18 EXPLANATIONS. 



zodiacal light, which has been thought a residuum 

 of the nebulous fluid of our system, we find geo- 

 logy taking us back towards a state of our globe 

 which cannot otherwise be explained. It was 

 clearly at one time in a state of igneous fluidity, — 

 the state in which its oblately spheroidal form was 

 assumed under the law of centrifugal force. Since 

 then it has cooled, at least in the exterior crust. 

 We thus have it passing through a chemical pro- 

 cess attended by diminishing heat. Whence the 

 heat at first, if not from the causes indicated in the 

 nebular hypothesis } But this is not all. In 

 looking back along the steps of such a process, 

 we have no limit imposed. There is nothing to 

 call for our stopping till we reach one of those 

 extreme temperatures which would vaporize the 

 solid materials ; and this gives us exactly that con- 

 dition of things which is implied by the nebular 

 cosmogony. 



Of particular objections it is not necessary to 

 say much. That there should be difiiculties attend- 

 ing such an hypothesis is only to be expected ; 

 but where general evidence is so strong, we should 

 certainly be scrupulous about allowing them too 



