

0. jFisher — Rival Theories of Cosmogony. 419 



may imagine that in a nebular mass cooling from the exterior, 

 the first change from a nebulous or gaseous state would be the 

 formation of a rain of condensed particles falling downwards, 

 which would continue until the whole mass became liquid, and 

 thus the heavier elements would begin to collect towards the 

 center. In this case the highest possible interior temperature 

 would be that at which the gaseous first assumed the liquid 

 condition under the pressure at the depth. 



Paradoxical as it appears, it is therefore possible that the 

 temperature in the interior may have been rendered higher by 

 a conglomeration of cold solid meteorites than by the cooling 

 of a nebula. 



We have no means of judging whether the meteorites would 

 come in rapidly or slowly, but in either case if we take no 

 account of the heat arising from impact, the amount produced 

 by condensation would be the same ; the only difference in the 

 two cases being that it would be generated in a less or greater 

 time. In the meanwhile a covering of a badly conducting 

 material would concurrently accumulate, preventing the rapid 

 escape of this heat, and at the same time increasing the pres- 

 sure, the compression, and the heat. 



To form an idea of the temperature which would be pro- 

 duced by the condensation of matter of surface density to the 

 density now existing at any given depth within the earth, not 

 taking into account its diffusion by conduction or otherwise, 

 we require to know the work which has been expended upon 

 it. Now we can estimate this in the following manner. Con- 

 ceive the earth to have been built up of meteorites falling in, 

 so that shell after shell accumulated until the globe attained 

 its present size. Then, fixing the attention upon a particular 

 unit volume, say a cubit foot, of the substance, and omitting 

 atmospheric pressure, it would successively be subject to every 

 degree of pressure from zero, when the shell of which it 

 formed a part was not covered up, until the present pressure 

 was reached, when it was buried to the depth at which it now 

 lies. If then we know the relation between the pressure and 

 the compression at every depth at the present moment, it will 

 give us the relation between the pressure and the compression 

 which that particular volume has obeyed during the course of 

 ages ; that is to say, we can judge how much compression any 

 given pressure would have produced in the substance under 

 the conditions involved. 



Laplace's law of density being based upon the assumption 

 that the increase of pressure within the earth is proportional 

 to the increase of the square of the density, in terms of a 

 pressure of one pound upon the square foot, this leads to the 



Am. Jour. Sci.— Fourth Series, Yol. XI, No. 66.— June, 1901. 

 29 



