260 
MESSRS. THOMAS MARTIN LOWRY AND PERCY CORLETT AUSTIN 
artificial mixtures of lsevorotatory turpentine with dextrorotatory camphor, as well as 
in some natural turpentine-oils. 
On the basis of Biot’s experiments, as well as of his own observations of the unequal 
dispersive power of different liquids, Arndtsen (‘ Ann. Chim. Phys., 5 1858, vol. 54, 
p. 421) put forward for the first time a precise explanation of “ the singular dispersion of 
the planes of polarisation of tartaric acid,” as follows 
“ If one should imagine two active substances which do not act chemically upon 
one another, of which one turns the plane of polarisation to the right, the other to the 
left, and, in addition that the rotation of the first increased (with the refrangihility of 
the light) more rapidly than that of the other, it is clear that, on mixing these substances 
in certain proportions, one would have combinations which would show optical pheno¬ 
mena precisely similar to those of tartaric acid, as M. Biot has already proved by his 
researches on different mixtures of turpentine and natural camphor. One might then 
regard tartaric acid as a mixture of two bodies differing only as regards their optical 
properties, of which one had a negative rotatory power, the other a positive rotatory 
power, and of which the rotations varied in different proportions with the refrangihility 
of the light.” 
This hypothesis, made more than 60 years ago, appears to us to afford a correct 
explanation of the peculiar rotatory dispersion of tartaric acid and its derivatives. 
In its support we submit (i.) the mathematical evidence that the form of the dispersion- 
curves is in harmony with this view, and (ii.) the chemical evidence that mixtures of 
isomerides in equilibrium actually exist, e.g., in the case of nitrocamphor, and that 
their behaviour is in accordance with that which must be postulated for the “ two kinds 
of optically-active molecules ” assumed by Arndtsen. 
(c) Anomalous Rotatory Dispersion produced by the Partial Compensation of two 
Simple Dispersions. 
The measurements described below afford strong support to Arndtsen’s theory, 
since it has been established by visual and photographic readings that the dispersion 
curves for tartaric acid, like those of its esters (‘ Trans. Chem. Soc.,’ 1915, vol. 107, 
pp. 1173-1195), can he represented over a wide range of wave-length and to a close degree 
of approximation by two terms of Drude’s -equation, thus 
/’] JC‘2 
This equation is a direct mathematical expression of the view that the anomalous 
rotatory dispersion of tartaric acid is produced by the counterbalancing action of two 
components of opposite rotatory power and unequal dispersion. It is, however, not in 
itself a conclusive argument for the presence of two kinds of optically active molecules 
