Dr, T. Sterry Hunt — On Chemical Geology. 51 



exposition of the principle, and of Mr. Hopkin's argument therefrom, for the 

 solidity of the globe, will be found in the fourth of Tyndal's lectures on Heat as a 

 Mode of Motioti. See, also, Sorby's Bakerian lecture for 1863, cited farther on. 

 Mr. Forbes must consider that just so far as he admits the condensing power of 

 the pressure of the superincumbent mass, he increases the difficulty of maintaining 

 that rocky mass in a liquid state. 



The condensing effect of pressure was by Dr. Young estimated to be sufficient 

 to reduce a mass of granite at the earth's centre to one-eighth its bulk at the sur- 

 face, which would give to the earth a mean density equal to twelve or thirteen 

 times that of water. This consideration has led a recent writer in the London 

 Athenmim to conclude with Herbert Spencer, that our earth and the other planets 

 may be only shells of varying thicknesses, enclosing a central cavity filled with 

 vaporous matter, by which hypothesis we may explain their apparently feeble den- 

 sities. See Mr. Spencer's essay on the Nebular Hypothesis in the Westminster 

 Review for July, 1858. It may be observed that his view, which supposes conden- 

 sation to have resulted in the formation of a solid shell around a gaseous nucleus, 

 is not incompatible with my scheme, which is simply opposed to a liquid interior. 

 See also the note of Mr. Barkas, in this Magazine for September last, page 426. 

 Leaving Mr. Forbes to settle these vexed questions, we may remark that in case 

 we suppose condensation of the gaseous globe to have commenced either at the 

 centre, or around a gaseous nucleus, it is probable that solidification from pressure 

 must have taken place long before the liquefaction of earthy matters was complete. 

 But if we adopt Mr. Forbes's second hypothesis, either that pressure would not 

 materially augment the density nor raise the melting point of the fused mass, what 

 grounds has he for assuming, as he does, that there occurred a separation of the 

 liquid into zones of different densities ? That metallic sulphids could be formed at 

 an elevated temperature, by condensation from an atmosphere containing an excess 

 of oxygen, is contrary to all that we know of chemical affinities ; sulphurous acid 

 and metallic oxyds would be the results so soon as the temperature fell below that 

 of dissociation. As for the noble metals, whose compounds with oxygen are de- 

 composed at elevated temperatures, their great volatility, as compared with earthy 

 and metallic oxyds, would keep them in the gaseous form till the last stage of pre- 

 cipitation of earthy oxydized matters, when by far the greater part of the globe 

 was probably solidified. Hence we now find them in the earth's superficial crust, 

 instead of being, as Mr. Forbes would suppose, carried to the centre of the planet. 

 Judging from what we know of chemical affinities, and of the proportions of 

 the elements now existing in the superficial parts of the globe, we cannot conceive 

 anything else than the production of a homogeneous oxydized silicated mass, upon 

 which, at a late period, would be precipitated the noble metals. From this mass, 

 while yet liquid, there might take place a separation of various crystalline compounds, 

 by a process analogous to that by which pure lead separates from the bath of the 

 argentiferous alloy in Pattison's process, as Fournet has already suggested (Geo!. 

 Lyonnaise, 1862, page 398). The last congealed and lighter portion of our globe, 

 with which alone we have to do, was, probably, a sort of mother-liquor from 

 which, during its slow cooling, compounds of various constitution and density may 

 well have crystallized. In furnace operations, it is true, we may obtain, besides 

 silicated slags, a dense stratum of reguline metals, sulphids or arsenids on the one 

 hand, and a lighter one of saline sulphates or chlorids on the other. But neither 

 of these classes of compounds was possible in the cooling globe, the reguline 

 matters for reasons just given, and the saline compounds, for reasons yet to be ex- 

 plained. 



I have in my lecture set forth that the earth's superficial crust must have been 

 composed of silicates of the metallic, earthy and alkaline bases, surrounded by a 

 dense acid atmosphere of hydrochloric, sulphurous and carbonic acids, besides 

 watery vapor, nitrogen and oxygen. These chemical combinations are such as 

 would naturally result from the affinities brought into play at the elevated tempera- 

 tures then prevailing, in virtue of which all those elements capable of forming fixed 

 and stable compounds with oxygen would be precipitated as oxyds. In these con- 

 ditions, as already said, no metallic sulphids would be formed, and the whole of 

 the sulphur would be found as sulphurous acid. In like manner the production of 

 alkaline chlorids under such conditions, is inconceivable, since in the conjoined 

 presence of oxygen, hydrogen, and silicon or silica, an alkaline silicate and hydro- 



