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gotten, what you get out of tlie mathematical mill depends upon what 
you put into it. The reasoning may be unimpeachable, but it merely 
proves that, if certain assumptions be granted, certain consequences 
will follow. It may be that Lord Kelvin himself, in the enthusiasm of 
enforcing his conclusions, did not always recall the foundations on 
which they rested, and it is to be suspected that many geologists 
read no more than the conclusions. 
Kelvin's argument was based necessarily upon a number of 
assumptions. At the present time, in the light of fuller knowledge, 
it is sufficient to note one, which in 1862 seemed little open to question. 
Kelvin recognized that, while the earth is certainly losing heat, " it is 
possible that no cooling may result from this loss of heat, but only an 
exhaustion of potential energy, which in this case could scarcely be 
other than chemical affinity between substances forming part of the 
earth's mass." This, however, he dismissed as " extremely im- 
probable," and proceeded on the assumption that heat is the only 
form of energy to be reckoned with. Since the discovery of radium 
we have learnt that the earth possesses a vast store of potential energy 
in a highly concentrated form then unsuspected. Strutt has cal- 
culated, from data of a very simple kind, that the observed tem- 
perature-gradient can be wholly accounted for by radio-activity, if 
the rocks to a depth of 45 miles contain as much radium as those 
at the surface. In other words, the heat generated by radio-active 
changes within this relatively thin crust will, on that supposition, be 
sufficent to compensate that lost at the surface. Clearly, therefore, 
the actual rate of cooling of the globe — if indeed it is cooling — must 
be far less than that adopted in Kelvin's calculation, and his estimate 
of the age of the earth must be enormously increased. 
This is not all. A study of the various radio-active elements 
contained in minerals and rocks has shown that it is possible, in 
certain favourable cases, to calculate directly their age in years. 
Some estimates of this kind have been made, and the results are 
liberal enough to satisfy the most exacting claims of what may be 
called the reformed Uniformitarian creed. 
With this turning of the tables one might suppose that the old 
controversy would come to an end. But the reversal of the situation 
is in fact more complete ; for meanwhile there has arisen a formidable 
minority of geologists who contend, on geological grounds, for estimates 
of time no more elastic than Lord Kelvin's. The question is still, in 
great part, one between geologists and physicists, but it is now the 
