As chemist of the United States Geological Survey, Professor Clarke has published ten official bulletins of work done in the laboratory under his charge, of which Bulletin 125, The Constitution of the Silicates, and Bulletin 148, Analyses of Rocks and Analytical Methods, by F. W. Clarke and W. F. Hillebrand jointly, are the most important. Other works are: Weights, Measures, and Money of All Nations, 1875; The Elements of Chemistry, a school text-book, 1884; and a Report on the Teaching of Chemistry and Physics in the United States, published by the United States Bureau of Education in 1881. A paper published by him in the Popular Science Monthly for 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.