evolutionists alone who are ready to accept the view here advanced of the derivation of the heavier elements from the lighter ones, in the course of the development of the system, can escape this conclusion by supposing that the substance in question was created in the planet after its separation from the central mass. But this assumption would not be required in the case of nitrogen, which remains a gas at high temperatures, and which actually exists in the nebulæ. The fact that it can be detected in the nebulæ, and not in the sun, although it doubtless abounds in both, may be accounted for by remembering that the spectrum of a nebula belongs to a different class from that of the sun, the former consisting of bright lines on a dark ground, indicating a luminous gas; while the latter consists of dark lines on a bright ground, indicating a body having an incandescent solid or liquid interior, the rays of which pass through a cooler gaseous atmosphere. Now, this antithesis in the constitution of the two bodies may explain why certain elements existing in both may be capable of spectroscopic determination only in one, owing to peculiar conditions supplied by the special nature of the substances themselves; for it is by no means probable that the spectroscope gives us an account of all the substances existing in the bodies examined by it.
While, therefore, there is nothing in the facts thus far discovered which is opposed to the theory that the terrestrial substances having high melting and volatilizing points have been developed out of substances which are gaseous at lower temperatures in the course of the evolution of planetary systems, these facts, so far as they bear at all upon the problem, are decidedly favorable to such an hypothesis. We certainly find such substances in our earth and in the intensely heated bodies of space, as well as in such meteoric aggregates as from time to time reach our planet, and we have not yet found any such in existing nebulæ. If the latter be conceived as gaseous, and the solar system as only a developed state of one of them, either some such hypothesis must be brought forward to explain the existence of such substances in the earth, or the original mass must be supposed to possess a sufficient degree of heat to maintain them in a gaseous form, which would be enormous, and, independently of the present theory of the origin of the nebulæ, altogether improbable. Of course, upon the view here taken, it would be wholly inadmissible. Prior to the stage in the history of a nebula at which the degree of molar aggregation is sufficient to occasion a great amount of friction among the particles, the temperature of the primary molecular aggregates must be nearly that of space, and it can rise only as increase of density and molar motion increases that friction and converts material motion into ethereal vibration. Nebulæ must therefore possess a long history, of which neither the telescope nor the spectroscope can furnish any record—the pre-luminous period—in which, of course, no gases can exist except those, like hydrogen and nitrogen, which maintain their gaseous form
