PKIMAET PEEIOD. 55 



the earth at the close of this the " primary period " may 

 now be considered. 



The early history of our globe forms one of the great 

 problems of geology, but there is evidence enough upon 

 which to form conclusions and show that the earth had 

 undergone some great and varied changes embracing 

 immense periods of time, and if the sciences of astronomy, 

 chemistry, mathematics, &c., be brought in to assist the 

 inquiry, some plain facts become evident. Astronomy 

 shows us orbs circulating round the sun (the planets), 

 strictly analogous to our earth, and furnishes us with their 

 densities ; these are found to be various, ranging from 

 about six times the density of water to half its density. 

 The planets appear to be at different stages of condensation, 

 and it is not unreasonable to suppose that our earth was 

 once of no greater density than the lightest of them. The 

 inner ring of Saturn is probably in a liquid state, for it is 

 transparent. The substance of which comets are composed 

 is a vapour, so rare and thin that it cannot be compared in 

 density to our atmosphere even, and yet these comets 

 preserve their identity, circulating through immense realms 

 of space with prodigious rapidity. Thus astronomy presents 

 analogies in favour of the supposition that the earth was once 

 gaseous, or at all events of much less density than at present ; 

 but mathematical inquiries go much further, and furnish 

 almost proof that the earth was once (if not now) in a 

 liquid state, for the exact form which a liquid ball would 

 assume upon rotating at the rate which the earth does, is 

 exactly that which the globe is found by measurement 

 to possess. 



Chemistry has ascertained that the heat of the earth is 

 far greater than any tract of the heavens through which it 

 passes ; that this heat could not have been communicated 

 by the sun's rays, for had it been so the surface would be 

 hotter than the interior, the very reverse of which is found 

 to be the fact, and the deeper we go down the higher the 

 temperature is. Chemistry shows further that this heat 

 can be completely accounted for by the condensation and 

 solidification of the earth itself; for the condensation of all 



