436 Dr. H. Stanley Allen on Optical Rotation, 



a resultant magnetic moment. It would, of course, be 

 possible to overcome this difficulty by a further assumption 

 that magnetic compensation arises from other parts of the 

 molecule. 



Optical Rotatory Power. 



The theory of: optical rotation for liquids and gases here 

 proposed appears to be in good agreement with the experi- 

 mental facts as summarized by Gray. 



1. Any substance of which the molecules are truly asym- 



metrical causes rotation. 



2. Antipodes, a molecule and its mirror image, rotate in 



opposite directions. 



3. Symmetrical molecules are not active. 



4z. Absorption systems in an active substance cause 

 anomalies in the rotary dispersion, although the origin 

 of the band may lie in a part of the molecule distant 

 from the central carbon atom. 



5. The magnitude of the activity is profoundly influenced 



by the molecular structure, and is roughly dependent 

 on the magnitude of the asymmetry of the molecule. 



6. Activity varies with temperature and pressure, and 



with the concentration of active molecules. 



From what has already been said it is obvious that the 

 first three results are in accord with the theory. The fourth 

 result follows alike from the electrostatic theories of Born 

 and of Gray and from the electromagnetic theory here 

 given. In the first case the rotation depends on a space 

 derivative of the electric force, in the second on a space 

 derivative of the magnetic force. The fifth result requires 

 somewhat closer examination. 



Guye * and Crum Brown f have discussed independently 

 the relation of optical activity to the character of the 

 radicles united to the asymmetric carbon atom. " It is 

 obvious that the amount of optical activity of a given com- 

 pound containing an asymmetric atom of carbon depends 

 upon the amount of difference in character among the four 

 radicles united to the asymmetric carbon atom, so that if two 

 of them are very nearly equal we come very near to a com- 

 pound of a symmetric carbon atom, in which the optical 

 activity is zero. The question suggests itself, How are we 

 to measure this difference of character?" Guye regarded 



* Guye, Compt. Rend. vol. ex. p. 714 (1890) ; vol. cxvl. pp. 1378, 

 1451 (1893). 



t Crum Brown, Proc. Roy. Soc. Ediu. vol. xvii. p. 181 (1890). 



