ARTICLES 247 



identical angles, the only difference between them consisting 

 in the right- and left-handed disposition of their hemihedral 

 facets and in the right- and left-handed directions of their equal 

 rotatory powers. 



Pasteur at once grasped the full significance of his discovery, 

 and concluded that in the molecules of these closely related 

 acids the atomic arrangements must be so nearly identical 

 that they differ only as an asymmetric * object and its image, 

 and he put the question, " Are the atoms of the dextro-acid 

 grouped in the form of a right-handed spiral or are they placed 

 at the angles of a regular tetrahedron or disposed according to 

 some other asymmetric non-superposable arrangement ? ' 



It is a striking mark of Pasteur's genius that, before leaving 

 the study of the phenomena of optical activity, he had carried 

 it as far as the knowledge of the time regarding the structure 

 of the chemical molecule allowed. 



About that period the idea of molecular structure was 

 only just beginning to take shape. This in its most complete 

 form we owe to Kekute, and even he for over twenty years 

 appears to have looked upon formulae as geometrical figures 

 drawn upon paper, and hence never to have regarded the 

 atoms as arranged in more than one plane. 



Chemical theory, however, only advances when a need 

 arises, and it was not till about 1870 that the inadequacy of 

 plane formulae began to be felt and the necessity for space 

 formulae recognised. 



As we have seen, Pasteur clearly realised that optical 

 activity, since it is shown by liquids and dissolved substances, 

 must in such cases be due to lack of symmetry in the molecule 

 itself and not to the way molecules are built up into crystals, 

 and stated with precision that this unsymmetric molecular 

 structure must be related in each pair of isomers as an " asym- 

 metric " object is to its mirror image. 



Adopting Kekule's idea of the tetra valency of carbon, 

 van't Hoff and Le Bel in 1874 independently defined for carbon 

 compounds the conditions under which such an asymmetric 

 structure could appear, namely, when a carbon atom was 

 attached to four different groups. 



1 The term " asymmetric," used by Pasteur, and, following him, by most chemists 

 up to the present time, is not strictly correct, inasmuch as enantiomorphous 

 objects are known which are not totally devoid of symmetry. 



