262 
MESSRS. THOMAS MARTIN LOWRY AND PERCY CORLETT AUSTIN 
form of the acid, whilst the other is a product of change, of opposite sign and unequal 
dispersion, but of a character sufficiently commonplace to give rise to a simple, instead 
of a complex, dispersion-curve when studied as a separate entity. This product might 
be an ion, a hydrate, a polymer, or an isomer of the original form of the acid ; but, 
since the amorphous acid and its liquid esters exhibit a full range of anomalies in the 
absence of any solvent, the first two possibilities are ruled out, and the alternatives are 
reduced to two, involving (i.) polymerisation or depolymerisation as suggested by 
Wendell (‘ Wiedemann’s Ann. Phys. Chem.,’ 1898, vol. 66, p. 1156), or (ii.) isomeric 
change. These alternatives may even be combined, since when isomeric change takes 
place, it is not unusual for one or other of the isomers to undergo association if the 
physical and chemical conditions are favourable. 
Arndtsen’s hypothesis may then be interpreted in the light of modern knowledge, 
by suggesting that tartaric acid and its esters afford yet another example of “ dynamic 
isomerism ” or reversible isomeric change. This phenomenon was discovered in 1877 
by Butlerow (' Liebig’s Ann. der Chem.,’ 1877, vol. 189, p. 77), who made use of it 
in order to account for the production of two types of derivatives from prussic acid, 
a result that could be explained most readily by assuming that the liquid acid was a 
mixture of the two parent-compounds, thus 
CHs.CiN -4- H.C:N^HN:C< ■> CH 3 .N:C< 
v__ J 
Methyl cyanide. Prussic acid. Methyl isocyanide. 
These two isomerides have not yet been isolated, although Butlerow was able to 
demonstrate the existence of a similar equilibrium between two isomeric olefines when 
isodibutylene was dissolved in strong sulphuric acid. If tartaric acid gave rise, when 
fused or dissolved, to a similar mixture of isomerides with suitable optical properties, 
Arndtsen’s hypothesis would afford a complete explanation of the anomalous dispersion 
of the acid. 
(e) Plastic and Fixed Derivatives of NitrocampJior and of Tartaric Acid. 
Nitrocamplior, which exists in solution in two optically active forms, of opposite sign 
and unequal dispersion, affords a still more striking example of dynamic isomerism. 
The ordinary form of the compound is laevorotatory, but its rotatory power in freshly 
prepared solutions changes from left towards right, giving rise to the phenomenon of 
mutarotation, or change of rotatory power with time (Lowry, ‘ Trans. Chem. Soc.,’ 
1899, pp. 75, 211) as the result of a reversible isomeric change, which can be expressed 
by the balanced equation : 
CH.NO, C:NO.OH 
c 8 h 14 < I x c 8 h 14 < | 
CO x co 
N itrocamphor. Pseudonitrocamphor. 
