232 SCIENCE PROGRESS 



this seems highly improbable. Most probably the position 

 involving least rotation will be such as to have the path of the 

 ray along one of the bonds. 



It is obvious that the twisting will be equal but in the oppo- 

 site direction in the other enantiomorph. 



The orientation of the radicals, and consequently also the 

 degree of twisting of the bonds, is, as has been indicated already, 

 most probably determined by two factors: (i) The affinity of 

 the central carbon to each of the four radicals, and (2) their 

 action upon each other. Both of these may indeed be grouped 

 together under the one heading of the chemical nature of the 

 groups. This being the case, it seems almost impossible to find 

 a quantitative relation between the degree of rotation produced 

 and the nature of the groups — at least, in the present state of 

 our knowledge ; and it is no wonder that all attempts at such a 

 co-ordination, based upon only one property of the groups, viz. 

 their mass (which alone lends itself to a quantitative treatment, 

 but which is nevertheless probably the least influential in the 

 matter under consideration), have entirely failed. 



With the help of this idea, it further becomes easier to under- 

 stand why the amount of rotation of one and the same substance 

 changes with the external conditions like temperature and 

 solvent. The amount of rotation changes for the simple reason 

 that the chemical nature of a group changes with the external 

 conditions. 



Now we shall consider the cases (1) Ca 3 b, (2) Ca 2 b 2 , and 

 (3) Ca 2 bc. 



If we take the first case, for example, we find that it is indeed 

 possible to imagine a direction for the path of the ray through 

 such a molecule, which will have the groups arranged in a 

 spiral round itself ; but that matters little. What we have to 

 decide is whether there is a direction possible for the ray, which 

 can avoid this twisting, and the consequent rotation of the plane 

 of polarisation. Because, if there is such a direction, then the 

 molecule being mobile will assume the corresponding position, 

 in accordance with the principle of " least resistance." And it is 

 not difficult to see that there is such a direction in each of the 

 three types under consideration. 



In the first case, such a direction is that of the bond between 

 the central carbon and b. 



In the second case, it is the direction joining the central 



