OPTICALLY ACTIVE COMPOUNDS 



69 



pursue a course which evidently would not lead them to cut one 

 another. It therefore appears that the T-R curves for an 

 active substance of the type with which we are dealing would not 

 have the appearance of Fig. 5, but rather that shown by the part 

 of Fig. 6 from D to V, the curves probably tending to some other 

 maximum, which so far cannot be reached. 



It is natural now to wish to extend this investigation to- 

 wards the low temperature end of the diagram. The rotation 

 of homogeneous ethyl tartrate for all colours of light diminishes 

 very rapidly as the temperature falls below zero : it is unlikely 

 that this should continue indefinitely, and, just as a maximum 



Fio. 6. 



The curve of greatest amplitude pertains to riolet light, that of least 



amplitude pertains to red light. 



was found at higher temperatures, a minimum of low value 

 is to be expected at low temperatures, a minimum doubtless 

 corresponding to that found by Frankland and Wharton [loc. 

 cit.) in the curves for ethyl dibenzoyl tartrate at medium 

 temperatures ; the behaviour of the dibenzoyl ester thus indica- 

 ting the behaviour of the parent ester at very much lower 

 temperatures. 



A definite decision on this point could not be come to from 

 an examination of T-R curves for a single colour of light, but 

 a much more definite conclusion can be drawn if light of different 

 refrangibilities be used. Thus, if the minimum actually ob- 

 served in ethyl dibenzoyltartrate at a temperature of about 60° 

 corresponds to the minimum which, presumably, exists in ethyl 



