ON OPTICAL ROTATORY DISPERSION. 
259 
distinguishes the work on tartaric acid from that on quartz, and gives to it its own 
peculiar importance. From the physical point of view the contrast is not great, 
and almost the whole of the advantage rests with the quartz, the large rotatory power 
of which renders possible an accuracy of measurement that is probably at least 100 
times greater than in the case of any other medium. But whereas the rotatory power of 
quartz is influenced only slightly by the experimental conditions, tartaric acid is subject 
to drastic changes of rotatory power as a result of very small changes of circumstance. 
In recent years this extreme sensitiveness has been regarded by many writers as an 
inherent quality of the physical property of optical rotatory power ; but all the new 
evidence goes to show that Biot was right in attributing it to chemical changes in the 
optically active liquid. One point must, however, be made clear immediately : whatever 
the nature of these chemical changes may be, they must proceed to equilibrium with 
very great rapidity, since even the most careful experiments (‘ Trans. Chem. Soc.,’ 1915, 
vol. 107, pp. 1177 and 1189) have failed to detect any lag in the adjustment of rotatory 
power in the tartaric esters when conditions have been altered. In this respect these 
changes may be compared with the dissociation of nitrogen peroxide, with the dissocia¬ 
tion and association of water, or even with the phenomena of ionisation in aqueous 
solutions, all of which appear to depend on very rapid or instantaneous reactions. 
Under these circumstances, only static methods of investigation are available, i.e., the 
liquid must be proved to be complex by recognising in it the attributes of a mixture, 
rather than by isolating its components and watching them change gradually into one 
another with lapse of time. 
( b ) Anomalous Rotatory Dispersion observed in Natural and Artificial Mixtures. 
What evidence is there, then, that tartaric acid gives rise to a complex product 
when it is dissolved or melted in order to convert it into an isotropic, optically active 
medium ? Biot found evidence of widespread chemical change in the progressive 
alterations of rotatory power that were produced alike by the addition of water or 
alcohol, of soda or potash, of ammonia, and of sulphuric and boric acids ; but this 
argument is obviously limited to the changes which accompany dissolution and cannot be 
applied to the mere fusion of the anhydrous acid. A more general argument is afforded 
by the experiments in which Biot attempted to neutralise the optical rotatory power 
of lsevorotatory turpentine by compensating it with a column of dextrorotatory oil 
of lemon (‘ Comptes Rendus,’ 1836, vol. 2, p. 543) ; not only was the compensation 
“ neither complete nor general,” but “ when the principal section of the analysing prism 
coincided with the original plane of polarisation, there was produced an extraordinary 
image of blue-violet colour, dark, and sensibly free from red : and, on turning the prism 
a little to the right or to the left of this position, the tint of the image varied in a con¬ 
trary sense to the refrangibility, things which are entirely different from those which a 
single one of the two essences could produce alone.” Similar effects were observed in 
2 o 2 
