1906 - 7 .] 
Optical Rotation of Two Salts. 
271 
XXVIII. — A Preliminary Note on the Optical Rotations (through- 
out the Spectrum), the Electrical Conductivities, and the 
Densities of Mixtures of Sodium-Potassium-Tartarate and 
Ammonium-Molybdate in Aqueous Solution. By James 
Robert Milne, D.Sc., Carnegie Research Fellow in Physics. 
(MS. received June 14, 1907. Read July 8, 1907.) 
I. Introduction. 
It has long been known that the rotative power of certain optically active 
salts in aqueous solution is greatly, and in some cases enormously, changed 
by the addition of certain non-active salts. Of late years a good deal of 
work has been done on this subject,* and a considerable amount of data 
has been collected; but, generally speaking, little explanation has been 
obtained of the precise causes of the observed phenomena. In some cases 
at least the reason for this no doubt lies in the fact that in such solu- 
tions there exist complicated molecular combinations, about which it may 
not be easy to obtain ordinary chemical evidence. This, however, only 
makes it the more important that we should utilise to the fullest extent 
such means of investigation as are readily available, and of these none is 
so good as that of optical activity. 
It seemed to me, therefore, that it would be well to try what could be 
done to extend the use of this method of investigation, by measuring the 
rotations of a few of these solutions not only for D light, but also for 
light of a number of other wave lengths chosen at intervals throughout the 
spectrum. In this way a great deal of additional optical information 
would be obtained ; for the usual method only, as it were, runs a single 
traverse over the unknown country whose topography is to be explored. 
Of course in some cases it has been found that this procedure is 
sufficient, and that, given the rotation of a substance for one wave length 
of the spectrum, the rotation for any other wave length can be deduced 
from that of the first by the use of a mathematical formula. Quartz is a 
very good example of this, for its observed rotations all fit very accurately 
into Boltzmann’s formula 
A , B 
a= u + v 
On the other hand, however, there 
were early discovered cases of strikingly abnormal rotation-dispersion ; for 
* References, and a good nummary of the chief results, may he found in H. Landolt’s 
Das Optische Drehungsvermogen (second edition). 
