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XLVIII. Optical Rotation, Optical Isomerism, and the Ring- 

 Electron. By H. Stanley Allen, M.A., D.Sc, University 

 of Edinburgh *. 



Introduction. 



WHEN a beam of plane-polarized light passes through 

 certain pure liquids and even certain vapours, there 

 is produced a rotation o£ the plane of polarization, which 

 implies that there must be some asymmetry in the molecule 

 of the substance concerned. This asymmetry has been 

 studied mainly from the chemical side, the difference between 

 isomeric molecules of the same structure being explained by 

 a different arrangement of atoms in space. On the physical 

 side progress has been less satisfactory, and although a 

 formal connexion between optical rotation and the electro- 

 magnetic theory of light was established by Drude |, it is 

 only recently that successful attempts have been made to 

 explain why asymmetry in molecular structure should in- 

 volve difference in the velocity of propagation of circularly 

 polarized light. In this paper the subject is approached 

 from a somewhat different standpoint from that usually 

 adopted, for it is assumed that the electron, in addition to its 

 electrostatic action, behaves like a current circulating in a 

 closed ring, and consequently acts as a small magnet. This 

 conception involves a modification of prevailing views, both 

 as regards the action of light on a system of electrons, and 

 also as regards the constitution of the molecule. The paper 

 therefore divides naturally into two parts. The first is con- 

 cerned with the propagation of an electromagnetic wave 

 through an assemblage of ring-electrons, whilst the second 

 (which may prove of more interest to the chemist) deals 

 with the question of the arrangement of the electrons in the 

 molecule of an optically active substance, assuming the 

 theory of the " cubical atom" developed by Gr. N. Lewis J 

 and Irving Langmuir§. 



* Communicated by the Author. 



t Drude, ' The Theory of Optics/ part ii. Chapters vi. & viii. 

 + G. N. Lewis, Journ. Airier Chem. Soc. vol. xxxviii. p. 762 (1916). 

 § I. Langmuir, Journ. Amer. Chem. Soc. vol. xli. pp. 868, 1543 

 1919). 



