ES 
B.— CHEMISTRY 39 
the discovery of the Bunsen burner in 1866. It was, however, certain 
that little progress could be made in elucidating the origin of optical 
rotatory power, or in predicting its magnitude, until the values of the 
rotatory power were known over a wide spectral range, instead of for a 
single casually determined point on the curve of rotatory dispersion. 
This opinion has received abundant confirmation from the subsequent 
demonstration that the substances which had provided the favourite 
materials for studies of optical rotatory power were those whose rotatory 
dispersion was most anomalous, since these substances are in fact (and 
perhaps inevitably) most sensitive to changes of solvent, concentration, 
or temperature. 
The ignorance then prevailing in reference to this important aspect of 
the subject is shown by the fact that, when Drude wished to test his 
equation for optical rotatory dispersion, he was only able to make use of 
data for quartz (38), since the rotatory dispersion of no one of the 
hundreds of optically active compounds prepared and studied by organic 
chemists was known with sufficient accuracy to be used for this purpose ; 
and his equation for magnetic rotatory dispersion was tested on data, for 
five wave-lengths only, for carbon disulphide and for creosote ! (39) 
Experiments carried out in order to supply the data required to 
determine the form of the curves of rotatory dispersion in organic com- 
pounds soon led to definite conclusions. Thus in 1913 I was able to 
show, with T. W. Dickson (40), that the optical rotations of ten simple 
alcohols, and the magnetic rotations of thirty-four simple organic com- 
pounds for eight wave-lengths in the visible spectrum could be expressed 
by one term of Drude’s equation : 
a =k] (22 — Ao”). 
In the following year we found (41) that two terms of opposite sign: 
ky ky 
canary Ce 2 i222 re? 
ea 
could be used in the same way to express the anomalous rotatory dis- 
persion of ethyl tartrate. This result confirmed the conclusion reached 
at a much earlier period by Biot (42) and by Arndtsen (43), that anomalous 
rotatory dispersion has its origin in the superposition of two partial 
rotations of opposite sign and of unequal dispersion. These partial 
rotations may be due to very diverse causes, ranging from the presence of 
two optically active absorption bands in the same molecule, to the case 
in which two liquids of opposite rotatory power and unequal dispersions 
are arranged in series in separate polarimeter tubes. This diversity has 
resulted in a certain amount of controversy as to the origin of the partial 
rotations which give rise to anomalous rotatory dispersion (44), but the 
essential facts represented by Drude’s equation are established beyond 
dispute. 
VALIDITY OF DrubDE’s EQuaTION. 
Since Drude’s equation is only applicable to transparent media, the 
limits of validity of the equation coincide with the conditions under which 
