770 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



atoms or groups, diii'ering from each other, two isomers in space may 

 be formed which not being antipodes are not optically active ; isomers 

 of which we have examples in fumaric and maleic acids. 



If we w^ish to seek for the origin of the stereochemical conception 

 in van't Hoff's mind, we must recognize first of all the preponderant 

 influence which the doctrines of Kekule had upon him, an influence 

 which had been increased during his stay in Bonn. With his doc- 

 trine of the valence and particularly of the tetravalence of carbon, 

 with the development given to the theory of the structure which 

 attained its apogee in the statement of the theory of the constitution 

 of aromatic compounds, Kekule had made of the University on the 

 Rhine a center from which radiated new ideas on organic chemistry, 

 and had grouped around him a crowd of brilliant pupils. 



A few years before, there had left Bonn for Italy a young Ger- 

 man chemist, W. Koerner, who was to publish in the same year his 

 classical researches on the determination of position of substituting 

 atoms in aromatic substances — another brilliant extension of Kekule's 

 theory. The coincidence is not without interest. The tetrahedral 

 model of van't Hoff was identical with that which Kekule had 

 already used in his demonstration. In each case Kekule had given 

 the model, but he lacked the confidence to use it in all the legitimate 

 deductions and representations which one could obtain from it. 



Naturally the van't Hoff innovation was much more radically revo- 

 lutionary, since for the first time structural formulas were employed 

 to indicate not only the order in which the atoms are linked among 

 themselves by their valences, but also, necessarily, their manner of 

 distribution in space. 



Pasteur, after having thoroughly studied the nature of these pe- 

 culiar isomers, had already pointed out intuitively that the cause of 

 this asymmetry must lie in a molecular asymmetry, since it does not 

 disappear, as in quartz, with the disappearance of the crystalline 

 form, but persists in the liquid or dissolved state. He had foreseen 

 that the atoms must be disposed in their molecules so as to give two 

 figures corresponding between themselves like the crystals, or, for 

 example, like a right and left spiral. 



But Pasteur could not reach the complete solution of the problem 

 for lack of a suitable model. Van't Hoff was to succeed when he ap- 

 plied the Kekulian model, and after having grasped at one careful 

 glance the already abundant data, he saw like a flash that the cause 

 which he sought was none other than the presence in all the optically 

 active compounds of an asymmetric carbon atom. If one seeks for 

 the cause of the differences between the ideas of van't Hoff' and of 

 Le Bel and of the less complete character of the latter, it may readily 

 be seen in the fact that the theories of valence and of structure had 

 not yet found in France the reception and diffusion which they 

 deserved. 



