22 
Proceedings of the Royal Society of Edinburgh. [Sess. 
III. — The Optical Rotation and Cryoscopic Behaviour of Sugars 
dissolved in (a) Formamide, (6) Water. By John Edwin 
Mackenzie and Sudhamoy Ghosh, M.Sc. (Research Student, 
University of Edinburgh). 
(MS. received September 22, 1914. Eead November 2, 1914.) 
THEORETICAL. 
The optical rotation of a solution of sucrose in water was first measured 
by Biot {Mem., 1819, ii, 41) in 1819. He introduced the term “optical 
saccharimetry ” for the method of estimation of sugar by measurement of 
its optical rotation. 
In 1846, Dubrunfaut {Ann. Chim. Phys., 1846, xviii, 99) observed that the 
specific rotation of a freshly prepared aqueous solution of glucose decreased 
from an initial value of about +110° to a constant value of +52°. The 
initial value being approximately double the constant value, he called the 
phenomenon “ bi-rotation.” This term proved unsuitable in the case of 
other substances where a similar change of rotation took place, the initial 
and final values being rarely in the proportion of 2:1; hence the expression 
“ multi-rotation ” came into use. A better term is that introduced by 
Lowry {Chem. Soc. J., 1899, Ixxv, 212), viz., “ mutarotation,” which is now 
in general use to indicate the change in rotation of a solution from its 
initial to its constant value at the same temperature. Mutarotation is a 
characteristic property of the sugars which display reducing properties, 
and of many optically active substances which occur in tautomeric or iso- 
dynamic forms. 
In the case of the sugars many theories of the mechanism of the change 
have been proposed. Some of these theories have had to be laid aside 
owing to the isolation of distinct modifications of the same sugar. Thus 
in the case of glucose two forms are now known, the one showing an 
initial rotation [a]|°=+110°, the other [a]D®=+19°, and both becoming 
constant with [a]|^=+52°. The former is the ordinary glucose, which 
was known to Saussure {Bulletin de Pharm., 1814, 6, 502) ; the latter 
was discovered by Tanret {Bull. Soc. Chim., 1896, [III], 15 , 195). The 
existence of two modifications of the same sugar led Lowry to advance 
the view that the mutarotation of glucose is caused by a balanced action 
between the highest and lowest rotating forms — a-glucose^^8-glucose. 
