2 3 o SCIENCE PROGRESS 



In the case of crystals, the arrangement taken into consideration 

 was that of the molecules or groups of molecules in the crystal 

 structure ; here, of course, the arrangement of atoms or groups 

 of atoms in the molecule itself will have to be considered. 



So the problem is to suggest an hypothesis as regards the 

 arrangement of atoms in the molecule, which will satisfy the two 

 conditions of showing the screw-spiral structure to be present 

 in the configuration of all optically active compounds, and show- 

 ing it to be absent in that of all the inactive compounds. The 

 following is an attempt to solve this problem with reference to 

 compounds of carbon and nitrogen. 



PART II.— COMPOUNDS OF CARBON 



It has been mentioned already, that Van't Hoff assumed the 

 tetrahedral grouping for the four radicals joined to a carbon 

 atom, the carbon itself being at the centre of the tetrahedron. 

 He did not commit himself as to the nature of the tetrahedron, 

 because it was unnecessary for his purpose ; the structure 

 becomes asymmetric and enantiomorphous, when all the four 

 radicals are different, whatever the nature of the tetrahedron, 

 and enantiomorphism by itself apparently seemed to him quite 

 sufficient to account for optical activity. But if it is not enough, 

 and if some sort of screw-spiral arrangement of the radicals has 

 to be postulated, we shall be obliged to make some further 

 assumptions about the tetrahedron. The assumptions suggested 

 below seem to the writer very plausible and dynamically sound. 



The linkages of the carbon may be pictured as the horns of 

 a snail ; they can be pushed out or pulled in, and can also be 

 twirled round, their orientation being determined by the four 

 radicals attached to them. The configuration of a substance 

 like methane or carbon tetrachloride may be represented by a 

 regular tetrahedron. The structure will possess its full number 

 of planes of symmetry, viz. six ; the distance of each radical from 

 the central carbon will be the same ; the angle between any pair 

 of linkages will be equal to that between any other pair; and 

 so on. 



This high degree of symmetry will gradually diminish as 

 more and more different groups appear. Thus, for example, in 

 a compound of the type C,a 3 ,b, the distance of all the a's from 

 the central carbon will be the same, but will be different from 



