number of moons, would be likely to retain some of his inner rings unbroken; that the earth would be likely to have a long day and Jupiter a short one; that the extreme outer planets would be not unlikely to rotate in a retrograde direction; and so on, through a long list of interesting and striking details. Not only, therefore, are we driven to the inference that our solar system was once a vaporous nebula, but we find that the mere contraction of such a nebula, under the influence of the enormous mutual gravitation of its particles, carries with it the explanation of both the more general and the more particular features of the present system. So that we may fairly regard this stupendous process as veritable matter of history, while we proceed to study it under some further aspects and to consider what consequences are likely to follow. Our attention should first be directed to the enormous waste of energy which has accompanied this contraction of the solar nebula. The first result of such a contraction is the generation of a great quantity of heat, and when the heat thus generated has been lost by radiation into surrounding space it becomes possible for the contraction to continue. Thus, as concentration goes on, heat is incessantly generated and incessantly dissipated. How long this process is to endure depends chiefly on the size of the contracting mass, as small bodies radiate heat much faster than large ones. The moon seems to be already thoroughly refrigerated, while Jupiter and Saturn are very much hotter than the earth, as is shown by the tremendous atmospheric phenomena which occur on their surfaces. The sun, again, generates heat so rapidly, owing to his great energy of contraction, and loses it so slowly, owing to his great size, that his surface is always kept in a state of incandescence. His surface-temperature is estimated at some three million degrees of Fahrenheit, and a diminution of his diameter far too small to be detected by the finest existing instruments would suffice to maintain the present supply of heat for more than fifty centuries. These facts point to a very long future during which the sun will continue to warm the earth and its companion planets, but at the same time they carry on their face the story of inevitable ultimate doom. If things continue to go on as they have all along gone on, the sun must by and by grow black and cold, and all life whatever throughout the solar system must come to an end. Long before this consummation, however, life will probably have become extinct through the refrigeration of each of the planets into a state like the present state of the moon, in which the atmosphere and oceans have disappeared from the surface. No doubt the sun will continue to give out heat a long time after heat has ceased to be needed for the support of living organisms. For the final refrigeration of the sun will long be postponed by the fate of the planets themselves. The separation of the planets from their