700 THE POPULAR SCIENCE MONTHLY. 



life could come to any perfection. The solidifying of the carbonic 

 oxide was the latest and the slowest of the atmospheric changes. 



It appears that during the epoch of the hydration of the lime-rocks 

 there occurred periods when the waters were gathered into seas, and 

 were sufficiently cooled for the existence of marine infusoria, mollusks, 

 and corals. Life, in some form, has ever been ready to spring into 

 being the moment that conditions and surroundings were suitable for 

 it. After the deposition, in those temporary oceans, of considerable 

 thicknesses of Cambrian or Silurian strata, mixed with organic remains, 

 some rent or upheaval has let the waters down to new beds of unslaked 

 material, which have heated, and, as it is termed, metamorphosed 

 those first fossiliferous deposits. 



The subsequent changes which the earth's crust has undergone — 

 aqueous, volcanic, and organic — the working up of the conglomerates 

 and sandstones, the depositing of the deep-sea beds, the overflowing 

 of the traps and lavas, the storing away of the carboniferous treasures, 

 are all the story of every hand-book of geology, and pertain no more 

 to one theory than another of the origin of the rocks. When the quar- 

 ries were once made and opened, the after-work was merely mechanics 

 and masonry. 



We have heretofore assumed that the gases which originally com- 

 posed the aerial envelope of the earth took up separate positions 

 therein, according to their specific gravities. This might seem to be 

 controverted by experiments on the diffusion of gases, in which those 

 of very different weights, as chlorine and hydrogen, will intimately 

 commingle, even against gravity, when brought into contact. This 

 may be true in the narrow compass of a laboratory experiment, and 

 yet not apply to any considerable thicknesses of the gases. Such a dif- 

 fusion, of one mile in depth of chlorine, would be equal to lifting up to 

 the hydrogen a shell of solid iron two feet thick. Whether we explain 

 the distinguishing principle of the constitution of gases as a mutual 

 repulsion of their molecules, or, according to a late theory, as an in- 

 cessant motion and clashing of atoms, there is nothing in either to 

 warrant the supposition of the lifting or overcoming any considerable 

 weight in the diffusion of gases. Under the first theory, diffusion, to 

 a limited extent, would be accounted for by the small residuum of 

 chemical or cohesive attraction that would remain between the atoms 

 when separated as they are in gases ; and, under the last theory, 

 by the mechanical impulsion of the molecules, through their hitting 

 against each other. Evidently, it is a principle which operates only 

 within narrow limits, and in the lower temperatures of the gases. The 

 sun gives no indications of such a commingling of its gaseous elements. 

 Spectrum analysis, when applied to its outer edges, shows first hydro- 

 gen, then the vapors of sodium and magnesium, and, lastly, those of 

 calcium and iron. The same fact and order of position are found to 

 exist in the more condensed layers of the sun-spots. 



