It is very true that on such estimates as those of Lord Kelvin, and according to what astronomers and geologists believed not more than twelve or even eight years ago, regarding the secular cooling of earth and sun--that, according to these, the time is by no means "unending long," and we may foresee, not so remotely, the end of the solar heat and light of which we are the beneficiaries. But the discovery of radium and the phenomena of radio-activity have profoundly modified these estimates, justifying, indeed, the acumen of Lord Kelvin, who always left the way open for reconsideration should a new source of heat and energy in general be discovered. We know now that, to consider the earth first, its crust is not self-cooling, or at any rate not self-cooling only, for it is certainly self-heating. There is an almost embarrassing amount of radium in the earth's crust, so far as we have examined it; a quantity, that is to say, so great that if the same proportion were maintained at deeper levels as at those which we can investigate, the earth would have to be far hotter than it is. Similar reasoning applies to the sun. Definite, immediate proof of the presence of radium there is not forthcoming yet, but that presence is far more than probable, especially since the existence of solar uranium, the known ancestor of radium, has been demonstrated. The reckonings of Helmholtz and others, based upon the supposition that the solar energy is entirely derived from its gravitational contraction, must be superseded. It would require but a very small proportion of radium in the solar constitution to account for all the energy which the centre of our system produces; and, as we have already seen, the earth is to no small extent its own sun--its own source of heat. The prospect thus opened out by modern physical inquiry supports more strongly than ever the conviction that the life of this world to come will be very prolonged. It is true that there is always the possibility of accident. Encountering another globe, our sun would doubtless produce so much heat as to incinerate all planetary life. But the excessive remoteness of the sun from the nearest fixed star suggests that the constitution of the stellar universe is such that an accident of this kind is extremely improbable. As for comets, the earth's atmosphere has already encountered a comet, even during the brief period of astronomical observation. This thick overcoat of ours protects us from the danger of such chances.