May 1 1, 1905J 



NA TURE 



31 



by its superior gravity. What would happen to it as it 

 descended towards the eartii's centre? 



The densities and pressures in the outer layers of the 

 earth, found by Laplace's law, are as follows : — 



The above table shows that before the mass has de- 

 scended 31-85 km. (i/2ooth of the radius) the pressure about 

 it would have become more than 8000 atmospheres, which 

 would force the molten fluid deep into the heated rock. 

 The rising temperature at that depth would also rapidly 

 dissolve the mass, and before the solid has sunk through 

 another equal space in the viscid liquid, and thus reached 

 a depth of 637 km., it seems almost certain that it would 

 be completely dissolved. 



It must be borne in mind that the solid is not much 

 denser than the liquid ; and as the liquid is highly viscous 

 the mass would sink slowly, while the increase of tempera- 

 ture and pressure would conspire together in the most 

 powerful manner to dissolve the mass and reduce it to the 

 same temperature and density as the enclosing liquid, which 

 would be forced into it on all sides by a pressure vastly 

 greater than any known in our laboratories. 



Even if we make the violent assuniption that the sink- 

 ing mass is a kilometre, or several kilometres, thick, it 

 is difficult to see how it could continue its downward 

 course, undissolved by temperature and pressure, below a 

 depth appro.ximating one-tenth of the radius, or 637 kilo- 

 metres. The sinking would be quite slow, owing to stiff- 

 ness of the fluid, and could hardly be accomplished to 

 this depth inside of several days, or more probably weeks. 



Moreover, before the mass reached a depth of 260 kilo- 

 nietres, or less than one-twentieth of the radius, the density 

 of the molten fluid would become 20 per cent, greater than 

 it was at the surface, owing to pressure ; and when the 

 solid mass was no denser than the surrounding fluid it 

 would cease to sink. Or, if it had acquired a small 

 velocity downward in the fall from the surface against the 

 viscous resistance ot the fluid, which is enormously in- 

 creased by the eddy arising from the condition of con- 

 tinuity, it might go down a little lower until the motion 

 was overcome by the buoyancy of the denser fluid below. 

 .'\ccordingly, so far as one can see, solidified crust in sink- 

 ing could by no possibility go lower than one-tenth of the 

 radius, which would hardly accomplish the building up 

 of a solid nucleus. 



In considering the effects of pressure in forcing molten 

 fluid into the sinking solid, we have not assumed that the 

 density would thereby be increased ; for at the great 

 temperature of the fluid it is obvious that the solid into 

 which the hot liquid entered would be dissolved, and heat 

 from the fluid would be conducted rapidly through the 

 solid mass. Thus no cause seems to be overlooked which 

 could invalidate our conclusion. 



It rests primarily upon the enormous pressures known 

 to exist at great depths in the earth, and their undeniable 

 effect in forcing the molten fluid into any possible solid 

 bodv, so as to prevent it- attaining any considerable depth 

 without dissolving : and upon the assumption that even 

 molten rock under such forces would take approximately 

 the density given by Laplace's law, -which hardly admits 

 of reasonable doubt. 



In considering these questions heretofore, the hypothesis 

 of incompressibility for the molten fluid has been tacitly 

 implied or assumed. Whether such an hypothesis is justi- 

 fied will appear differently to different minds, but for our 



NO. T854, VOL. 72] 



part we cannot hesitate in rejecting it on account of the 

 known porosity of all matter, and its observed yielding 

 and condensation under great forces. 



On account of the difliculty in handling liquids, especially 

 when at high temperatures, they have not been so care- 

 fully investigated in the laboratory as solids ; but there 

 remains scarcely any doubt that under planetary pressure 

 they would all yield like sponges. 



In indicating his interest in the paper on planetary 

 pressures i^ksitoiwrnische Nachricliten, No. 3992), one of 

 the most eminent British mathematical physicists has 

 pointed out that to his mind the present writer has under- 

 estimated the probability that the earth has a metallic 

 nucleus. I have since pointed out in a letter to the editor 

 of N.iTURE (April 13, p. 559) that pressure, and not metallic 

 constitution, is the true physical cause of the earth's 

 rigidity ; for under such pressure any kind of matter would 

 assume a hardness greater than that of steel ; and as the 

 material is above the critical temperature of every sub- 

 stance it is really gaseous, and would expand with in- 

 credible violence if the pressure could only be relieved. 



In the Astronomische Nachricliten, No. 3992, I have 

 shown that in any mass of considerable size, so condensed 

 that the pressure amounts to millions of atmospheres, 

 circulation at great depth becomes practically impossible, on 

 account of the friction due to the increasing pressure as 

 we descend within the mass. The pressure and friction 

 which prevent circulation also prevent separation of the 

 elements according to their densities. While it ma}" ' 

 not be possible to say that there is not an increasing 

 amount of metals, such as iron, towards the centre of the 

 earth, it is, I think, clear that there is no distinctively 

 iron nucleus ; for the existence of such a nucleus would 

 imply that the earth's mass had unimpeded circulation 

 when in a fluid state, all of which is to the last degree 

 improbable. 



When the earth was less condensed it was at lower 

 temperature, and the elements may not have been fused : 

 and as condensation advanced, and the temperature rose, 

 the friction due to pressure operated with increasing in- 

 tensity to destroy circulation, which would thus be re- 

 stricted to the subsidence of compact masses decidedly 

 denser than the surrounding fluid. .\s the fluid was 

 necessarily at high temperature, a compact mass would 

 soon be dissolved, and further circulation of its elements 

 practically cease. 



It seems, therefore, very difficult to escape the con- 

 clusion that the earth's interior is a magma of all the 

 elements, the increasing density towards the centre being 

 due primarily to pressure. If any separation of the metals 

 from the rocks took place, it could only be near the sur- 

 face where the pressure is slight ; but because the rocks 

 predominate at the surface, we must not conclude the same 

 material does not exist abundantly in the great central 

 nucleus of the globe. 



The difference in the point of view here adopted and 

 that held by the older school of physicists is based primarily 

 upon the eft-ects of pressure. -While there is a certain dis- 

 appointment in negative results, they are sometimes useful 

 in leading us to new conceptions, and perhaps we may 

 hope that further study of these difficult questions wilt 

 produce results admitting of general acceptance. It should 

 be added that the pressures for the interior of the earth, 

 calculated in the Astronomische Nachrichten, No. 3902, 

 would not be very greatly modified by any other admissible 

 law of density. 



The researches of Radau and Darwin (c/. Monthly 

 Knfices. Roy. Astron. Soc. December. i8qq, pp. 122-3-) 

 have shown that, so far as the mathematical conditions 

 are concerned, the law of density within the earth might 

 depart considerably from that of Laplace. But on physical 

 grounds, inclndiiif; the incontestahly steady rise of pressure 

 towards the earth's centre, ivhatevcr he the exact law of 

 density, and especially the observed yielding and con- 

 densation of all matter under such forces, T hold that the 

 true law is essentially that of Laplace, and any departure 

 from it in the actual arrangement of the matter of the 

 fflobe is likely to be extremely small and unimportant. 



T. J. J. See. 

 U.S. Naval Observatory, Mare Island, California, 

 March 31. 



