ON OPTICAL ROTATORY DISPERSION. 
253 
Biot for the anhydrous acid, and by Arndtsen Ann. Chim. Phys.,’ 1858, vol. 54, 
p. 415) for very strong alcoholic solutions ; this negative rotation for dark-blue light 
became positive for light of longer wave-lengths, reaching a maximum dextrorotation 
in the yellow region of the spectrum. Winther (' Zeitschr. physikal. Chem.,’ 1902, 
vol. 41, pp. 181-189), who made a detailed study of the effects of temperature and 
concentration on the rotatory power of the acid in aqueous and in alcoholic solutions, 
extrapolated to 100 per cent, tartaric acid, and concluded that the maximum might 
be displaced still further, disappearing at the red end of the spectrum when the tem¬ 
perature of the anhydrous acid fell below 20° C. This conclusion was not confirmed 
by Bruhat (‘ Trans. Faraday Soc.,’ 1914, vol. 10, p. 89), whose extrapolation gave a 
maximum between the yellow and the red, whilst his observations of the glassy acid at 
15° C. indicated that a maximum could still be seen in the red region of the spectrum. 
New ground has been broken in the detailed investigation by Winther and by 
Patterson of the tartaric esters which show similar anomalies to those which Biot 
had discovered in tartaric acid.* Very noteworthy also is the achievement of Bruhat 
(i loc . tit.), who by means of special apparatus succeeded in measuring the rotatory power 
of fused and superfused tartaric acid for several wave-lengths at temperatures from 
180° to 15° C., thereby bringing the acid into line with its esters as they had been investi¬ 
gated 12 years earlier by Winther. The investigation of tartar emetic and of the 
related compounds of arsenic and bismuth has also given interesting results, which 
are described in detail below (pp. 284 and 285). 
2. Rotatory Dispersion in Quartz and in Tartaric Acid. 
Biot’s measurements of the optical rotatory power of tartaric acid indicated the 
existence of two types of rotatory dispersion, which may be described provisionally as 
the “ quartz type ” and the “ tartaric acid type.” Since these correspond to some 
extent with the later classification of rotatory dispersion as normal and anomalous, or 
as simple and complex, it will be desirable to set out the essential features of these 
various methods of classification. 
(a) Biot’s Two Types. 
Biot divided optically active substances into two groups according as they obeyed 
the law of inverse squares, a = Z;/\ 2 , or showed large deviations from this law. The 
maximum in the optical rotatory power of tartaric acid, which he observed in the green 
region of the spectrum, was merely an incidental feature in the exceptional behaviour 
of the acid, and did not receive any of the emphasis which has since been placed upoii 
* For a study of the form of the dispersion-curves for ethyl and methyl tartrates see Lowry and Dickson, 
* Trans. Chem. Soc.,’ 1915, vol. 107, pp. 1173-1187 ; Lowry and Abram, ibid., pp. 1187-1195. 
