244 SCIENCE PROGRESS 



So let us see whether a compound N, a, b, c has such a 

 structure, when we represent it by the tetrahedral formula. In 

 order to find out whether a given configuration will be optically 

 active or not, we have simply to ask the question (as we have 

 done before in the case of carbon-compounds), whether it is 

 possible for the plane-polarised ray to find a direction through 

 the molecule, such that there will be no forces round it, that will 

 tend to twist the plane of polarisation. If there is no such 

 direction, then the molecule will lie in such a position as will 

 produce minimum rotation. But if there is such a direction, 

 then the molecule, being mobile, will take the corresponding 

 position, in accordance with the principle of " least resistance." 



Now, clearly there is such a direction possible in the con- 

 figuration under consideration. Suppose the ray passes along 

 one of the three bonds; then it is clear that the remaining two 

 groups can in no sense be described as lying on a spiral round 

 the ray, and hence there will be no rotation. 



Then there arises the further question, whether the isomerism 

 of the two enantiomorphous configurations : 



a*-- 



will at all be made manifest in any of the other physical or 

 chemical properties. By analogy of carbon-compounds, it seems 

 probable that the two configurations will be identical in physical 

 and chemical properties. The evidence of facts has so far been 

 of a very indecisive character; and it is too early yet to form 

 any conclusion in the matter. But if we look at all the cases ! 

 of alleged differences in properties of such isomers, one fact at 

 once strikes our notice, viz. that in all the cases the groups 

 attached to the nitrogen are of a very complex character. 



1 For examples see Stewart's Stereochemistry, p. 264. 



