ON OPTICAL ROTATORY DISPERSION. 
261 
in these liquids since the two compensating terms in the equation may be due to (i.) two 
electrons with opposite influence on the rotatory power, as in the case of quartz, where, 
however, they do not give rise to anomalous dispersion, (ii.) two radicals of opposite 
activity united hi one molecule, as in the cases of Lmenthyl d-camphor /3-sulphonate 
(Tschugaeff, £ Ber. Deutsch. Chem. Gesell.,’ 1911, vol. 44, p. 2023 ; 1912, vol. 45, 
p. 2759; compare c Trans. Faraday Soc.,’ 1914, vol. 10, p. 73) and Gmenthyl 
d-diphenylmethylacetoacetate (Rupe and Kagi®, £ Ann. der Chem.,’ 1920, vol. 420, 
p. 38) ; (iii.) two molecules of opposite rotatory power, either easily convertible, as in 
the case of the isomeric nitro-camphors (Lowry, ‘ Trans. Faraday Soc.,’ 1914, vol. 10, 
p. 100), or fixed, as in the artificial mixtures of Biot (‘ Comptes Rendus,’ 1836, vol. 2, 
p. 543), and of von Wyss (‘ Wiedemann’s Ann. Phys. Chem.,’ 1888 [2], vol. 33, p. 567). 
To decide between these three possibilities, further consideration is needed both of the 
chemical and of the physical properties of the solution as set out in the following 
paragraphs. 
(d) Dynamic Isomerism as an Explanation of Anomalous Rotatory Dispersion. 
The view that the two terms in the equations showing the effect of wave-length on 
the rotatory power of tartaric acid and its derivatives are due to two electrons, as in the 
case of quartz, is rendered improbable by the fact that substances of similar type do 
not show this effect. Thus, if tartaric acid be regarded as a dicarboxylic acid of the 
sugar-group, belonging to the C 4 series and containing two asymmetric carbon atoms, 
it might be expected that the methyl-glucosides, which belong to the C 6 series and 
contain five asymmetric carbon atoms, would give even more complex dispersion-curves ; 
actually, however, their dispersion can be expressed accurately by a simple one-term 
formula. The hypothesis of two radicals of opposite optical activity is even less easy 
to apply to tartaric acid, unless some form of molecular rearrangement is first postu¬ 
lated, since the two active radicals of which it is composed are not only of the same 
sign, but are of identical structure, and would therefore give identical rotatory dispersions. 
It is, indeed, impossible to discover, in the simple structural formula 
HO.CO.CHOH.CHOH.CO.OH, 
commonly assigned to tartaric acid, any justification for its anomalous rotatory 
power, and some change of molecular structure appears to be inevitable if its 
peculiar physical properties are to be accounted for, since the complex dispersion of 
the acid is just as exceptional amongst simple organic compounds as would be the 
appearance of a bright blue or green colour in a simple compound of the alcohol or 
sugar group. If then, some form of molecular rearrangement must be assumed, no 
simpler hypothesis can be adopted than that of Arndtsen, which suggests that the 
rearrangement is incomplete, so that one of the two compensating factors required to 
account for the complex or anomalous dispersion of the medium is merely the original 
