EVOLUTION OF THE CHEMICAL ELEMENTS. 533 



mation, many of which, nevertheless, would prove sufficiently stable 

 to preserve the new form of aggregation after the temperature should 

 go down, and, instead of reverting to their former condition on the 

 cooling of the system, would assume successively the liquid and the 

 solid states, and become constituent parts of and distinct substances 

 in the cooled-off planets. 



This theory of the origin of all those elementary terrestrial sub- 

 stances which require great heat to convert them into gas is supported 

 by some facts. In the first place, none of the gases of these sub- 

 stances have been discovered in any of the nebulae. The only two 

 terrestrial substances, thus far determined with any certainty, are 

 hydrogen and nitrogen. The latter of these exists in a free state in 

 the earth's atmosphere, forming about four fifths of its volume and 

 three fourths of its weight. The former does not exist in a free state 

 in the atmosphere, in consequence of its strong affinity for oxygen, 

 which is present there in excess, and whose union with it forms the 

 waters of the globe. Both of these substances are gases at all tem- 

 peratures producible by artificial means, and have only very recently 

 been made to assume the liquid and solid states by the use of ex- 

 traordinary devices. The other definite line which the spectrum of 

 certain nebula3 presents is near to that of barium, but is conceded 

 not to be the barium-line. It is, therefore, an unknown substance, 

 and nothing can be said of its properties. Its proximity to the barium- 

 line in the spectrum can not certainly be taken to indicate any special 

 resemblance to that metal ; and it is probably a gas at low tempera- 

 tures, like hydrogen and nitrogen. 



In the second place, as to these two last-named substances, one of 

 them, hydrogen,, is present in nearly or quite all the self-luminous 

 bodies whose spectra have been observed, where it seems to occupy a 

 position far out in the upper atmosphere. As to nitrogen, its presence 

 in such bodies is doubtful, so far as the spectroscope is able to inform 

 us ; but, as it exists in such quantities in the earth's atmosphere, the 

 belief is strong, especially among those who accept the nebular hy- 

 pothesis, that the failure to discover it there is due to our imperfect 

 methods, or to our ignorance of the manner in which the phenomena 

 of the spectroscope are to be interpreted. The recent triumph of 

 science, in the discovery of oxygen in the sun, serves to show how 

 easy it is to overlook phenomena all the while perceptible, and 

 gives great hope that not only nitrogen, but many other substances, 

 will yet be found there, which have hitherto escaped observation. 

 The fact that an element exists in the earth may not be proof that it 

 must exist in the sun, even on the assumption that the sun is the 

 parent of all the jDlanets, but it is certainly strong presumptive 

 evidence that it is also there. It is, however, much stronger proof 

 that it existed in the general mass, as late at least as when the earth 

 was formed out of it, and therefore in the original nebulae. Those 



