CHIEF CHEMIST, UNITED STATES GEOLOGICAL SURVEY. [_Concluded._] It is evident, from what has been already said, that chemistry and physics are near akin--indeed, they can hardly be separated. Avogadro’s law and spectrum analysis are but two illustrations of the relationship, but many other examples are equal to them in importance. Take, for instance, the action of light upon chemical substances; it may provoke union of elements, or effect the decomposition of compounds; upon the latter phenomenon the art of photography depends. That salts of silver are chemically changed by light was the fundamental observation, and upon this fact most photographic processes, though not all, are founded. Thus light, working as a chemist in the laboratory of the photographic plate, has become the useful servant of all arts, all sciences, and all industries--an indispensable aid to invention and research. On this theme a volume might be written; a bare reference to it must be sufficient here. Still another branch of chemistry, recently developed but essentially an extension of the theory of valence, is also due to the study of optical relations. That different crystalline bodies differ in their behavior toward polarized light has long been known, and the polariscope is recognized as an instrument of great value in chemical research. To the analysis and valuation of sugars and sirups it is most effectively applied, and commercial transactions of great magnitude depend in part upon its testimony. Here is practical utility, but the development of theory is what concerns us now. The discovery of isomerism, of the fact that very different compounds might contain the same elements united in the same proportions, was easily interpreted by the theory of valence in a fairly complete and satisfactory way. In the structural formulæ the different atomic groupings were clearly shown, but with one essential limitation--the arrangement was in a single plane. That is, the linking of the atoms was considered, but not their relations to tridimensional space. For the study of reactions, for the classification of compounds, the structural symbols sufficed; but human thought is not so easily satisfied, and more was soon required. One class of isomers was unexplained, and an explanation was demanded.