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.