ON DYNAMIC ISOMERISM. 1038 
dispersion will open up a new and fruitful field for the investigation of 
dynamic isomerism.’ The importance of this aspect of the subject is 
shown by the conspicuous part which it played in a general discussion 
on ‘Optical Rotatory Power,’ held before the Faraday Society on 
March 27, 1914, to which the Chairman and Secretary of this Com- 
mittee contributed papers. Preliminary experiments, which will he 
described in a subsequent Report, have shown (1) that ethyl tartrate, 
the typical example of anomalous rotatory dispersion, is probably a 
mixture, and (2) that nitrocamphor, the typical example of dynamic 
isomerism, gives rise to anomalous rotatory dispersion in certain 
solvents. 
Dynamic Isomerism, Metamerism, Tautomerism, and. Desmotropy. 
Attention has recently been directed (Proc. Chem. Soc., April 4, 
1914) to the importance of maintaining strict accuracy in the use of 
terms to describe the phenomena of reversible isomeric change. 
Briefly, it may be said that all the essential facts in reference to 
the conception of equilibrium between isomerides are set out in 
Butlerow’s classical, but almost forgotten, paper, ‘ Ueber Isodibutylen ’ 
(Annalen, 1877, 189, 44). The name dynamic isomerism was intro- 
duced in 1899 (Trans. Chem. Soc., 75, 235) as a paraphrase of Butle- 
row’s description of ‘ a condition of equilibrium depending on incessant 
isomeric change ’; but the adjective isodynamic had already. been sug- 
gested by Armstrong in 1889 (Watts’ Dictionary, ‘Isomerism’) to 
describe those isomerides ‘which change their type with exceptional 
facility in the course of chemical interchanges.’ The word metameric 
had been used in this sense in 1833 by Berzelius to describe isomerides 
which were readily converted into one another, but the usefulness of 
‘the word was destroyed by a misguided attempt to transfer it to another 
usage. 
The hypothesis of tautomerism was introduced by Laar in 1885 
(Ber. 18, 648; 19, 730) to account for the facts which had already (as 
time has shown) been explained adequately by Butlerow. Laar asserts 
that, in every case of tautomerism, the different formule. suggested 
by the reactions of the substance represent, ‘ not isomeric, but identical 
bodies ’; the term cannot, therefore, be applied to any case of isomer- 
ism, however readily the isomerides may be converted into one another. 
It is impossible to say whether tautomerism exists; but it has at 
least been proved by the work of Knorr that the two substances repre- 
‘sented by the formule 
CH,:CO:CH,:CO,Et and CH,:C(OH) :CH:CO,Et 
are not tautomeric, but have a real existence as well-defined isomeric 
compounds, which only change into one another under definite physical 
and chemical conditions. 
. The word desmotropy was introduced by Jacobson (Ber. 1887, 20, 
1732, footnote; 1888, 21, 2628, footnote) in 1887, when it had become 
evident that Laar’s theory of tautomerism had broken down in the 
very case to which it had been most frequently applied, namely, the 
labile isomerism which results from the contiguity of a double bond 
and an acidic hydrogen atom. Jacobson considered ‘that the known 
