194 SEE— FURTHER RESEARCHES ON [April 24, 



shell of stony material. In the paper which he recently presented 

 to the International Seismological Association in session at the 

 Hague, September 21-26, 1907, he estimates the depth of the stony 

 layer as 1,500 kilometers, which is nearly one fourth of the earth's 

 radius. 



This view that interior of the earth is metallic has been en- 

 tertained by many eminent physicists, including Lord Rayleigh ; 

 but it is beset with many difificulties. We shall here mention three 

 of the principal objections : 



1. If this constitution of the earth be admitted, the curve of 

 density will have a sudden break at a depth of about one fourth of 

 the radius ; and, as the pressure increases rapidly as we go down- 

 ward, it seems improbable that the density of the outer layer could 

 remain uniform and then change suddenly at a depth of one fourth 

 of the distance to the center. Such discontinuity in nature seems 

 highly improbable for the density, since there probably is no sensible 

 discontinuity in the laws of pressure and temperature. 



2. If the central nucleus is metallic, it follows that the denser 

 elements have separated from the rest of the mass. As the matter 

 has been essentially solid and highly rigid, owing to the pressure, 

 ever since the globe attained anything like its present dimensions, 

 this sinking would not be possible, because the resistance to the 

 motion would be much too great. Thus owing to resistance to 

 motion arising from rigidity we can not admit a separation of the 

 denser from the lighter elements of such a globe. If the metals 

 were all so deep down, it would be hard to account for the veins 

 found in the crust by any kind of eruptive process, since the globe 

 is never fissured to a depth of anything like one fourth of the 

 radius. 



3. If in addition to these mechanical objections we recall that 

 deep down the pressure is so great as to cause an interpenetration 

 of all the elements, whatever be the temperature, but especially 

 under the high temperature known to prevail in the interior of the 

 globe, so that no aggregation or crystallization of substances would 

 be possible, and the nucleus would therefore be a magma of all the 

 elements, it becomes inconceivable that the metals could separate 

 from the stonv elements bv sinking, while the latter floated to the 



