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 in some fashion of mechanical power into heat. To take our own sun as an example, it is now an assured doctrine that the heat so necessary for our welfare is sustained by the gradual contraction of the solar volume. The energy available for transformation into heat in this process seems sufficient to supply the radiation of the sun, not only for ages such as those which we reckon in the human period, but even throughout a lapse of time so vast as that which geology demands for the formation of the earth's crust. But it is certain that the quantity of possible light and heat to be dispensed by the sun is limited in amount. The sun cannot shine on for ever. A time must assuredly come when the mighty orb at present so brilliant will have no more potency for the radiation of light than is at present possessed by the earth or the moon.

In like manner it can be shown that the materials constituting the sun have not always been luminous. We cannot indeed say with certainty by what influence their brightness was originally kindled. It probably arose from a collision, or an approach to a collision, between two dark masses which happened to come to an encounter with enormous velocities in their progress through space. It is, however, plain that the ages during which the sun has been brilliant form only an incident, so to speak, in the infinite history of that quantity of matter of which the solar system is constituted. Notwithstanding the millions, or thousands of millions, of years for which that matter has existed, it has perhaps only once become so heated, owing to circumstances which we may describe as accidental or casual, as to have acquired the ample light-dispensing power of a sun. It is, however, possible that such periods of light-radiating capacity should have