ANALYSIS OF CRYSTAL-STRUCTURE BY X-RAYS 403 



and other optically active bodies, the angle through which the 

 plane of polarisation of light is rotated is, to a close approxima- 

 tion, inversely proportional to the square of the wave-length 

 of the light employed (Biot's Law), in the case of tartaric acid 

 this is by no means true, for the specific rotation of tartaric 

 acid shows a well-defined maximum in the green portion of 

 the spectrum. Since Biot's time many experiments have been 

 performed to determine the cause of this anomaly, but, beyond 

 the explanation put forward by Arndtsen in 1858, no real 



H O 



(6-) 



Fig. 6. — Showing enantiomorphous arrangements of atoms in tartaric acid. 



physical basis for the phenomenon has been forthcoming. 

 Arndtsen imagined a mixture of two substances, one of which 

 was dextro- and the other Isevo-rotatory, and, in addition, 

 that the rotation of the first increased, as the wave-length of 

 the light decreased, more rapidly than that of the other. It 

 is clear that such a combination, dextro-rotatory in certain 

 circumstances and laevo-rotatory in others, would simulate the 

 properties of tartaric acid. For while aqueous solutions of 

 ordinary tartaric acid are dextro-, the crystals of the same 

 acid are Isevo-rotatory. And, as we pass from dilute solutions 



