226 FROM NEBULA TO NEBULA 



only feasible hypothesis is that the primordial nebula 

 was strictly gaseous and that the sun contracted to his 

 present volume from a maximum radius no greater than 

 thirty million miles, equivalent to a radiating longevity 

 of less than 3,000,000 years, liberally estimated. 



So much for the quantitative deficiency of the con- 

 traction theory, now as to its qualitative merits. Al- 

 though all astronomers, it appears, accept the theory, 

 there is a division of opinion among them as to whether 

 the sun is getting hotter, as time goes on, or colder. Such 

 an elementary disagreement as this certainly does not in- 

 spire confidence, especially when neither view accords 

 with the phenomena demanding elucidation. The sort of 

 sun geology prescribes is one which, though having evi- 

 dently suffered many vicissitudes, has nevertheless man- 

 aged to preserve a rather uniform average of tempera- 

 ture throughout geological time. That is to say, if we 

 conceive geological history to be divided into periods of 

 equal duration, ancient, medieval and modern, the mean 

 temperature of the sun during each of these was virtually 

 the same as it was in the others. A secularly cooling or 

 warming sun, such as Helmholtz's hypothesis contem- 

 plates, even were it quantitatively sufficient, is therefore 

 qualitatively unacceptable. Geologists, moreover, in- 

 form us that genial and frigid ages have alternated with 

 fair regularity in each of these enormous periods 

 another essential qualification on which the Helmholtzian 

 lamp casts no light. 



Until about the year 1880 it was supposed as a mat- 

 ter of course that because the sun is, in theory, conceived 

 to be shrinking on account of his cooling, his temperature 

 must be consequently falling. It was reserved for J. 

 Homer Lane, however, to propound the weird paradox 

 that the more that luminary cools the hotter he gets ! I 

 quote Sir Robert Ball's exposition of the idea (Story of 

 the Heavens, p. 522) : 



And now for the remarkable consequence, which seems to 

 have a very important bearing on astronomy. As the globe con- 

 tracts, a part of its energy of separation is changed into heat; 

 that heat is partly radiated away, but not so rapidly as it is pro- 



