This computation would have carried little weight with the physicists had it not chanced that another computation of their own was soon made which had even more startling results. This computation, made by Lord Kelvin, was based on the rate of loss of heat by the earth. It thus resembled the previous solar estimate in method. But the result was very different, for the new estimate seemed to prove that a period of from one hundred to two hundred millions of years has elapsed since the final crust of the earth formed.
With this all controversy ceased, for the most grasping geologist or biologist would content himself with a fraction of that time. But the case for the geologist was to receive yet another prop from the studies of radio-activity, which seem to prove that the atom of matter has in store a tremendous, supply of potential energy which may be drawn on in a way to vitiate utterly all the computations to which I have just referred. Thus a particle of radium is giving out heat incessantly in sufficient quantity to raise its own weight of water to the boiling-point in an hour. The demonstrated wide distribution of radio-active matter--making it at least an open question whether all matter does not possess this property in some degree--has led to the suggestion that the total heat of the sun may be due to radio-active matter in its substance. Obviously, then, all estimates of the sun's age based on the heat-supply must for the present be held quite in abeyance. What is more to the point, however, is the fact, which these varying estimates have made patent, that computations of the age of the earth based on any data at hand are little better than rough guesses. Long before the definite estimates were undertaken, geologists had proved that the earth is very, very old, and it can hardly be said that the attempted computations have added much of definiteness to that proposition. They have, indeed, proved that the period of time to be drawn upon is not infinite; but the nebular hypothesis, to say nothing of common-sense, carried us as far as that long ago.