for the exhibition of this element. Oxygen also reveals itself in spots in combination with titanium. Other elements may be detected in a similar manner when the investigation is more complete.
Another important consideration must not be lost sight of. The dark lines of the solar spectrum can only show the chemical nature of the gases and vapour which lie above the brightly luminous photospheric surface. What lies below the photosphere we have at present no means of learning, but it is not difficult to believe that some of the heavier metals, which fail to give spectroscopic indications of their existence, may enter into its composition.
There is every indication, therefore, that the present incomplete record of the terrestrial elements in our tables of solar elements does not necessarily represent an actual difference of composition of the sun and the earth, but arises chiefly from the incompleteness of the investigation, and from the restricted parts of the sun and its radiation which are available for spectroscopic analysis. As remarked by Rowland many years ago, if the earth were heated up to the same temperature as the sun, its spectrum would probably resemble that of the sun very closely.
The sun may properly be regarded as a typical specimen of the thousands of stars which present us with spectra which cannot be distinguished from that of the sun itself, when taken as a whole. In the case of stars, however, we can deal only with the integrated light, and the presence of helium, for example, in such stars can only be inferred from analogy with the more favourably situated sun.
Turning to stars which have spectra differing from that of the sun, all modern work accords with the belief that the possibility of arranging them in a practically continuous sequence is indicative of their having arrived at different stages in a process of evolution. The inclusion of solar stars in the evolutionary scheme may therefore be considered to imply similarity of chemical constitution. Independent stellar evidence, however, is not wanting.
As in the case of the sun, the majority of stellar lines have now been identified with terrestrial elements. This important result is largely due to the study of the variations in
