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 substances which require great heat to convert them into gas is supported by some facts. In the first place, none of the gases of these substances have been discovered in any of the nebulæ. 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 temperatures producible by artificial means, and have only very recently been made to assume the liquid and solid states by the use of extraordinary devices. The other definite line which the spectrum of certain nebulæ 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 temperatures, 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 hypothesis, 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 planets, 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 nebulæ. Those