250 
MESSRS. THOMAS MARTIN LOWRY AND PERCY CORLETT AUSTIN 
plate of opposite sign acting as a compensator (‘ Mem. Acad. Sci.,’ 1817, vol. 2, 
pp. 103-114). When tartaric acid was compared with quartz,however, no such parallelism 
was observed, the rotations for the chief colours being as follows :— 
Red. 
Orange. 
Yellow. 
Green. 
Blue. 
Indigo. 
Violet. 
Quartz. 
Tartaric acid . . . 
O 
18-99 
38-7 
O 
21-40 
40-29 
O 
23-99 
42-51 
O 
27-86 
46-11 
O 
32-31 
44-40 
o 
36-13 
42-9 
O 
40-88 
39-38 
(‘ Mem. Acad. Sci.,’ 1838, vol. 15, p. 236). Similar phenomena were observed when 
tartaric acid was dissolved in alcohol (ibid., p. 245) ; but Biot found that “ when it 
combines with basic substances in the same media, it loses its special action and imprints 
on the products the properties common to all other bodies endowed with rotatory 
power ” ( £ Mem. Acad. Sci.,’ 1838, vol. 16, p. 229 ; paper read November 27, 1837). 
Biot’s experiments on tartaric acid and the tartrates covered so wide a range that 
the whole of the work described in the present paper may be regarded as a logical exten¬ 
sion of his investigations, aided on the physical side by modern optical theories and 
by modern apparatus, and on the chemical side by structural formulae and by stereo¬ 
chemical notions which were only struggling for recognition even in the closing years of 
Biot’s life. The contributions which he made to the fundamental problem of deter¬ 
mining the form of the curves of rotatory dispersion are discussed below in a separate 
paragraph, but reference may be made here to a few of the many topics covered by his 
investigations. 
(a) Influence of Water on the Rotatory Rower of Tartaric Acid. 
Biot discovered that ££ in aqueous solutions of tartaric acid at a given temperature, 
the rotatory power of the acid calculated for each simple ray is always of the form 
A + Be, where e represents the proportion by weight of water in the solution ” ( £ Mem. 
Acad. Sci.,’ 1838, vol. 15, p. 216 ; compare ibid., p. 207), the specific rotation being 
therefore a linear function of the concentration. This linear law was first described in 
a sealed communication deposited in the Archives of the Academy on August 25, 1834, 
and opened on December 7, 1835. A note added on the latter date ( £ Comptes Rendus,’ 
1835, vol. 1, p. 459) stated that the law was an approximation which did not apply to 
dilute solutions. Biot made use of the linear law to construct a diagram ( £ Mem. Acad. 
Sci.,’ 1838, vol. 15, fig. 4, facing p. 652), in which the rotatory power for light of different 
colours is shown as a series of straight lines inclined at different angles to the axis of 
concentration. This diagram shows a marked similarity to the “ characteristic diagram ” 
constructed 75 years later by Armstrong and Walker ( £ Roy. Soc. Proc.,’ 1913, series 
A, vol. 88, pp. 388-403), the chief difference being that the rotations for green light are 
