Optical Activity of Solutions. 825 



the present exposition. Of course, in a subject of" such 

 complexity it is hardly likely that any such simple theory 

 can give a complete account of the whole matter ; but it is 

 nevertheless interesting to notice one or two points where 

 the present theory fails. 



Firstly, according to our formula the specific rotation has 

 a minimum value 



corresponding to the concentration 



1 — asd n 





a(r—sl—di) 



which may conceivably be experimentally attainable. The 

 minimum value of the rotation would thus on this theory 

 appear to be entirely independent of the particular solvent, 

 unless indeed the constant a involves some account of that 

 substance. Although approximately verified in one or two 

 instances, this relation does not seem to be generally true 

 for most of the substances for which the minimum is 

 attainable. 



Again, with some few substances in particular solvents 

 it is found that the rotation decreases with increasing con- 

 centration, but that the minimum value occurs before a 

 change of sign takes place. This could not happen according 

 to our formula. 



4. Conclusion, 



In spite of these few discrepancies the experimental 

 results may be said to be satisfactorily verified by the 

 theoretical formula deduced. The apparent anomalous 

 behavour of the few substances which do not conform to the 

 general rules above laid down will form the subject of 

 further investigation, as it probably has its origin in some 

 additional (probably chemical) actions involved in the 

 phenomena ; actions which are consistent with but not 

 necessarily dependent on those discussed. In the meantime, 

 however, direct experimental evidence of an additional and 

 independent nature may be obtained for the present theory, 

 or against it. The most general method of investigating its 

 appropriateness is by a detailed analysis of the other optical 

 properties of such solutions ; for it appears on this theory 

 that the variation in the rotativity of a dissolved substance 

 is not due to any direct modification of the rotatory power 



