REVIEWS 461 



dimensions would be reduced about one tenth. "At the center the 

 compressibility similarly measured would be very small, viz., 2.5 X 

 io~', while the condensation would be large, viz., 0.744." 



In the absence of direct measurements on the compressibility of 

 rocks, the author computes its value from the values of Young's modu- 

 lus and the modulus of rigidity which have been obtained in some 

 instances, and compares the result with the theoretical compressibility 

 of surface locks deduced from Laplace's law. Respecting the results, 

 he remarks that "it is certainly not a little remarkable how closely 

 this value ranges with those found by experiment. It is of the same 

 order of magnitude but rather smaller than the average." He adds : 

 We find here a somewhat strong presumption in favor of the view that 

 the earth consists throughout of matter not very dissimilar from what we know 

 at the surface, and that the internal densities are due rather to condensation 

 than to the presence of heavier substances such as metals. But it is not a 

 proof of this. 



Respecting the alternative view that the greater density toward the 

 center is due to heavy metals, Fisher says : 



We may probably dismiss the supposition that these all fell in first, and 

 only regard them as segregated from a uniform mass of some kind, and hav- 

 ing gravitated towards the center. This implies a condition of liquidity. If 

 the materials were solid this separation could not have occurred. Now the 

 only force that we know of that could cause the denser materials to move bv 

 a kind of convection towards the center is gravity ; and in a solid gravity 

 would not have that effect. Moreover, it must not be forgotten that gravity 

 continually diminishes as we go deeper into the earth, and that at the center 

 bodies have actually no weight. It is greatest at the surface, and if not com- 

 petent to segregate downwards the heavy particles of a rock at the surface, 

 which we know it is not, still less could it have that effect near the earth's 

 center. 



Neither can we attribute this segregation to pressure ; for pressures act 

 equally upon the surface of heavy or light materials. If we had a layer of 

 mixed shot and sand, no steady pressure laid upon it would force the shot to 

 the bottom and bring the sand to the top. 



It seems, therefore, that the view that the denser materials in the interior 

 consist of heavy metals necessitates a condition of liquidity of the whole, which 

 accords more readily with the nebular than with the meteoric theory of its 

 origin. For we 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 



