A HUNDRED YEARS OF CHEMISTRY. 59 



A HUNDRED YEARS OF CHEMISTRY. 



By F. W. CLARKE, 



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 decom- 

 position of compounds; upon the latter phenomenon the art of pho- 

 tography 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 indus- 

 tries — 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 essen- 

 tially 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 chem- 

 ical 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 com- 

 pounds might contain the same elements united in the same pro- 

 portions, was easily interpreted by the theory of valence in a fairly 

 complete and satisfactory way. In the structural formulae the dif- 

 ferent 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 tri- 

 dimensional space. For the study of reactions, for the classifica- 

 tion 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 de- 

 manded. 



A typical example of the difficulty was offered by tartaric acid, 

 which exists in two forms differing crystallographically and optic- 



