24 Proceedings of the Royal Society of Edinburgh. [Sess. 
pronounced maximum (J.C.S., 1909, 95, 322), Walden’s results for ethyl 
tartrate in cinnamic aldehyde give only a very slight indication of the 
occurrence of this phenomenon in dilute solution. 
We may now turn to the question with which this paper is primarily 
intended to deal, namely, the course of the T-R curves for tartrates at 
low temperatures. There are various ways of investigating the problem. 
Firstly, a substance, homogeneous or in solution, can be examined directly 
at low temperatures : but this procedure presents a number of difficulties 
such as the likelihood of solidification of the active substance or of the 
solvent, or the difficulty of obtaining and maintaining low temperatures. 
Secondly, the active compound may be dissolved in some solvent which 
brings into the region of ordinary temperatures the T-R curves, which, 
in the homogeneous substance, could only be directly observed at very low 
temperatures. Thus in the case of ethjd tartrate it has already been 
shown ( J.C.S . , 1908, 93, 360 ; Winther, Zeitschr. phys. Chem., 1907, 60, 578) 
that in ethylene bromide the rotation is very considerably depressed, 
wherefore most probably, in this particular solvent, the whole T-R curve 
is displaced towards the right of the diagram — that is, in the direction of 
high temperatures; or, to put it otherwise, ethylene bromide brings into 
view, at the ordinary temperature, the part' of the diagram a b c (fig. 1), 
which, in homogeneous ethyl tartrate, could only be observed at decidedly 
lower temperatures. The behaviour of ethyl tartrate in ethylene bromide 
at moderate temperatures ought therefore to reveal the behaviour of the 
homogeneous ester at temperatures lower still. Thirdly, the behaviour 
of some related substance can be studied either in the homogeneous 
condition or in solution. Thus in passing from ethyl tartrate to mffiutyl 
tartrate — a slight change of constitution — the general T-R curves are 
slightly displaced towards the left, towards lower temperatu es ; whereas 
on converting ethyl tartrate into ethyl diacetyl tartrate — a considerable 
change of constitution, — the displacement of the general T-R curves is 
very much greater and is in the opposite direction. 
Applying, to begin with, the second of these methods, ethyl tartrate was 
examined in ethylene bromide solution. Unfortunately, in attempting to 
get readings at as low a temperature as possible, the solution was cooled 
rather too far, so that the solvent crystallised out and spoiled the experi- 
ment at an early stage. From the data on p. 34, however, it will be seen 
that the rotation for violet is, in the absolute sense, very much lower than 
for the other colours examined, and that the rotation increases rapidly 
with rise of temperature, so that the effect of this solvent is very much 
the opposite of that of cinnamic aldehyde, whence it may be concluded 
