CHAPTER XVIII. 



THE PEOBABLE ORIGIN AND AGE OF THE SUN'S 

 HEAT. — Continued. 



Not obliged to assume that Gravitation is the only Source of the 

 Sun's Heat. — How the Mass obtained its Temperature. — No 

 Limit to the Amount of Heat which may have been produced. 

 — Age of the Sun in Relation to Evolution. — Note on Sir 

 William Thomson's Arguments for the Age of the Earth. 



Are we really under the necessity of assuming that 

 the sun's heat was wholly, or even mainly, derived from 

 the condensation of his mass by gravity? According 

 to Helmholtz's theory of the origin of the sun's heat 

 by condensation, it is assumed that the matter com- 

 posing the sun, when it existed in space as a nebulous 

 mass, was not originally possessed of temperature, but 

 that the temperature was given to it as the mass 

 became condensed under the force of gravitation. It 

 is supposed that the heat given out is simply the 

 heat of condensation. But it is quite conceivable that 

 the nebulous mass might have been possessed of an 

 original store of heat previous to condensation. 



It is quite possible that the very reason why it 

 existed in such a rarified or gaseous condition was its 

 excessive temperature, and that condensation only 

 began to take place when the mass began to cool down. 

 It seems far more probable that this should have been 

 the case than that the mass existed in so rarefied a 

 condition without temperature. For why should the 



