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Proceedings of the Royal Society of Edinburgh. [Sess. 
instance, the case of aqueous solutions of tartaric acid, in which the 
maximum rotation is found as usual at the violet end of the spectrum 
when the solutions are weak, but becomes displaced at greater concentra- 
tions, and, travelling along the spectrum, finally arrives at the red end. 
Such abnormal cases show that the information to be obtained from 
spectro-polarimetry will by no means be necessarily a repetition of, or 
mere variant on, that which can be obtained by polarimetry alone. 
The present note is only of a preliminary nature, for the purpose of 
giving a first instalment of data ; for it has been found that already certain 
interesting conclusions may be established. 
II. Description of the Experiments. 
The active and the inactive salts employed were respectively sodium- 
potassium - tartarate and ammonium - molybdate, and both were of 
Kahlbaum’s manufacture. The solutions were prepared by weighing out 
the required quantities of the two salts, and placing these quantities in a 
graduated flask, which was then filled up to the mark with distilled 
water at 25° C. The flasks employed were specially graduated by myself, 
so as to give true indications at the temperature mentioned, which is that 
at which all the measurements were carried out. 
The method of experiment was as follows. Each solution was made up 
to always be a third normal as regards the tartarate, and some simple 
fraction of a third normal as regards the molybdate. Each solution was 
measured not only for its optical rotation at various parts of the spectrum, 
but also for density and for electrical conductivity, separate portions being 
always set aside for each purpose. The object of the two latter kinds of 
measurement was to obtain if possible other indications of the changes in 
the solution, with a view to comparing them with the optical indications. 
In carrying out the work the following order of solutions was observed. 
First of all a i N solution of the tartarate alone was made up and measured. 
Then a solution was made up containing ^ N tartarate and N molybdate 
and similarly measured. The next solution contained J- N tartarate and 
N molybdate, and so on; the successive concentrations of molybdate in 
terms of I- N being 0, -J, J, f, and 1. 
1. The Density Measurements. 
Little requires to be said about these. Ostwald’s form of Sprengel 
pyknometer was employed. It was filled with the solution and placed in a 
thermostat bath at 25° C. until no further movement was observed on the 
