quite possible to account for the radiation of the sun on strictly scientific principles, even if we discard entirely the contributions due to meteoric matter. As the sun parts with its heat it must contract, in virtue of the general law that all bodies contract when cooling; but in the act of contraction an amount of heat is produced. By this the process of cooling is greatly retarded. It can, indeed, be shown that, if the sun contracts so that his diameter decreases one mile every twenty-five years, the amount of heat necessary to supply his radiation would be amply accounted for. At this rate many thousands of years must elapse before the diminution in the sun's diameter would be large enough to be appreciable by our measurements.
Looking back into the remote ages, we thus see that the sun was larger and larger the further back we project our view. If we go sufficiently far back, we seem to come to a time when the sun, in a more or less completely gaseous state, filled up the whole solar system out to the orbit of Mercury, or earlier still, out to the orbit of the remotest planet. If we admit that the present laws of Nature have been acting during the past ages to which we refer, then it does not seem possible to escape the conclusion that the sun was once a nebulous mass of gas such as the nebular theory of Laplace would require.
It will also throw some light upon this retrospective argument for the nebular theory if we briefly consider the probable past history of the earth. It is known that the interior of the earth is hotter than the exterior. It has been suggested that this interior heat may arise from certain chemical actions which are at present going on. If this were universally the case, the argument now to be brought forward could not be entertained. I believe, however, most physicists will agree in thinking that the interior heat of the earth is an indication that the earth is cooling down from some former condition in which it was hotter than it is at present. The surface has cooled already, and the interior is cooling as quickly as the badly conducting materials of the earth will permit. "We are thus led to think of the earth as having been hotter in past time than at present. The further we look back the greater must the earth's heat have been. "We can not stop till the earth was once red-hot or white-hot, till it was molten or a mass of fiery vapor. Here, again, we are led to a condition of things which would certainly seem to harmonize with the doctrines of the nebular theory.
The verdict of science on the whole subject can not be expressed better than in the words of Newcomb: