452 Rev. MaxiveU H. Close — Extent of Geological Time. 



intelligible matter ? I submit that if we try to trace the line of 

 causation into a cold nebula, when we have got there we shall find 

 ourselves in a cul-de-sac, and can go no further. Moreover, we are 

 not entitled to take it as an ultimate fact without the strongest 

 reasons. It is not the custom of the physicists to take refuge in 

 Agnosticism as long as they can possibly avoid it. In other matters 

 thej' will trace the steps of causation as far back as they possibly 

 can. Let us then, instead of going back into a cold nebula which 

 cannot be accounted for, take the other turn into a heated one, for 

 which there is "a thoroughly intelligible source," in the relative 

 motion of cosmical masses which we know to be an actual fact. 

 We do not know that a cold nebula is a fact in this stage of the 

 universe. All the nebulae of which we know are apparently glowing 

 with heat, and considering the apparent tenuity of them, and the 

 enormous dimensions of some, it is highly probable that their heat 

 is the result of the falling together of their parts. As to the 

 gravitation attraction being the sole force that was to cause the 

 molecular falling together of the materials of the nebula, we do not 

 know that this was so. The shapes of some nebulae and the 

 behaviour of others by no means encourage the belief that the 

 gravitation attraction is the sole connexion, between their parts. 



But as to the gravitative attraction itself, granting that it was the 

 origin of the potential energy of the solar system nebula, do we know 

 that the unit of gravitation is constant throughout the enormous 

 interstellar space that has been traversed by our system since it first 

 began to fall together? Eecent physical speculations contemplate 

 gravitation as being not an inherent essential property of matter 

 itself. If it were so, we might well suppose it to be unchangeable, 

 as inertia most probably is. But it is conceived of as depending 

 upon the action of an external gravific medium, which might or 

 might not be there. And there seems to be nothing to lead us to 

 believe that the energy of that medium must be uniform through- 

 out time and space. Some physicists conceive of that medium as 

 being itself not amenable to the law of the conservation of energy ; 

 there is no inconsistency whatever in this. But it is rather a 

 startling position to take up, and to avoid the necessity of doing 

 so, Mr. Tolver Preston suggests that the gravific medium may be 

 comparable in its structure to a gas with an exceedingly great 

 yet limited mean excursion of its particles. This would limit the 

 range of gravitation between two material particles, though we have 

 now nothing to do with that. And he suggests that the light and 

 heat which is supposed to be entirely dissipated and lost in space 

 may not be really so — the luminiferous ether may be all the while 

 paying back to the gravific medium, in some way unknown to us, 

 the energy which it is expending in producing the heat of the 

 countless suns. This would certainly be more in analogy with what 

 we see around us of the transformation of energy. Now, let us join 

 with this Poisson's idea that the so-called temperature of space — the 

 whole radiation of the stars at any place — is not uniform, and it 

 results from speculations made by physicists themselves, not in the 



