ON OPTICAL ROTATORY DISPERSION. 
287 
boric acid was in large excess, or less exactly by a linear formula representing a tangent 
to the hyperbola. This linear formula was found to apply also to the enhancement of 
rotatory power in a glassy mixture of amorphous tartaric and boric acids ; four such 
mixtures gave rotations (for the neutral tint) ranging from -f-31° to +13°, and these 
when extrapolated gave for pure glassy tartaric acid a negative rotation —2-9°, agreeing 
closely with the value —3*28° observed experimentally (‘Ann. Chim. Phys.,’ 1850, 
vol. 28, p. 374). 
Preliminary experiments on an equimolecular mixture of tartaric and boric acids 
(15 grams of tartaric acid and 6-2 grams of boric acid in 100 c.c. of solution) showed 
that the dispersion did not fulfil the requirements of Biot’s law, but could be expressed 
by a simple formula with a normal dispersion constant [M] = 24-835/(A 2 —0-0271). 
A more exact series, Table XX.(a), including readings for 18 lines in the visual and five 
lines in the photographic region of the spectrum, showed, however, that the simple 
formula was again only an approximation ; but the negative term in the complex 
formula is very small (only about of the positive term), and would probably disappear 
altogether if a sufficient quantity of boric acid were used to convert the tartaric acid 
wholly into boro-tartaric acid. Table XX. (6), which shows the effect of 1-| mols. of 
boric acid on tartaric acid of half the strength used for Table XX. (a) affords further 
justification for this view ; the dispersion is here so nearly simple that the only hint 
of complexity is that given by a few negative errors in the red and in the extreme violet 
region of the spectrum. 
The study of boro-tartaric acid, like that of tartar emetic, illustrates in a very striking 
manner the way in which the complex rotatory dispersion of tartaric acid is simplified 
when it is converted into “ fixed ” derivatives, even when these are more complex in 
their chemical structure. The actual structure of these derivatives has been the subject 
of much speculation, and must be regarded as still very uncertain ; but we hope to be 
able to carry out a chemical study of this problem, which will form a suitable sequel 
to the physical investigations which are described in the present paper. 
12. Summary. 
1. The rotatory power of tartaric acid for a senes of 9 wave-lengths has been 
determined in aqueous solutions of 11 different concentrations ranging from 5 to 55 per 
cent, by weight, and also for 21 and 26 wave-lengths respectively at 2 other concentrations. 
2. The optical rotatory power of tartaric acid, like that of its methyl and ethyl esters, 
is expressed to a close degree of approximation by the formula 
__k 2 _ 
A 2 -A/ ~ A 2 —A/' 
3. The rotatory power of sodium tartrate agrees very closely with Biot’s law, 
« — k /\ 2 , but requires for its exact expression a two-term formula similar to that 
