of the chloride of sodium—common salt. Apparently in this one instance the oxide is the less stable compound.
But if, as we have endeavored to prove, there is a necessity of accounting, in accordance with this theory, for the various compounds and phenomena with which geology makes us familiar, then it is in the highest degree essential that experiment and research be prosecuted in this new field. And there must be no hesitation in accepting the conclusions to which they lead. Should the nebulous origin of one planet be thus established by internal and inductive evidence, then the nebular theory of the formation of worlds, which has heretofore been received as only a provisional hypothesis, must be accepted as having a scientific basis. If the earth has once been a self-luminous body, in all respects excepting size, like the sun of to-day, it follows from analogy that the other planets have likewise been minor suns which have become extinguished by the burning out of their materials. To an observer on any unseen world among the stars, our sun should have appeared in those times as a brilliant double, or multiple star, around which nine lesser companions have shone out for a season, and then one after the other folded themselves up in darkness.
Furthermore, the study of this subject may throw light on many cosmical problems—may tell us in earth-periods, if not in years, how old the sun is when his glowing vapors begin to condense into dark clouds; and perhaps, too, something of his future prospects as a luminary. It is remarkable that the spectrum has never shown any indications of free oxygen in the atmosphere of the sun. Is not the absence of this element further corroborated by the fact that the solar spots, which there is evidence to believe are condensing clouds of iron and calcium, do not glow with fierce burning, as they would if oxygen were present? Does not the enormous volume of the sun's uncombined hydrogen indicate that it has not found, then, the element of its strongest 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 so dense as they would be if composed of the lightest compound substance that we know of. If oxygen had been there in excess, it would long ago have burned and condensed their elements, 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 combustions or elemental agitations, they have quietly stratified into gaseous worlds, retaining in great part their original heat? So far
