252 MESSRS. THOMAS MARTIN LOWRY AND PERCY CORLETT AUSTIN 
is broadly correct, since aqueous solutions of sodium tartrate show a closer approxima¬ 
tion to the requirements of Biot’s Law than in the case of any other substance that we 
have yet investigated ; the agreement is, however, even here not exact, and the complex 
character of the rotatory dispersion of the acid is shown, although in a much less striking 
way, in the salts derived from it. 
(d) Influence of Boric Acid. 
Of wider general interest is Biot’s discovery of the remarkable exaltation of rotatory 
power which is produced by the addition of boric acid to tartaric acid. The properties 
of the boro-tartaric solutions were first described in outline in his sealed note to the 
Academy (‘ Comptes Rendus,’ 1835, vol. 1, p. 458, compare £ Mem. Acad. Sci.,’ 1838, 
vol. 16, p. 271), but a detailed description was given ten years later as part of a long 
memoir 44 On the employment of polarised light to study various questions of chemical 
mechanics” (‘Aim. Chim. Phys., 5 1844, vol. 11, pp. 82-112; see also ‘Ann. Chim. 
Phys.,’ 1860, vol. 59, pp. 229-256). It is characteristic of the thoroughness with which 
Biot worked, that he extended his observations to include not only aqueous solutions, but 
also glassy amorphous mixtures of the two anhydrous acids (‘ Ann. Chim. Phys., 5 1850, 
vol. 28, p. 368). The exaltation of rotatory power which Biot observed on the addition 
of boric to tartaric acid appears in many other substances which resemble tartaric acid 
in containing two hydroxyl-groups attached to adjacent carbon-atoms. It is generally 
accompanied by an increase of electrical conductivity, and in recent years has been 
made the basis of a general method of determining the configuration of hydroxylic- 
compounds of the sugar group (see especially Magnanini, 4 Zeitschr. physikal. Chem., 5 
1890, vol. 6, p. 67 ; 4 Berichte Deut. Chem. Ges., 5 1891, vol. 24 ref., p. 894 ; Boeseken, 
4 Berichte,’ 1913, vol. 46, p. 2612 ; Irvine, 4 Trans. Chem. Soc., 5 1914, vol. 105, p. 898 ; 
1915, vol. 107, pp. 1221 and 1230). 
Later workers, whilst confirming the accuracy of his general conclusions, have added 
many details to the broad outlines of Biot’s work. Thus Arndtsen ( 4 Ann. Chim. 
Phys., 5 1858, vol. 54, p. 411), during a summer spent in Paris, showed that Biot’s 
maximum in the green disappeared at the violet end of the spectrum at concentrations 
below 20 per cent, of tartaric acid ; Krecke ( 4 Archives Neerlandaises, 5 1872, vol. 7, 
p. 107) showed that even in a 50 per cent, solution the maximum vanished at the violet 
end of the spectrum when the temperature was raised to 50° C. ; and Wendell 
( 4 Wiedemann’s Ann. Phys. Chem.,'1898, vol. 66, pp. 1149-1161) confirmed the observa¬ 
tions of Arndtsen and of Krecke as to the displacement of the maximum towards 
the violet by dilution and by heating, and the increase of rotatory power which accom¬ 
panies these changes. Lereschkin ( 4 Berichte Deut. Chem. Ges., 5 1899, vol. 32, pp. 
1180-1184), working in the opposite direction, observed in a supersaturated solution at 
20° C. a negative rotatory power, analogous with that which had been recorded by 
