186 CHEMISTRY OF THE EARTH. 



and, altliough it lias met with opposition, appears to be that which ac 

 cords best with our present knowledge of the chemical and physical con- 

 ditions of matter, such as we must suj^pose it to exist in the condensing 

 gaseous mass which, according to the nebular hypothesis, shouhl form 

 the center of our sohir system. Taking this, as we have already done, 

 for granted, it matters little whether we imagine the different planets 

 to ha^^e been successively detached as rings during the rotation of the 

 l)rimal mass, as is generally conceived, or whether we admit with Oha- 

 cornac a process of aggregation or concretion operating within the 

 primal nebular mass, resulting in the production of sun and plan<?ts. 

 In either case we come to the conclusion that our earth must at one time 

 have been in an intensely heated gaseous condition such as the sun 

 now presents, self-luminous, and with a process of condensation going 

 on at first at the suriace only, nntil by cooling it must have reached the 

 IJoint where the gaseous center was exchanged for one of combined and 

 liquefied matter. 



§ 9. nere commences the chemistry of the earth, to the discussion of 

 which the foregoing considerations have been only preliminary. So 

 long as the gaseous condition of the earth lasted, we may su])pose the 

 whole mass to have been homogeneous ; but when the temperature be- 

 came so reduced that the existence of chemical compounds at the center 

 became possible, those which were most stable at the elevated te^Jipera- 

 ture then prevailing, would be first formed. Thus, for example, while 

 compounds of oxygen with mercury, or even with hydrogen, could not 

 exist, oxides of silicon, aluminium, (jalcium, magnesiuai, and iron might 

 be formed and condensed in a liquid form at the center of the globe. 

 By i)rogressive cooling, still other elements would be removed from the 

 gaseous mass, which would form the atmosphere of the non-gaseous 

 nucleus. We ma^' suppose an arrangement of the condensed matters 

 at the center according to their respective specific gravities, and thus 

 the fact that the density of the earth as a whole is about twice the mean 

 density of the matters which form its solid surface may bo explained. 

 Metallic or metalloidal compounds of elements, grouped differently from 

 any compounds known to us, and far more dense, may exist in the cen- 

 ter of the earth. The condensing effect of pressure as we approach the 

 center of the globe has, however, been regarded by some as far more 

 than sufficient to account for the considerable mean density of the planet, 

 and, according to Dr. Young, would be sufficient to reduce a mass of 

 granite, transported to the earth's center, to one-eighth of the bulk 

 which it occupied at the surface, which would give to the earth a mean 

 density equal to twelve or thirteen times that of water. This considera- 

 tion has led a recent writer to conclude, with Herbert Spencer, that our 

 earth and the other planets may be only shells of varying thickness, 

 inclosing a central cavity filled with vaporous matter, by which hypoth- 

 esis may be explained their apparently feeble density. It is, however, 

 a matter of indifference, so far as our argument is concerned, whether 

 the process of condensation commenced around such a central cavity, 

 or at the center of the globe itself. 



§ 10. The processes of combination and cooling having gone on until 

 those elements which are not volatile in the heat of our ordinary fur- 

 naces were condensed into a liquid form, we may here inquire what 

 would be the result, upon the mass, of a further reduction of tempera- 

 ture. It is generally assumed that in the cooling of a liquid globe of 

 mineral matter, congelation would commence at the surface, as in the 

 case of water : but water offers an exception to most other liquids, in- 



