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PHYSICAL CONDITIONS GOVERN APPEARANCES OF SPECTRA

A question frequently asked is this: if the yellow and red stars have been developed from the blue stars, why do not the thousands of lines in the spectra of the yellow and red stars show in the spectra of the blue stars? Indeed, why do not the elements so conspicuously present in the atmosphere of the red stars show in the spectra of the gaseous nebulæ? The answer is that the conditions in the nebulæ and in the youngest stars are such that only the simplest elements, like

hydrogen and helium, and in the nebulæ nebulium, which we think are nearest to the elemental state of matter, seem to be able to form or exist in them; and the temperature must lower, or other conditions change to the conditions existing in the older stars, before what we may call the more complicated elements can construct themselves out of the more elemental forms of matter. The oxides of titanium and of carbon found in the red stars, where the surface temperatures must be relatively low, would dissociate themselves into more elemental components and lose their identity if the temperature and other conditions were changed back to those of the early helium stars. Lockyer's name is closely connected with this phenomenon of dissociation. There is no evidence, to the best of my knowledge, that the elements known in our Earth are not essentially universal in distribution, either in the forms which the elements have in the Earth, or dissociated into simpler forms wherever the temperatures or other conditions make dissociations possible and unavoidable.

The meteorites, which have come through the atmosphere to the Earth's surface, contain at least 25 known terrestrial elements. That they have not been found thus far to contain all of our elements is not surprising, for we should have difficulty in finding a piece of our Earth weighing a few kilograms which would contain 25 of our elements. We have not found any elements in meteorites which are unknown to our chemists. Our comets, which ordinarily show the presence of not more than three elements, carbon, nitrogen and oxygen, give certain evidence of sodium in their composition when they approach fairly near to the Sun; and the great comet of 1882, when very close to the Sun, developed in its spectrum many bright lines not previously seen in comet spectra, which Copeland said were due to iron. That the comets do not show a greater number of elements is not in the least surprising: they are not condensed bodies, and we think that their average temperature is low, too low generally to develop the luminous vapors of the more refractory elements. If their temperatures, approximated those which exist in the stars, their spectra would probably reveal the presence of many of the elements which exist in the meteorites. Of course the proof of this is lacking.