THE AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 227 



adds to the burden of explication. Both hj-potheses ultimate!}- appeal 

 to the same source, the gravdtative descent of the earth's substance. 

 Their differences lie in the modes of action assumed, respectively, and 

 these modes are determined by the antecedent conditions of aggrega- 

 tion. Has it been demonstrated that these antecedent conditions were 

 of the one kind and not of the other ? 



Lord Kelvin obviously assumes a nebulous state of the earth as the 

 controlling antecedent condition. It is not quite clear whether he 

 adopts the complete gaseous theory of Laplace, including the earth- 

 moon gaseous ring, or not. Apparently, however, he has not adopted 

 the gaseous earth-moon ring, but has substituted therefor a meteor- 

 oidal ancestry for the earth, for he says (p. 706): 



"Considering the almost certain truth that the earth was built up of 

 meteorites falling together, we may follow in imagination the whole 

 process of shrinking from gaseous nebula to liquid lava and metals, 

 and solidification of liquid from central regions outward." 



A little further on he speaks of "the gaseous nebula which at one 

 time constituted the matter of our present earth." 



Without feeling quite certain that 1 am not in error, I interpret 

 these sentences to mean that the matter of the earth was in a meteor- 

 oidal condition just previous to its falling together, and that it passed 

 into the gaseous condition as a result of the heat of impact, and that 

 from thence it shrank into the liquid and later into the solid state. If 

 this be correct, it would be interesting to learn on what grounds the 

 older hypothesis of a nebulous ring, once regarded as a quite sure 

 assumption, has been abandoned, and whether the reasons for that 

 abandonment do not bear adversely also on this modified phase of the 

 gaseous hypothesis. The strongest objection recently urged against 

 the Laplacean gaseous ring is the apparent inabilitj' of the feeble 

 gravit}^ of such a ring to overcome the high molecular velocities of its 

 lighter constituents at the high temperatures necessary to maintain 

 the refractory material of the earth in a gaseous condition.^ In addi- 

 tion to this radical objection to the gaseous earth-moon ring, there is 

 the extreme probability that, if formed, it would cool below the tem- 

 perature of volatilization of rock substance before it would concentrate 

 into a globe. 



The studies to which reference has just been made seemed to show 

 that even in the globular form it is doubtful if the earth could be 

 volatilized without the dissociation of its water and the loss of its 

 hydrogen by molecular projection away from the earth. The inquiry 

 seemed even to raise a doubt whether the vapor of water, as such, or 

 the atmospheric gases could be retained at the temperature of rock 

 volatilization; indeed, it seemed that the oceanic and atmospheric con- 

 stituents might even be in jeopardy at the temperature of white-hot 



*"A group of hypotheses bearing on climatic changes," Jour. Geol., Vol. V, No. 7, 

 October-November, 1897, pp. 658-668. 



