372 Mr. J. Croll on Geological Time, and the probable 



to the fact that the sun's density may increase greatly towards 

 his centre. But we should require to make extravagant assump- 

 tions regarding the interior density of the sun and the propor- 

 tion of original heat to that produced by condensation before 

 we could manage to account for anything like the period that 

 geological phenomena are supposed by some to demand. 



There seems to be an impression on the mind of a great many 

 geologists, that, notwithstanding all that has been advanced re- 

 garding the limitation to the age of the sun's heat, there may 

 yet be found some possible way of accounting for a duration of 

 sun-heat equal to a few million of centuries. 



If the sun has really given out some two or three hundred 

 million years' heat, one of three things must follow : (1) the 

 sun must have been in possession of that enormous store of 

 energy prior to the commencement of the geological history of 

 our globe ; or (2) he must have received it after the commence- 

 ment ; or (3) a part must have been originally possessed by the 

 sun prior to the commencement, and the rest derived afterwards. 

 We must admit that one or other of these three suppositions is 

 true; for these exhaust the whole field of the possible. But if 

 the sun had originally possessed the amount of energy supposed, 

 then his volume would have extended beyond our earth's orbit, 

 and, of course, our earth could not at that time have existed as a 

 separate planet. That amount of energy could not have been im- 

 parted to the sun since the commencement of our globe's geo- 

 logical history. It could not under any conditions have been 

 communicated by chemical agency, nor by any means conceiv- 

 able except by that of meteors or other bodies falling into the 

 sun with enormous velocity. And if we suppose gravitation to 

 be the agent that gave these bodies their velocity, then it is 

 found by calculation that gravitation falls far short of affording 

 the necessary amount of velocity. If we adopt the third suppo- 

 sition, that part of the energy was originally possessed by the 

 sun, and part imparted, this will not remove the difficulty ; for 

 as the energy imparted by gravitation could be only but a small 

 fraction of the amount required, the hundreds of millions of 

 years' heat demanded, with the exception of that portion im- 

 parted, must have been in the possession of the sun at the com- 

 mencement of our globe's geological history. But the sun cannot, 

 as we have seen, have been a sphere with the earth revolving 

 round it as a planet if it was in possession of such a store of 

 energy ; for its volume would have more than filled the entire 

 space encircled by the earth's orbit. 



It is quite possible, however, that a meteor may reach the sun 

 w r ith a velocity far greater than that which it could acquire by 

 gravitation ; for it might have been moving in a direct line to- 



