J 34 



SCIENCE PROGRESS. 



atoms must always occupy the periphery of the molecule 

 because of their readiness to react, and from the forms of 

 crystals he concluded that all molecules must possess three 

 dimensions. These speculations only served to illustrate 

 the sterility of theories which cannot be submitted to definite 

 practical tests. 



Meanwhile, in 1873, Wislicenus had published in greater 

 detail the proofs that the two active lactic acids possess the 

 same structural formula, and had again urged, as he had 

 done earlier, that the difference in their properties must 

 be accounted for by a difference in the space relations of 

 their atoms. At this point, and inspired by this doctrine, 

 Van't Hoff took up the subject in 1874; a little later in 

 the same year Le Bel's first paper appeared ; and the results 

 of Pasteur, of Kekule, and of Wislicenus appeared as con- 

 nected parts of one harmonious whole, when seen in the 

 light of the theories thus almost simultaneously published, 

 and now recognised as the basis of modern stereochemistry. 



The basis of Van't Hoff's theory was a tetrahedral con- 

 figuration for the groups attached to a carbon atom ; this 

 shows two configurations for the active lactic acids (fig. 5, 



PLATE I. 



Plate I.), and in general two configurations for CR'R"R'"R 1V 

 which become identical when two R-groups become iden- 



