274 
MESSRS. THOMAS MARTIN LOWRY AND PERCY CORLETT AUSTIN 
which depend on extrapolation beyond the range of the experimental data are printed 
in italics. 
From Table V., and from the graphical representation of fig. 4, the following con¬ 
clusions may be drawn :—- 
(1) The reversal of sign (which has been observed hitherto only in the anhydrous 
acid, in alcoholic and in super-saturated aqueous solutions or in the ultra¬ 
violet region of the spectrum) is here recorded with the help of the camera, in 
the extreme violet region of the spectrum in unsaturated aqueous solutions 
containing only 50 grams of tartaric acid in 100 c.c. of the solution (e = 0-5875 
or P = 41-25 per cent.). 
(2) The maximum rotation, which has usually been said to vanish on dilution, is here 
retained on the less refrangible side of the violet mercury line even at a con¬ 
centration of only 5 grams per 100 c.c. This fact is of interest in connection 
with the statement of Arndtsen that at concentrations below 20 per cent, by 
weight “ the dispersion becomes normal in so far as the angle of rotation 
increases continuously with the refrangibility of the light; yet the rotation 
increases so little from the ray F to the ray e, that one cannot doubt that it 
would have a maximum in the violet part of the spectrum ” (‘ Ann. Chim. 
Phys.,’ 1858, vol. 54, p. 412). Our observations prove the reality of this 
hypothetical maximum, whilst, at the same time, they show the futility of any 
definition of “ normal dispersion ” which depends merely on the removal of 
the maximum from the easily-visible into the violet or ultra-violet region of 
the spectrum ; a similar usage of this term by Winther has already been 
criticised (Lowry, ‘ Trans. Chem. Soc.,’ 1915, vol. 107, p. 1195) on the ground 
that it implies a physiological definition of a purely physical property. Anoma¬ 
lous dispersion depending on the imperfect compensation of two simple rotatory 
dispersions of opposite sign may, as a matter of actual fact, be rendered normal 
by displacing the maximum to infinite wave-length in the infra-red ; but any 
displacement in the opposite direction only exaggerates the invisible maximum 
in the ultra-violet region.* 
7. Tartrates of the Alkali-Metals. 
In his sealed note, communicated to the Academy on August 25, 1834, Biot referred 
for the first time to the rotatory power of the tartrates in the following terms —“ The 
combinations of tartaric acid with solid bases give products endowed with rotation 
towards the right ; but the relative intensity of these rotations for the different simple 
rays obeys the general law of this phenomenon, to which tartaric acid alone is a marked 
exception, at least, among all the bodies which I have been able to study hitherto ” 
(‘ Comptes Rendus, 1835, vol. 1, p. 458). A detailed study of the tints produced by 
* For a series of curves illustrating this point, see ‘ Trans. Chem. Soc.,’ 1915, vol. 107, p. 1200. 
