66 CHEMISTRY OF THE ROCKS. 



with fierce burning as they would if oxygen were present? 

 Does not the enormous volume of the sun's uncombined hydro- 

 gen indicate that it has not found there the element of its strong- 

 est affinity ? And is there not reason to believe that the heat and 

 light supplies of our great luminary will last all the longer for 

 the absence of this most extravagant fire-generator ? 



Again, the four outer planets of our system have specific 

 gravities varying but little from that of water. Considering 

 central condensation from pressure, it is probable that they are not 

 more dense than they would be if composed of the lightest com- 

 pound substance that we know of. If oxygen had been there in 

 excess, it would long ago have burned and condensed their ele- 

 ments, whatever they might be, into most stable and solid forms. 

 This gas therefore cannot have formed any considerable part of 

 their constitution. Is it not then a probable supposition that 

 these distant planets are composed of some non-combining and 

 inactive elements like nitrogen, and that, undisturbed by com- 

 bustions or elemental agitations, they have quietly stratified into 

 gaseous worlds, retaining in great part their original heat ? So 

 far as the spectroscope gives any indications of their constitu- 

 tion, it shows them to be composed of gases unknown in the 

 earth. 



As we have stated, the four outer planets are very nearly of 

 the specific gravity of water ; then come the innumerable aste- 

 roids, filling the place of a missing planet, and of which we 

 know but little ; then three planets that are five and a half times 

 as dense as water;: and lastly, Mercury, over eight times as 

 dense. Does not this increasing density of the planets from the 

 outer to the inner, imply that they have been successively 

 formed on the exterior of one great parent globe, and received 

 each its proportion in the main of denser elements, as it was 

 later born ? That this effect should appear somewhat in groups 

 of the planets, is owing probably to the absence or excess of 

 oxygen among their components. 



But if this is so, what shall we say of hydrogen, the lightest 

 of all the gases, which seems to be most abundant the nearer 

 to the center of the system ? To explain this notable exception, 



