732 Sir J. J. Thomson on some Optical Effects 



We can explain by displacements of this kind the very 

 interesting fact that the optical rotation by a solution of an 

 active substance may depend not merely in magnitude, but 

 also in sign on the nature of the solvent : thus, for example, 

 a solution of d acetic acid in water is dextro rotatory, while 

 when dissolved in a mixture of acetone and ether it is la3vo 

 rotatory (Landolt). If a solvent made some progress 

 towards ionizing the substance by weakening the attachment 

 of one of the atoms (say D) to the carbon atom, it might 

 make the electron 8 which binds D to the carbon atom move 

 from 8 to a position more remote from the carbon atom. 

 This change, as fig. 6 shows, might reverse the sign of the 

 rotation. 



The general conclusions to which we are led by the 

 preceding investigation, is that the electrical system which 

 is instrumental in producing optical rotation are the electrons 

 which couple the atoms in the molecule to the central carbon 

 atom, and that the most important quantities in the expression 

 for the rotation are (1) the rigidities of those electrons, i. e. 

 the intensity of the forces restoring them to their position 

 of rest when displaced from it ; and (2) the distances of these 

 electrons from the central carbon atom. For it is on these 

 quantities that the dynamical and geometrical asymmetries- 

 depend respectively, and as we have seen, both these 

 asymmetries are essential for rotation in solutions. If it 

 were not for differences in (1) there would be no dynamic 

 asymmetry, for the electrons have all the same mass. 

 When, however, they are pulled back when displaced from 

 their position of equilibrium with forces of different inten- 

 sities, the effect will be much the same as if the electrons 

 had different masses. Thus, if the frequency of the vibrations 

 of the electron when displaced from its position of equilibrium 

 is i%i and p is the frequency of the light, the behaviour of 

 the electron under the forces in the light, wave will be much 

 the same as if the electron were free but had a mass 

 (p 2 — nfl/p 2 times itslnormal mass. When n 2 is large com- 

 pared with p, the effective mass is approximately n 2 jp 2 times 

 the normal mass, and is thus much greater. Thus the dyna- 

 mical asymmetry of the molecule will be measured, not as 

 on Guye's theory * by differences in the atomic weights of 

 the atoms attached to the carbon atom, for these on our view 

 have no direct bearing on the rotation, but on the differences 

 in the frequencies of the electrons which bind the atoms 

 to the carbon atom. 



* Ann. Chim. Phys. (6) xxv. p. 145 (1892). 



