that is not admitted, we are driven to a greater heat at some time more than 20,000 years ago.  A greater heating all over the surface than 100 deg.  Fahrenheit would kill nearly all existing plants and animals, I may safely say.  Are modern geologists prepared to say that all life was killed off the earth 50,000, 100,000, or 200,000 years ago?  For the uniformity theory, the further back the time of high surface-temperature is put the better; but the further back the time of heating, the hotter it must have been.  The best for those who draw most largely on time is that which puts it furthest back; and that is the theory that the heating was enough to melt the whole.  But even if it was enough to melt the whole, we must still admit some limit, such as fifty million years, one hundred million years, or two or three hundred million years ago.  Beyond that we cannot go."[60]

It will be observed that the “limit” is once again of the vaguest, ranging from 50,000,000 years to 300,000,000.  And the reply is, once more, that, for anything that can be proved to the contrary, one or two hundred million years might serve the purpose, even of a thorough-going 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?