248 SCIENCE PROGRESS 



Van't Hoff, in addition, adopted the tetrahedron as the 

 formal representation of the carbon atom in this new aspect. 



It ought, however, to be repeated that the state of know- 

 ledge at the time of Pasteur's work did not enable him to put 

 forward anything like a complete theory, which could only 

 come after ideas of molecular grouping had become much 

 more precise. 



The views of van't Hoff and Le Bel rapidly gained accep- 

 tance, for not only did these chemists state the basic principle, 

 but they showed that it applied to most known active sub- 

 stances, and enabled the possible existence of others to be 

 predicted. 



The field of experimental inquiry thus opened was so vast 

 that for many years the study of optical activity was almost 

 entirely restricted to carbon compounds. 



The possibility of other multivalent elements serving as 

 the nucleus of asymmetric groupings must, however, have 

 been present to the minds of all chemists approaching the pro- 

 blem, and the element nitrogen soon attracted attention in this 

 connection. General considerations led to the conclusion that 

 in trivalent nitrogen compounds the combined atoms or groups 

 would, under the influence of molecular attraction, arrange 

 themselves symmetrically round the nitrogen atom and thus 

 would lie with it in one plane, which is therefore a plane of 

 symmetry. 



Consequently no trivalent nitrogen compound should show 

 optical activity or be resolvable into optically active com- 

 pounds. This conclusion has been established by experiment, 

 all attempts to resolve such compounds having hitherto proved 

 unsuccessful. 



On the other hand, five different groups attached to a 

 nitrogen atom might be disposed so as to form an enantio- 

 morphous grouping, the pentavalent nitrogen atom thus act- 

 ing as a centre of optical activity in the same way as a carbon 

 atom. 



The correctness of this deduction was first established by 

 Le Bel, who showed that a solution of isobutyl propyl ethyl- 

 ammonium chloride, when subjected to the action of moulds, 

 became laevo-rotatory, the dextro-rotatory isomer being prefer- 

 entially destroyed. It was brilliantly confirmed by Pope and 

 Peachey, who succeeded in isolating the two oppositely active 



