ASTRONOMY AND GEOLOGY EDDINGTON 197 



that has a direct bearing on geology I can not say, since I have 

 nothing to guide me as to the course of its subsequent checkered 

 history. I do not say that the earth was a gaseous body when it first 

 became recognizable as an independent planet, but I am convinced that 

 its material was at one time merged in a completely gaseous sun. 



It may be of interest to indicate why it seems so probable that 

 a star in its early diffuse state is gaseous and not meteoric. The 

 stars are known to be of closely similar mass. There are occasional 

 exceptions, but probably 90 per cent of them are between one-half 

 and five times the sun's mass. We have no explanation of this 

 uniformity if they are initially merely aggregations of solid meteors ; 

 but we have a very exact explanation if they are gaseous. In 

 fact this critical mass round which the actual masses of the stars 

 cluster so closely is predicted by the theory of equilibrium of 

 spheres of gas, using only well-known physical constants deter- 

 mined in the laboratory. The crucial factor is radiation pressure, 

 which is inappreciable in smaller masses, and almost suddenly 

 takes control between one-half and five times the sun's mass. There 

 can be little doubt that large radiation pressure, tending to overcome 

 gravity, conduces to instability, so that larger masses have small 

 chance of survival. Somewhere about one-half the sun's mass the 

 radiation pressure no longer counts seriously, so that there is no 

 tendency for the primitive material to break into smaller units. 



The existence of radioactive minerals on the earth seems to supply 

 another reason for believing that its material was originally sub- 

 jected to high temperature or to physical conditions of a different 

 order from those now prevailing. In radioactivity we see a mecha- 

 nism running down which must at some time have been wound up. 

 Without entering into any details, it would seem clear that the 

 winding-up process must have occurred under physical conditions 

 vastly different from those in which we now observe only a running 

 down. The only possible guess seems to be that the winding- 

 up is part of the general brewing of material which occurs under 

 the intense heat in the interior of the stars. 



The trend of this argument has been against the Chamberlin- 

 Moulton hypothesis, and in favor of some form. of nebular orgin 

 of the solar system. It is, of course, accepted that the details of 

 the original nebular hypothesis of Laplace require modification. 

 Also the word nebula is meant to signify diffuse gaseous material 

 in general, and has no immediate connection with those objects 

 which we see in the sky, and call nebulae more particularly. There 

 is still controversy as to what process of evolution is represented by 

 the spiral nebulas which are seen in such numbers — what they will 

 ultimately turn into; but the controversy is whether the spiral 



