228 THE AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 



lava. Without insisting that these molecular inquiries are demonstra- 

 tive for they only profess to be preliminary — they seem, at least, to 



justify the radical inquiry whether the hypothesis that the earth was 

 once a gaseous nebula can be entertained with any confidence, in the 

 light of modern molecular physics. As an aT)stract proposition in 

 physics addressed to physicists, would Lord Kelvin feel free to assert 

 that the water now on the surface of the earth would be retained 

 within its gravitative control if the earth were heated so that its rock 

 substance was volatilized? May I be pardoned for inquiring whether 

 Lord Kelvin has not joined the company of geologists and neglected 

 some of the physical considerations that bear pertinently on the 

 problem in hand? 



But passing this point and striking hands with Lord Kelvin in assum- 

 ing "the almost certain truth that the earth was built up of meteorites 

 falling together," what imperative reason is there for inferring a gas- 

 eous or even a white-hot liquid condition as a result? It goes without 

 saving that the energy of impact of the falling meteorites would be 

 sufficient, under assumable conditions, to give rise to the liquid condi- 

 tion, and much more, but the actual condition that would be assumed 

 by the earth woidd be dependent wholly on the rate at which the 

 meteorites fell in. If they fell in simultaneously from assumable dis- 

 tances an intensely hot condition may be predicated with all the confi- 

 dence of logical certitude. If they fell at as great intervals as they do 

 to-day, a low surface temperature may be predicated with equal cer- 

 tainty. If they fell in at some intermediate rate, an intermediate ther- 

 mal state of the surface must be postulated. No physical deduction 

 can be more firm than that the temperature of the surface of the earth 

 would be rigorously dependent on the rate of infall, so far as the infiu- 

 enee of infall alone is concerned. Before a white-hot condition can be 

 regarded as a safe assumption, it must be shown that the meteoroids 

 would necessarily fall together at a highly rapid rate; otherwise the 

 heat of individual impacts would be lost concurrently, as is now the 

 case, and would not lead to general high temperature. 



Now, has Lord Kelvin, or any other of our great teachers in physics 

 or in astronomy, followed out to a final conclusion, by the rigorous 

 processes of mathematics, the method and rate of aggregation of a 

 multitude of meteorites into a planet, so as to be able to authoritatively 

 instruct us as to the rapidity at which the ingathering would take 

 place? Can the problem be solved at present with any such close 

 approximation to precision as to determine whether a liquid or a gas- 

 eous state would or would not ensue ? I assume that the most probal)le 

 hypothesis relative to the distribution and movements of the meteorites 

 is one that assumes that they consisted of a swarm or belt revolving 

 about the sun in the general neighborhood of the present orbit of the 

 earth; in other words, some form of meteoroidal substitute for the 



