Later, in his chairman's address before the Chemical Subsection of the American Association, in 1878, he had this subject in mind, and mentioned it as part of the work of the section "to attract public attention to the subjects that interest us, and to do what we can to secure for chemistry a wider appreciation and greater means for development.... If the general public," he said later in his address, "is not interested in chemistry, it is because we as chemists have neglected a part of our duty. We have but to speak, in order to command the public ear."
Another side of scientific advancement to which Professor Clarke's working life has proved him much attached is presented in this address at the American Association meeting of 1878, and more minutely as to the particular point we have in mind in an article on Laboratory Endowment, in the tenth volume of the Popular Science Monthly. In the association address he insisted strongly upon the physical side of chemical research, stated briefly as the study of the phenomena which occur during the reactions in chemical experiments, or of the transformations of energy, and upon the importance of the co-ordination of studies separately pursued to the systematic and permanent advancement of the science; for which purpose he considered endowed laboratories for research extremely desirable. In such laboratories adequate corps of thorough specialists should co-operate in those investigations which individuals could not undertake; every worker should be assigned to definite, positive duties, the accurate and careful performance of which would eventually be sure to advance exact knowledge. The work would be hard routine, and the real value of the institution would be independent of everything sensational, and would rest upon considerations of the most severely practical kind. As an example of such work he mentioned the study of the connection between the composition of a substance and its physical properties. Supposing this taken up systematically by a well-organized body of investigators, the first step would be to determine, carefully and with the utmost rigor, the physical properties of the elements. Each one of these substances would have to be isolated in quantity and in a chemically pure condition, such as has never been attained as to some—a labor which would of itself involve a great amount of research. Then would come the measurement of physical relations, thermal, electrical, optical, magnetic, mechanical, and so on; and the determination of all their "constants" under widely varied conditions, notably of pressure and temperature; labors which would in many cases involve the comparative testing of various methods of research, and often the invention of new experimental processes. The number of elements and of their compounds which should be taken up in some regular order, series by series, would afford almost illimitable fields of research to large numbers of students; all of whom, if laboring under some plan of systematic co-operation, might contribute directly and efficiently to the perfection of the science. "One chemist might undertake to furnish certain of the elements in a perfectly pure condition; another might carefully determine under varying circumstances their densities and rates of expansion; a third could work up their latent and specific heats; a fourth their electrical relations, and so on. Failure to attain grand results would be impossible. Doubtless the labor would prove irksome and monotonous, but the reward would be sure. In five years, more would be done toward rendering chemistry an exact science than can be accomplished in a century by means of chemical investigations at present most in vogue." Chemists engaging in work of this kind would have to make sacrifices, for it would offer little promise of sensational reputations to be gained through dazzling discoveries, and would have to look to the ultimate glory of the science for their chief reward.
Professor Clarke has not omitted to practice what he thus preached; and while he has not failed to win honors in other fields of the science, has made it the chief work of his scientific life to advance toward solution one of the physical problems of chemistry indicated above. He has taken as his special field of research the "constants," and of these, the one which is perhaps the most fundamental of all, the revision of the atomic weights—not by experiments of his own so much as by comparison and criticism of the work of all who have undertaken the task, eliminating errors and finding from the sum of the whole what is the nearest deducible approach to accuracy. In 1872 he sent to the Smithsonian Institution a compilation entitled A Table of Specific Gravities, Boiling Points, and Melting Points for Solids and Liquids. This was accepted by Prof. Joseph Henry, who made it the first publication of a projected series to be called The Constants of Nature. To this series Professor Clarke has since contributed Tables of Specific Heats, of Expansions, and a Recalculation of the Atomic Weights. A new edition of the Specific Gravities was issued in 1886, and a second edition of the Atomic Weights in 1897. For the past five years Professor Clarke has contributed an annual report on atomic-weight determinations to the Journal of the American Chemical Society, giving each year a consistent table of values brought thoroughly down to date. These tables are now used in all parts of the world as standards for reference.
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.
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.