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MESSES. THOMAS MARTIN LOWRY AND PERCY CORLETT AUSTIN 
it.* On tlie other hand, the extreme sensitiveness of the rotatory power of tartaric 
acid to changes of temperature, concentration and solvent was regarded by Biot as 
one of the chief anomalies in the behaviour of this exceptional substance. Biot had 
laid stress on the fact that the optical rotatory powerf of a substance was usually hide- 
pendent of the conditions under which it was observed ; thus the rotatory power of 
turpentine was not affected by diluting it with other essential oils (‘ Mem. Acad. Sci.,' 
1817, vol. 2, p. 115), or with ether (ibid., p. 116), and it even retained its optical activity 
when examined as a vapour in a column 30 metres in length (ibid., pp. 126-133, 
compare Gernez, ‘ Ann. de l’Ecole Norm.,’ vol. 1, p. 1) ; cane sugar, too, showed a 
very similar rotatory power when examined in the amorphous solid state as “ barley 
sugar ” and when this same product was dissolved in water (‘ Mem. Acad. Sci.,’ 1835, 
vol. 13, pp. 126-132). The marked influence on the rotatory power of tartaric acid of 
dilution with water or of addition of boric acid was therefore attributed to chemical 
changes just as definite as those involved in the conversion of the acid into its salts. 
This early view, which had fallen into disrepute for many years, has now become promi¬ 
nent again, more especially as an explanation of variations of rotatory power in those 
cases of anomalous rotatory dispersion of which tartaric acid is still the chief type. 
(b) Normal and Anomalous Rotatory Dispersion.I 
Only a short time elapsed before Biot’s law of inverse squares was recognised as 
being inexact. Biot had suspected almost from the first that small variations of dis¬ 
persive power might exist in different compounds ; in 1836 he obtained clear evidence 
of this fact by balancing against one another columns of turpentine and of oil of lemon 
which produced approximately equal and opposite rotations, when he found that “ the 
compensation of the deviations, although very close for all the rays, was, however, 
neither complete nor general ” (‘ Comptes Rendus,’ 1836, vol. 2, p. 543) ; cane-sugar 
* For early examples of this emphasis, see Arndtsen, ‘ Ann. Chim. Phys.,’ 1858, vol. 54, p. 409 ; Krecke, 
‘ Arch. Neerlandaises,’ 1872, vol. 7, p. 114 ; Landolt, ‘ Liebig’s Annalen,’ 1877, vol. 189, p. 274. 
t Biot described as the molecular rotatory power of substances (‘ Mem. Acad. Sci.,’ 1838, vol. 15, p. 95 ; 
compare ‘ Mem. Acad. Sci.,’ 1835, vol. 13, p. 116) the rotation that would be produced by a column 1 mm. 
thick and of unit density; this is one-hundredth part of what is now called the specific rotatory power of 
the substance. 
1: The term “ normal dispersion ” was used by Arndtsen in 1858 (‘ Ann. Chim. Phys.,’ 1858, vol. 54, 
p. 412) to describe the case in which “ the angle of rotation increases continuously with the refrangibility 
of the rays." The term “anomalous (rotatory) dispersion” appears to have been introduced in 1877 by 
Landolt (’ Liebig’s Ann. der Chemie,’ 1877, vol. 189, p. 274), who described under this heading (1) a 
maximum which travels from the violet to the green region of the spectrum as the concentration of the 
aqueous solution of tartaric acid increases, and (2) a reversal of sign in the rotatory power of the anhydrous 
acid and of its alcoholic solution. Krecke, five years earlier ( ; Arch. Neerland.,’ 1872, vol. 7, pp. 98, 110 
and 114), had referred less specifically to the “ remarkable anomalies ” which are observed in the optical 
properties of tartaric acid. 
