1918-19.] Researches in Optical Activity. 25 
that the former is represented by the part b c of the general curves (fig. 1), 
whereas the latter is represented by the region h i k. But it is clear that 
the depressing influence of ethylene bromide is not sufficient to take us 
into a region of minimum rotation, and since ethylene bromide is either 
the most powerful, or almost the most powerful, depressing solvent known 
for this ester, we were forced to turn next to the third method of investiga- 
tion, namely, that of examining the T-R curves of some other tartrate, in 
which there is reason to suppose that the change of constitution has 
brought about a considerable shifting of the family of T-R curves. 
Now it has been shown by Frankland and Wharton ( J.C.S. , 1896, 69, 
1587, also 1309) that the rotation, for yellow light, of ethyl dibenzoyl- 
tartrate exhibits a distinct minimum rotation at 604° ; moreover, this 
rotation is low in value ([a]D= —59*36°), and it seems clearly possible 
that the T-R curves for this ester at ordinary temperatures represent 
the behaviour of ethyl tartrate — the parent ester^at very much lower 
temperatures. The examination of an active substance for a single 
colour of light can only yield an indication in regard to this question, 
but an examination of the T-R curves for various colours of light 
makes a definite decision possible. Thus if, for example, the minimum 
in ethyl dibenzoyltartrate correspond to the minimum which exists in 
ethyl tartrate at about 180°, namely, to the region k l m in fig. 1 , then 
we should expect the dispersion to be the same in both cases, namely, 
positive ; the rotation for violet should be greater, in an absolute sense 
than for red. But if the minimum were such as is to be expected from 
a continuation of the ethyl tartrate curves towards low temperatures, 
or for the curves for ethyl tartrate in ethylene dibromide, the rotation 
for red might be expected to have a higher absolute value than the 
rotation f r violet, the dispersion then being negative. In this way it 
is possible to decide to which region of the ethyl tartrate curves those 
for ethyl dibenzoyltartrate correspond. Instead, however, of the ethyl 
derivative, we prepared m>butyl dibenzoyltartrate, and examined its 
rotation over a range of temperature in the homogeneous condition as 
well as in solution in cinnamic aldehyde and in ethylene bromide. The 
results are plotted in fig. 3, and it will be observed that the form of 
the curves is in agreement with the second suggestion made above. The 
absolute value of the rotation diminishes from red to violet: the dis- 
persion is negative. There is a distinct minimum in each of the curves, 
and this minimum passes slightly towards lower temperatures as the 
refrangibility of the light becomes greater, although the displacement is 
comparatively small. The rotation tends to rise somewhat rapidly as 
