266 DISCUSSIONS IN COSMOLOGY. 



The amount of heat thus produced by gravitation 

 would suffice for 20,237,500 years. 



The conclusions are based upon the assumption that 

 the density of the sun is uniform throughout. But it 

 is highly probable that the sun's density increases 

 towards the centre, in which case the amount of work 

 performed by gravitation would be something more 

 than the above. 



Testimony of Geology as to the Age of Life 

 on the Globe. 



At this point, in reference to the age of our globe, 

 geology and physics are generally supposed to come 

 into direct antagonism. For if it be true, as phy- 

 sicists maintain, that gravitation is the only possible 

 source from which the sun could have derived its 

 store of energy, then the sun could not have main- 

 tained our globe at its present temperature for more 

 than about 20 millions of years. " On the very highest 

 computation which can be permitted," says Professor 

 Tait, " it cannot have supplied the earth, even at the 

 present rate, for more than about fifteen or twenty 

 million years." * The limit to the age of the sun's 

 heat must have limited the age of the habitable globe. 

 All the geological history of the globe would neces- 

 sarily be comprehended within this period. If the 

 sun derived its heat from the condensation of its mass, 

 then it could not possibly be more than about twenty 

 million years since the beginning of the Laurentian 

 period. But twenty million years would be considered 

 by most geologists to represent only a comparatively 

 small portion of the time which must have elapsed 

 since organic life began on our globe. 



* " Recent Advances in Physical Science," p. 175. 



