172 Notices of Memoirs — The EarWs Interior. 



incumbent pressure, we sliould then find by calculation that this 

 zone could not extend deeper than 400 miles, since below this depth 

 the matter would be so heavy that its density can only be explained 

 upon the supposition that it consists of metallic compounds, and as 

 the density of the lower zones would still go on increasing up to the 

 centre of the earth, the natural inference would be that the whole of 

 the great central mass of our planet, situated at a distance of some 

 450 miles, or less, below the surface, is in reality formed of metals 

 and their compounds. 



Whether this great central metallic nucleus is solid or fluid may 

 next be inquired into. According to Bunsen's theory previously 

 alluded to, it ought to be solid, for, owing to the enormous pressure 

 to which it would be exposed, the solidification of the molten sphere 

 should first commence at the centre. This view would, no doubt, 

 be quite correct if the earth were known to be composed of highly 

 compressible non-metallic materials ; but since this is not the case, 

 and since, as before alluded to, the experimental data already ob- 

 tained indicate that neither the metallic nor the less compressible 

 substances become more refractory in proportion to the increase of 

 pressure, we are at present, at least, more justified in assuming that 

 this central nucleus must also be in a fluid condition, and the more 

 so, not only because it is known that, as a rule, metallic compounds 

 are much more fusible than rock silicates, but also as the well-known 

 high temperature of the earth's interior would also, by its expanding 

 action, tend to counteract the elfects of the pressure. 



In summing up the results of this inquiry, the balance of evidence 

 appears to me to be decidedly in favour of the hypothesis that the 

 interior of our earth is a mass of molten matter arranged in con- 

 centric zones according to their respective densities, and the whole 

 inclosed within a comparatively thin external crust or shell, and 

 that our globe consists of (1st) an external solid crust not exceeding 

 fifty miles in thickness — the upper third or more of which consists 

 chiefly of stratified sedimentary rocks which rest upon some, to us 

 at present unknown, species of igneous rock, which at one period 

 had formed the lower part of the primeval crust ; (2ndly) below 

 this again a zone or sheet of molten rock extending all round the 

 sphere, and reaching to a depth not exceeding 400 miles below the 

 solid crust; and (3rdly) a dense metallic nucleus, the outer part 

 of which consists of the compounds of the metals with sulphur, 

 arsenic, etc., whilst in the very centre we should expect metals in 

 an uncombined condition, or alloyed with one another, to pre- 

 dominate. 



Having now completed the task of giving a concise exposition of 

 the present state of our knowledge respecting the nature of the 

 interior of our planet, a few words may be added in conclusion 

 by way of apology for introducing a scientific subject of so ex- 

 tremely speculative a nature. Although the first thing in science 

 is to collect as many facts, i.e. truths, as possible, it is nevertheless 

 absolutely necessary, in order to utilize these truths and not allow 

 them to degenerate into a mere chaos of disconnected facts, to from 



