A history without dates must seem very unsatisfactory to a historian, and the question will naturally arise whether we can assign any definite time to the various critical events recorded in the evolution of the earth. At present we can only make more or less plausible estimates. Thus, from a consideration of the thickness of the sedimentary crust, and the rate at which sediments are now being deposited, it has been asserted that the interval which separates us from the close of the Archæan era may amount to about twenty-six millions of years. Professor Joly, basing his argument on the undoubted fact that the ocean derives the greater part of its salt from the dissolved material contributed to it by rivers, comes to the conclusion that the ocean first came into existence about one hundred millions of years ago. As regards the birth of the moon, Sir George Darwin has given a minimum limit of fifty-four millions of years, but he adds that it may have taken place many hundreds of millions of years before this. Lord Kelvin has attempted to determine the time which has elapsed since the earth first acquired a solid crust. If we only knew the rate at which the earth is cooling we might calculate back to this time with some assurance of certainty, always, however, on the assumption that the earth is simply a hot body cooling like any other hot body--such, say, as a red-hot cannonball. But a few years ago it began to be seriously suspected that this assumption was a very doubtful one, for a new element--radium--was discovered in 1898, which possesses the remarkable property of spontaneously liberating heat, and this not in small quantities, but at an astonishing rate. One gramme of radium, for example, gives out enough heat in one hour to raise the temperature of one gramme of water to boiling point; hour after hour, year in, year out, this wonderful substance is setting free the energy it contains, and will continue to do so until, some thousands of years hence, it has exhausted its store. If this element should happen to exist in sufficient quantity within the earth, then the earth could not be said to be cooling just like a piece of hot iron, and the increase of temperature we experience as we descend towards the interior of the earth might possibly be due to the heat set free from radium. Indeed, the argument is not confined to the earth; it may apply also to the sun, and much of the heat we derive from that luminary may be provided by bursting atoms of radium. This was pointed out by Sir George Darwin and Professor Joly in 1903.