2 Dr. J. Croll on the Origin of Nebula;. 



sipating into space during past ages? Difficult as the ques- 

 tion at first sight appears to be, it is yet simplified and brought 

 within very narrow limits when we remember that there are 

 only two conceivable sources. The sun must have derived his 

 energy either from Gravitation, or from that other source to 

 which I directed attention several years ago*, Motion in Space. 

 All other sources of energy put together could not have sup- 

 plied our luminary with one thousandth part of that which he has 

 possessed. We are therefore compelled to attribute the sun's 

 heat to one or other of these two, or to give up the whole in- 

 quiry as utterly hopeless. The important difference between 

 the two is that the store of energy derivable from Gravitation 

 could not possibly have exceeded 20 to 30 million years' sup- 

 ply of heat at the present rate of radiation ; whereas the store 

 derivable from Motion in Space, depending on the rate of that 

 motion, may conceivably have amounted to any assignable 

 quantity. Thus a mass equal to that of the sun, moving with 

 a velocity of 476 miles per second, possesses in virtue of that 

 motion energy sufficient, if converted into heat, to cover the 

 present rate of the 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 a paper published 

 in the i Quarterly Journal of Science ' for July 1877, I have 

 discussed the geological evidence for the age of the earth at 

 considerable length, and have pointed out that the time which 

 has elapsed since life began on the globe cannot have been less 

 than 60 million years. This estimate is based upon a rough 

 estimate of the thickness of rock which has been removed by 

 subaerial denudation since the earliest epoch of which geolo- 

 * Phil. Mag. May 18G8. 



