534 "THE POPULAR SCIENCE MONTHLY. 



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 nebulfe. The fact that 

 it can be detected in the nebulae, and not in the sun, although it doubt- 

 less 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 lu- 

 minous 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 cer- 

 tain elements existing in both may be capable of spectroscopic deter- 

 mination 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 sub- 

 stances 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 sub- 

 stances 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 nebulse. 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 pos- 

 sess 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 nebulae, 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 ethe- 

 real vibration. Nebulae 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 



