We come now to the consideration of a subject which has a most important bearing on the question of stellar evolution, viz. the genesis and dissociation of the chemical elements. The evolution of one element from another is, it is true, as yet but a mere hypothesis, but it is an hypothesis well supported by a host of facts and considerations, and held by a large number of our leading chemists and physicists. “The demonstrated unity of force,” says Professor F. W. Clarke,[65] “leads us by analogy to expect a similar unity of matter; and the many strange and hitherto unexplained relations between the different elements tend to encourage our expectations.” The hypothesis throws much light on some obscure points in stellar evolution. In regard to this, Professor Clarke justly remarks that “it is plain that the nebular hypothesis would be doubled in importance, and our views of the universe greatly expanded, if it could be shown that an evolution of complex from simple forms of matter accompanied the development of planets from the nebulæ. Evolution could look for no grander triumph.” In fact, it is difficult to understand how our sun and the stars could have been evolved from nebulæ without assuming an evolution of the chemical elements. The true nebulæ show the presence of only two elements, nitrogen and hydrogen, but our sun contains more than a dozen of distinct elements, and the planets more than three times that number. How, then, could all these have arisen out of nebulæ composed simply of nitrogen and hydrogen? The matter is plain if we assume an evolution of the elements. [Footnote 65: _Popular Science Monthly_ for January 1873.] The stars have been classed into four groups, which, as Professor Clarke has remarked, indicate different stages in the process of evolution. The first class, containing white stars like Sirius, show the predominance of hydrogen and a scarcity of the metallic elements. In the second class the metallic elements become more numerous and the hydrogen less distinct; while in the third class hydrogen is difficult to detect.[66] This seems to show a gradual development of the chemical elements as the star cools and grows older. I shall now give a brief account of the views expressed on the subject by some of our leading physicists and chemists. [Footnote 66: See also on this point Mr. Lockyer’s “Bakerian Lecture,” _Proc. Roy. Soc._ No. 266, p. 21.]