due to the Scattering of Light by Electrons. 731 



, ... IJk HxlO 1 ,. , 



t lie intrinsic rotation would be radians or about 



10 4 ... 



;.- degrees. This has to be multiplied hjpP/n 2 , where n is the 



natural frequency of the vibration of the tetrahedron about 

 the axis aft. But we see that,' even allowing for a small 

 value of p 2 /n 2 , the system we have considered is able to 

 produce rotations comparable with those excited by optically 

 active substances. If the specific rotation is multiplied by 

 the molecular weight of the active substance, the product is 

 a measure of the rotation due to a single molecule, a quantity 

 which is much more likely to throw light on the properties 

 of the molecule than the rotation due to a gramme of the 

 substance. 



The volume of the tetrahedron which measures the con- 

 tribution to the optical rotation of one molecule, when the 

 electrons 78 rotate round aft, vanishes in the following- 

 cases : — 



If 78 is parallel to aft. 



If 78 intersects aft. 



If 78 is at right angles to aft. 



From the third of these conditions it follows that no 

 optical rotation will be produced if «, ft, 7, 8 are at the 

 corners of a regular tetrahedron, there must therefore be 

 some lack of symmetry in the distribution of these electrons. 

 The rotation vanishes whenever yS is in a plane at right 

 angles to aft. Thus it would vanish if the two atoms or 

 radicles which the electrons 7 and 8 bind respectively to the 

 central carbon atom were identical ; for then by symmetry 

 a and ft would both be in the plane bisecting 78 at right 

 angles. 



A shift in the position of one or both of the electrons 7, 8 

 might change the sign of the optical rotation produced by 

 the molecule. Thus, for example, when the electrons are 

 FLr. 5. Fig. 6. 





distributed as in fig. 5, the molecule would behave like a 

 positive screw, while if they were as in fig. 6, it would 

 behave like a negative one. 



3B2 



