248 THE POPULAR SCIENCE MONTHLY. 



quite possible to account for the radiation of the sun on strictly scien- 

 tific 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 radia- 

 tion 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 nebu- 

 lous 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. "VVe 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 : 



At the present time we can only say that the nebular hypothesis is indi- 

 cated by the general tendencies of the laws of Nature; that it lias not been 

 proved to be inconsistent with any fact; that it is almost a necessary conse- 

 quence of the only theory by which we can account for the origin and conserva- 

 tion of the sun's heat ; but that it rests on the assumption that this conservation 

 is to be explained by the laws of Nature as we now see them in operation. 



