January, 1873 (Volume II), on Evolution and the Spectroscope, showed that the evolution of the planets from nebulæ was possibly accompanied by an evolution of the chemical elements. This was nearly a year in advance of Lockyer's first paper suggesting the same general view. The discussion of this subject was taken up again in the eighth volume of the Monthly (February, 1876), in an article, Are the Elements Elementary? in which the author, after showing how subtle connections significant of unity run through them all, inquired: "If the elements are all in essence one, how could their many forms originate save by a process of evolution upward? How could their numerous relations with each other, and their regular serial arrangements into groups, be better explained? In this, as in other problems, the hypothesis of evolution is the simplest, most natural, and best in accordance with facts. Toward it all the lines of argument presented in this article converge. Atomic weights, specific volumes, and spectra, all unite in telling the same story, that our many elements have been derived from simpler stock." These views were admitted to be speculative but not baseless. "Science is constantly reaching forward from the known to the unknown, partly by careful experiment and partly by the prophetic vision of thought." Then, speculation upon such questions "is not altogether unprofitable. The time spent in conjectures and surmises is not wholly wasted, for it is impossible to follow up any of the lines of thought thus opened without reaching some valuable suggestions which may pave the way to new discoveries. New truth, in one direction or another, is sure to be reached in the long run. So, then, we may proceed to theorize in the most barefaced manner without entirely quitting the legitimate domain of science." An article on The Present Status of Mineralogy, in the thirty-second volume of the Monthly, presents the mutual bearings of that study and chemistry and geology.
Professor Clarke contributed the chapter Element to the last edition of Watts's Dictionary of Chemistry. He was made president of the Washington Chemical Society in 1885, and of the Philosophical Society of Washington in 1896. He organized and had charge of Government exhibits, on behalf of the Department of the Interior, at the expositions of Cincinnati, Chicago, Atlanta, Nashville, and Omaha. He is a corresponding member of the British Association, of the Edinburgh Geological Society, and of the New York Academy of Sciences.