418 0. Fisher — Rival Theories of Cosmogony. 



All these were measured in C. G. S. units, in which the unit 

 of pressure is one dyne per square centimeter. To convert the 

 value of the compressibility 2*3847 X 10" G which we have 

 obtained theoretically for atmospheric pressure into the same 

 C. G. S. units, we must divide it by 1-014 X 10 6 and the result is 



l/&= 2-352 X 10- 12 . 



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. But 

 it will be noticed that Dr. Knott says of his experiments that 

 they were made on ".fairly solid rock" and our calculation 

 clearly would refer to rocks thoroughly solid, for the specific 

 gravity of surface rock which enters into our calculations has 

 no reference to rigidity. We might, therefore, expect our 

 value to be small. 



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. 



Let us next consider what the alternative view of the greater 

 density towards the center being due to heavy metals involves. 

 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 having gravitated towards the center. This 

 implies a condition of liquidity. If the materials were solid 

 this separation could not have occurred. low the only force 

 that we know of that could cause the denser materials to move 

 by 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 

 competent 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 mate- 

 rials. 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 



