STEREOISOMERISM AND OPTICAL ACTIVITY 247 

 N N 



V 



X 



N 



The two resulting substances are enantiomorphs, as are also 

 their originals. But now the molecule has become more 

 complex, so that it is no longer possible for the plane-polarised 

 ray to find a direction through the molecule, so as to avoid 

 having the atoms or groups arranged in a spiral round itself, 

 because the pyramid is irregular, 1 the four groups a, b, c, and 

 d being all different. Hence the configurations will be optically 

 active, one being dextro- and the other laevo-rotatory. 



Now passing on to the other types, we find that in the type 

 Na 3 bx no stereo-isomers are possible, and none are known. 

 The case of trimethyl-ethyl-ammonium-iodide, which was at first 

 thought to be a case of stereo-isomerism, is now shown to be 

 only one of dimorphism. 



As regards optical activity, that also is not possible, as the 

 molecule certainly gives a smooth path to the ray in at least 

 two directions. Suppose, for instance, the ray lies along x — N, 

 then the line joining the four radicals a, a, a, and b will not be 



1 The conception of the nature of the linkages is the same here as in the case 

 of carbon. In the case of carbon, however irregular the spatial distribution of the 

 linkages and the radicals, the resulting figure could always be accurately described 

 as a tetrahedron. But here the distribution of the linkages and the radicals will 

 often make the formation of a pyramid impossible. Still, in the sequel, the word 

 "pyramid" has been used loosely to describe the resulting polyhedron in all 

 cases. 



