Nature of the Earth's Interior. 171 



casion to describe the mode of doing so, and therefore we must 

 content ourselves with accepting as facts the results of such in- 

 vestigations, which prove that the total weight of our planet is 

 approximately five and a half times the weight of a similar globe of 

 pure water. 



Knowing thus that the mean density (or specific gravity, as it is 

 also called) of the earth is 5^, and also from direct experiment that 

 the mean density of the entire solid matter or rocks forming its 

 external crust, cannot be higher than about 2^, or less than half that 

 of the entire sphere, it naturally follows that the central parts must 

 be infinitely more heavy than the surface, in order to account for so 

 high a mean figure as 5|^ ; indeed, it has been calculated that if we 

 suppose the earth is composed of three concentric portions, of equal 

 thickness, and respectively increasing in density towards the centre 

 in arithmetical progression, that we should have an outer crust of 

 specific gravity of 2^, like our ordinary rocks ; an intermediate zone 

 of specific gravity 12, or as heavy as quicksilver, and a central 

 nucleus of about twenty times the density of water, or as heavy as 

 gold. 



This admitted increase in density has sometimes been erroneously 

 represented as entirely due to the effects of the enormous pressure of 

 the superincumbent mass, a supposition which is, however, quite 

 untenable, since the tendency of all the numerous experiments made 

 in this direction has been to prove that no substances exist which 

 can be compressed or condensed to an indefinite extent, since what 

 may be termed their approximative maximum density is soon ob- 

 tained, beyond which point the effects of pressure become so much 

 smaller in proportion to the extra force applied, that at last the 

 further condensation effected by still greater pressure becomes all but 

 inappreciable. Besides this, it must not be forgotten that the crust 

 of the earth is a species of dome, like the shell of an egg, which can 

 support itself without resting or floating upon its fluid contents, and 

 further that the earth's high internal heat, by causing the materials 

 which compose it to expand, must also counteract the effects of the 

 superincumbent pressure ; so that when all these facts are taken 

 into due consideration, it appears evident that the materials which 

 actually form the mass of the interior must be infinitely denser than 

 any of the rock matter met with at the surface, and that they must 

 also be of a metallic nature, since no other bodies are known which 

 could at all fulfil these conditions of high density. 



If, now, we imagine that the earth's interior be composed of a 

 series of concentric zones or layers, made up of substances which 

 are of more and more dense nature in proportion as they are situated 

 nearer the centre, and also take for granted that the external one 

 is rock of a density of 2|, a calculation will show that the centre 

 or nucleus will then be about specific gravity 10, or about as heavy 

 as silver. Supposing now that the zone of molten lava, which we 

 have already been led to conclude as existing at a depth of some 

 50 miles from the surface, has a density of 3, or say even 4, in order 

 to make the fullest allowance for the condensing efi'ects of super- 



