PROBABLE ORIGIN OF NEBULJ-:. 299 



sun's radiation for 50 million years. Twice that 

 velocity would give 200 million years' heat ; four times 

 that velocity would give 800 million years' heat, and 

 so on without limit. 



It is, however, not enough that we should have in 

 the form of motion in space energy sufficient. We must 

 have a means of converting this motion into heat — of 

 converting motion of translation into molecular motion. 

 To understand how this can be effected, we simply 

 require the conception of Collision. Two bodies 

 moving towards each other will have their motion of 

 translation converted into molecular motion (heat) by 

 their encounter. 



To which of these two causes must we attribute the 

 Sun's heat ? It is certain that gravitation must have 

 been a cause ; and if we adopt the nebular hypothesis 

 of the origin of our solar system, then from 20 to 30 

 million years' heat may thus be accounted for. But 

 we know from geological evidence that the sun has 

 been dissipating his light and heat at about the present 

 rate for a much longer period. In Chapter XVI. the 

 geological evidence for the age of the earth was dis- 

 cussed at considerable length, and it was pointed out 

 that the time which has elapsed since life began on the 

 globe cannot have been less than 60 million years. 

 Although we have good grounds for believing that 60 

 million years have elapsed since life began on the globe, 

 yet the lapse of time may really have been very much 

 longer. We are justified, therefore, in concluding that 

 our globe has been receiving from the sun for the past 

 60 million years an amount of light and heat daily not 

 very sensibly less than at present. This shows that 

 gravitation alone will not explain the origin of the 

 sun's heat, and that a far more effective cause must be 

 found. Now the only other conceivable cause exceed- 

 ing that of gravity is, of course, motion in space. 



