due to the Scattering of Light by Electrons, 729 



motlitied expression, would be much smaller than that 

 calculated by (8) which, as we have seen, is already far too 

 small. Again, from this formula the rotation ought to tend 

 to be smaller for heavier molecules than for light ones ; there 

 is no indication of this in the very numerous determinations 

 which have been made of specific rotation. 



Thus we do not get sufficient movement to account for 

 the rotation produced by optically active substances if we 

 suppose the whole molecule to rotate as a rigid body under 

 the electric forces in the wave of light. We may, however, 

 regard the molecule as made up of two parts, the first part 

 consisting of the atoms in the molecule, the second of the 

 electrons which bind these atoms together. Thus, with a 

 tetrahedral arrangement of the atoms round the carbon atom, 

 the first system will consist of the carbon atom at the 

 centre and four other atoms or radicles, A, B, C, D at 

 the corners of the tetrahedron (fig. 4). If we make the 



Fig. 4. 



e 



M' 



usual assumption that A, B, C, and D are bound directly to 

 the carbon atom, then between and each one of these atoms 

 there must be electrons whose attractions on the positively 

 electrified atoms will bind them together. These structural 

 electrons will form a tetrahedral arrangement which will 

 occupy a definite position relative to the tetrahedron A, B v 

 0, D, and if disturbed from this position will vibrate about 

 it with a definite frequency. When the light waves fall 

 upon the molecule what I imagine to happen is, that in 

 consequence of the relatively enormous mass of the atoms 

 themselves there is but an insignificant displacement of the 

 atomic tetrahedron, but that the much lighter tetrahedral 

 arrangement of the electrons suffers a much greater amount 

 of rotation, and that the motion of the electrons consequent 

 upon this rotation gives rise to the rotation of the plane of 

 polarization. 



We have thus to regard the system of electrons as that to 

 which we must apply the preceding analysis. It may be 

 urged against this that since the electrons are all of the same 

 mass, and the electrical centre of the system coincides with 

 the mass centre, the analysis shows that the system would 

 be Ear too symmetrical to give rise to optical rotation. This 



Phil. Man. S. 6. Vol. 40. No. 240. Dec. 1920. 3 B 



