C. Lloyd Morgan — Geological Time. 159 



condensation, the same effect -would be produced : lieat would be 

 evolved. According to the Nebular hypothesis, the primitive fire- 

 mist, or nebulous mass, out of which our system has been elaborated, 

 once extended beyond the orbit of the sun's most distant planet. 

 During the condensation from that nebulous state to the present 

 condition of the sun, a definite amount of heat must have been 

 given out by that luminary. This amount has been approximately 

 calculated. It would suffice for the radiation of some twenty millions 

 of years, or if, as is probable, the sun's density increases towards 

 its centre, for that of a somewhat longer period. If gravitation, 

 therefore, be the only possible source of sun heat, all geological 

 phenomena will have to be comprised within a period of less than 

 twenty millions of years. But it is highly improbable, as Mr. Croll 

 has pointed out, that the nebulous mass should have existed in the 

 gaseous state, without possessing a high temperature to begin with. 

 Nay, rather, it is most probable that it was to its excessive tempera- 

 ture that its intensely rarefied condition was due. How, then, was 

 this high initial temperature produced ? The answer must be a 

 purely hypothetical one. But there is one way in which this 

 temperature may have been produced. 



Our system is believed by astronomers to be moving onward 

 through space, probably in the direction of the constellation Her- 

 cules, and from analogy we may well presume that other suns are 

 likewise in motion. Sirius, for example, has a relative motion to 

 that of our sun, which tends to increase the distance between these 

 bodies by about twenty miles in every second of time. Other 

 celestial globes must be approaching each other. Taking an hypo- 

 thetical case, Mr. Croll tells us that, " Two bodies, each one half the 

 mass of the sun, moving directly towards each other with a velocity 

 of four hundred and seventy-six miles per second, would by their 

 concussion generate in a single moment fifty million years' heat." 

 " Why may not the sun have bepn composed of two such bodies ? " 

 he adds, " And why may not the original store of heat possessed by 

 him have all been derived from the concussion of these two bodies?" 

 " Two such bodies coming into collision with that velocity would be 

 dissipated into vapour by such an inconceivable amount of heat as 

 would thus be generated ; and when they condensed on cooling, they 

 would form one spherical mass like the sun." 



Thus, although the consideration of the sun as our source of heat 

 would seem at first to limit the history of the earth, as the habita- 

 tion of life, to some fifteen or twenty millions of years, a method by 

 which the sun-heat may have sufficed for a hundred million of years 

 is at any rate not inconceivable. 



On the other hand, Mr. Norman Lockyer •'' thinks it likely that 

 many of the substances which we believe to be elements, because we 

 have not been able to decompose them, are really compounds ; and 

 that, during the early periods of a star's lifetime, their components 

 existed in an uncombined state, the dissociation being perhaps due 

 to intense heat ; when the heat was so far reduced that it was no 

 longer able to keep the elements apart, chemical combination took 



