ON OPTICAL ROTATORY DISPERSION. 
265 
rotatory power with the parent substance, but like it exhibit a perfectly simple rotatory 
dispersion, with a dispersion-constant \ 0 2 = 0*0627. 
After the mathematical evidence set out under (c) above, the discovery of these fixed 
compounds is the most important evidence that has yet been put forward in support of 
Arndtsen’s hypothesis. If it is difficult to discover in the formula commonly assigned to 
tartaric acid any physical basis for the anomalous optical properties of the acid and of 
so many of its derivatives, it would be at least equally difficult to discover either in 
boro-tartaric acid or in tartar emetic any factor which would account for the disap¬ 
pearance of the anomalies, apart from the view which has already been advanced that 
in these compounds the plastic acid has been fixed in one of its labile forms. 
4. Experimental Methods. 
The experimental work described in the present paper was undertaken with the 
object of applying to the problems investigated by Biot the exact methods of measuring 
rotatory dispersion which have been developed during the past 15 years, and which 
have already been applied (as described in the preceding paper of this series) to the 
exact determination of the optical rotatory power of quartz. Thus Section 5 describes 
a critical investigation of the relationship between the rotatory power and concentration 
of aqueous solutions of tartaric acid, and a detailed study of the deviations from Biot’s 
linear law, as revealed by exact measurements for a series of eight wave-lengths in the 
visible region of the spectrum. Section 6 describes an investigation of the relationship 
between optical rotatory power and wave-length for a series of aqueous solutions of 
tartaric acid ; this was undertaken in order to test in the case of tartaric acid the equa¬ 
tions which had already been proved to be adequate to express the anomalous rotatory 
dispersion of the tartaric esters. The remaining sections of the paper deal with the 
influence of various chemical agents on the rotatory power of tartaric acid, in extension 
of the early pioneering work of Biot. 
The optical apparatus used in the experiments on tartaric acid was the same as that 
which has already been described in the preceding paper of the present series. An 
important improvement has, however, been effected in the matter of light-sources. In 
the experiments on quartz the cadmium lines were read with the help of light derived 
from an open arc burning between electrodes of a cadmium-silver alloy (Lowry, ‘ Phil. 
Mag.,’ 1909, vol. 18, pp. 320-327). Five years later a description was given of “ An 
enclosed cadmium arc for use with the polarimeter ” (Lowry and Abram, ‘ Trans. 
Faraday Soc.,’ 1914, vol. 10, pp. 103-106) ; this arc was of an experimental type, requiring 
the continuous use of a Gaede pump to maintain the vacuum, and merely served to 
show how very valuable an efficient lamp of this type would be in all optical experiments 
which demand intense sources of monochromatic light. During the period covered by 
the experiments now described we have had the privilege of using the enclosed cadmium 
arc designed by Dr. Sand and described by him at the Manchester meeting of the British 
Association (‘ B.A. Report,’ 1915, vol. 85, p. 386). This lamp has proved to be per- 
2 P 
vol. ccxxn.— A. 
