462 REVIEWS 



collect towards the center. In this case the highest possible interior temper- 

 ature 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 pressure, the compression, and the heat. 



To form an idea of the temperature which would be produced 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 bj' conduc- 

 tion or otherwise, we require to know the work which has been expended 

 upon it. Now we can estimate this in the following manner. Conceive the earth 

 to have been built up of meteorites falling in, so that shell after shell accumu- 

 lated until the globe attained its present si2e. Then, fixing the attention 

 upon a particular unit volume, say a cubic foot, of the substance, and omit- 

 ting 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 sub- 

 stance 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 result, that the pressure at the depth where the density 

 is^ is equal to 5.9X10'' (^^ —.y^) [where ^=density of surface rock and/= 

 density of rock at the depth under consideration]. 



If we accept Laplace's law, this expresses a fact, whether the increase 

 of density is due to condensation by pressure or to increased density in the 

 intrinsic nature of the matter. But if we assume that the increase of density 

 is caused solely by the pressure, then the above relation gives the amount of 

 pressure which would reduce matter of density .f to matter of density p under 

 circumstances existing within the earth. It will therefore remain true if the 



